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 <head>
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  <title>P1403R0: Experience Report: Implementing a Coroutines TS Frontend to an Existing Tasking Library</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
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/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


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/*                                    Print                                   */
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1403R0<br>Experience Report: Implementing a Coroutines TS Frontend to an Existing Tasking Library</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Draft Proposal, <time class="dt-updated" datetime="2019-01-16">2019-01-16</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="wg21.link/P1403">wg21.link/P1403</a>
     <dt>Issue Tracking:
     <dd><a href="https://github.com/ORNL/cpp-proposals-pub">GitHub</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:dshollm@sandia.gov">David S. Hollman</a>
     <dt>Audience:
     <dd>SG1, EWG, WG21
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#revision-history"><span class="secno">1</span> <span class="content">Revision History</span></a>
     <ol class="toc">
      <li><a href="#p1403r0"><span class="secno">1.1</span> <span class="content">P1403R0</span></a>
     </ol>
    <li><a href="#background"><span class="secno">2</span> <span class="content">Background</span></a>
    <li><a href="#implementation"><span class="secno">3</span> <span class="content">Implementation</span></a>
    <li><a href="#benchmarks"><span class="secno">4</span> <span class="content">Benchmarks</span></a>
    <li><a href="#appendix-source-code"><span class="secno">5</span> <span class="content">Appendix: Source code</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="revision-history"><span class="secno">1. </span><span class="content">Revision History</span><a class="self-link" href="#revision-history"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="p1403r0"><span class="secno">1.1. </span><span class="content">P1403R0</span><a class="self-link" href="#p1403r0"></a></h3>
   <ul>
    <li data-md>
     <p>Initial version</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="background"><span class="secno">2. </span><span class="content">Background</span><a class="self-link" href="#background"></a></h2>
   <p>Kokkos is a performance portability library for HPC applications.  It provides abstractions for writing algorithms that are generic over the details of their execution and storage, and provides backends for execution with CPUs, GPUs, and other accelerators.</p>
   <p>While most of Kokkos is focused on providing loop-level abstractions (like <code class="highlight"><c- n>for_each</c-></code> and <code class="highlight"><c- n>reduce</c-></code>), it also provides a facility for fine-grained task DAG execution.  The application-level interface for Kokkos tasking takes the form of a typical <code class="highlight"><c- n>async</c-></code>/<code class="highlight"><c- n>future</c-></code> programming model with some important caveats.  Most importantly, futures in Kokkos cannot be waited on; instead, the user must respawn the current task with the desired future as a dependence.  The respawned task will start over from the beginning when the given dependence is ready, and it is up to the user to store any state needed across respawns and to return to the previous point of progress before the respawn was requested.  This is perhaps best illustrated by way of the ubiquitous recursive Fibonacci example:</p>
<pre class="language-c++ highlight"><c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>Scheduler</c-><c- o>></c->
<c- k>struct</c-> <c- n>Fib</c-> <c- p>{</c->
  <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- b>long</c-><c- p>;</c->
  <c- n>using</c-> <c- n>future_type</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>BasicFuture</c-><c- o>&lt;</c-><c- b>long</c-><c- p>,</c-> <c- n>Scheduler</c-><c- o>></c-><c- p>;</c->
  <c- n>using</c-> <c- n>team_member_type</c-> <c- o>=</c-> <c- kr>typename</c-> <c- n>Scheduler</c-><c- o>::</c-><c- n>member_type</c-><c- p>;</c->

  <c- k>const</c-> <c- n>value_type</c-> <c- n>n</c-><c- p>;</c->
  <c- n>future_type</c-> <c- n>deps</c-><c- p>[</c-><c- mi>2</c-><c- p>];</c->

  <c- n>KOKKOS_INLINE_FUNCTION</c-> <c- c1>// things like __device__, when appropriate</c->
  <c- b>void</c-> <c- n>operator</c-><c- p>()(</c-><c- n>team_member_type</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>member</c-><c- p>,</c-> <c- n>value_type</c-><c- o>&amp;</c-> <c- n>result</c-><c- p>)</c-> <c- p>{</c->
    <c- k>auto</c-> <c- n>sched</c-> <c- o>=</c-> <c- n>member</c-><c- p>.</c-><c- n>scheduler</c-><c- p>();</c->
    <c- k>if</c-><c- p>(</c-><c- n>n</c-> <c- o>&lt;</c-> <c- mi>2</c-><c- p>)</c-> <c- p>{</c->
      <c- c1>// recursive base case:</c->
      <c- n>result</c-> <c- o>=</c-> <c- n>n</c-><c- p>;</c->
    <c- p>}</c->
    <c- k>else</c-> <c- k>if</c-><c- p>(</c-><c- n>deps</c-><c- p>[</c-><c- mi>0</c-><c- p>].</c-><c- n>is_ready</c-><c- p>()</c-> <c- o>&amp;&amp;</c-> <c- n>deps</c-><c- p>[</c-><c- mi>1</c-><c- p>].</c-><c- n>is_ready</c-><c- p>())</c-> <c- p>{</c->
      <c- c1>// this is the respawn case, since the dependences will only</c->
      <c- c1>// be ready after respawn:</c->
      <c- n>result</c-> <c- o>=</c-> <c- n>deps</c-><c- p>[</c-><c- mi>0</c-><c- p>].</c-><c- n>get</c-><c- p>()</c-> <c- o>+</c-> <c- n>deps</c-><c- p>[</c-><c- mi>1</c-><c- p>].</c-><c- n>get</c-><c- p>();</c->
    <c- p>}</c->
    <c- k>else</c-> <c- p>{</c->
      <c- c1>// Spawn tasks for Fib(n-1) and Fib(n-2) and store their futures</c->
      <c- c1>// in a member variable</c->
      <c- n>deps</c-><c- p>[</c-><c- mi>0</c-><c- p>]</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>task_spawn</c-><c- p>(</c->
        <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskSingle</c-><c- p>(</c-><c- n>sched</c-><c- p>,</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>High</c-><c- p>),</c->
        <c- n>Fib</c-><c- p>{</c-><c- n>n</c-> <c- o>-</c-> <c- mi>2</c-><c- p>}</c->
      <c- p>);</c->
      <c- n>deps</c-><c- p>[</c-><c- mi>1</c-><c- p>]</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>task_spawn</c-><c- p>(</c->
        <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskSingle</c-><c- p>(</c-><c- n>sched</c-><c- p>),</c->
        <c- n>Fib</c-><c- p>{</c-><c- n>n</c-> <c- o>-</c-> <c- mi>1</c-><c- p>}</c->
      <c- p>);</c->
      <c- c1>// Aggregate the dependences into one future</c->
      <c- k>auto</c-> <c- n>fib_all</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>when_all</c-><c- p>(</c-><c- n>deps</c-><c- p>,</c-> <c- mi>2</c-><c- p>);</c->
      <c- c1>// Respawn this task dependent on the aggregate future</c->
      <c- n>Kokkos</c-><c- o>::</c-><c- n>respawn</c-><c- p>(</c-><c- n>this</c-><c- p>,</c-> <c- n>fib_all</c-><c- p>,</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>High</c-><c- p>);</c->
    <c- p>}</c->
  <c- p>}</c->
<c- p>};</c->
</pre>
   <p>With only a couple of hours of work, we were able to use the Coroutines TS to create a wrapper to this interface that afforded the following code:</p>
<pre class="language-c++ highlight"><c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>Scheduler</c-><c- o>></c->
<c- k>struct</c-> <c- n>FibCoroutine</c-> <c- p>{</c->
  <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- b>long</c-><c- p>;</c->
  <c- n>using</c-> <c- n>coroutine_scheduler_type</c-> <c- o>=</c-> <c- n>BasicCoroutineScheduler</c-><c- o>&lt;</c-><c- n>Scheduler</c-><c- o>></c-><c- p>;</c->

  <c- n>value_type</c-> <c- n>n</c-><c- p>;</c->

  <c- kr>typename</c-> <c- n>coroutine_scheduler_type</c-><c- o>::</c-><c- n>template</c-> <c- n>coroutine_return_type</c-><c- o>&lt;</c-><c- b>long</c-><c- o>></c->
  <c- n>operator</c-><c- p>()(</c-><c- n>coroutine_scheduler_type</c-><c- o>&amp;</c-> <c- n>sched</c-><c- p>)</c-> <c- p>{</c->
    <c- k>if</c-><c- p>(</c-><c- n>n</c-> <c- o>&lt;</c-> <c- mi>2</c-><c- p>)</c-> <c- p>{</c->
      <c- n>co_return</c-> <c- n>n</c-><c- p>;</c->
    <c- p>}</c->
    <c- k>else</c-> <c- p>{</c->
      <c- k>auto</c-> <c- n>f_2</c-> <c- o>=</c-> <c- n>sched</c-><c- p>.</c-><c- n>spawn</c-><c- p>(</c-><c- n>FibCoroutine</c-><c- p>{</c-><c- n>n</c-><c- o>-</c-><c- mi>2</c-><c- p>},</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>High</c-><c- p>);</c->
      <c- k>auto</c-> <c- n>f_1</c-> <c- o>=</c-> <c- n>sched</c-><c- p>.</c-><c- n>spawn</c-><c- p>(</c-><c- n>FibCoroutine</c-><c- p>{</c-><c- n>n</c-><c- o>-</c-><c- mi>1</c-><c- p>});</c->
      <c- k>auto</c-> <c- p>[</c-><c- n>v1</c-><c- p>,</c-> <c- n>v2</c-><c- p>]</c-> <c- o>=</c-> <c- n>co_await</c-> <c- n>sched</c-><c- p>.</c-><c- n>when_all</c-><c- p>(</c-><c- n>f_1</c-><c- p>,</c-> <c- n>f_2</c-><c- p>);</c->
      <c- n>co_return</c-> <c- n>v1</c-> <c- o>+</c-> <c- n>v2</c-><c- p>;</c->
    <c- p>}</c->
  <c- p>}</c->
<c- p>};</c->
</pre>
   <p>The implementation of this wrapper required no modification to Kokkos itself, and benchmarks showed zero (or less!) overhead for this interface at runtime.  This wrapper code is presented below in its entirety.  It has been included in contiguous form (along with driver code to run the benchmarks) in an appendix, in case the section below is difficult to read as is.</p>
   <p>The code for the wrapper was prepared by team members with little or no experience using the Coroutines TS.  We present this as further evidence that the Coroutines TS is sufficiently baked and should be merged into the C++20 draft.</p>
   <h2 class="heading settled" data-level="3" id="implementation"><span class="secno">3. </span><span class="content">Implementation</span><a class="self-link" href="#implementation"></a></h2>
   <p>The basic implementation strategy employed involved wrapping the Kokkos <code class="highlight"><c- n>Scheduler</c-></code> abstraction in a coroutine-aware scheduler with similar semantics:</p>
<pre class="language-c++ highlight"><c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>Scheduler</c-><c- o>></c->
<c- k>struct</c-> <c- n>BasicCoroutineScheduler</c-> <c- p>{</c->
</pre>
   <p>It holds an instance of the Kokkos scheduler so that the coroutine scheduler can delegate to it:</p>
<pre class="language-c++ highlight">  <c- n>Scheduler</c-> <c- n>m_scheduler</c-><c- p>;</c->
</pre>
   <p>We use a Kokkos future to communicate the user’s suspension points as dependences in the Kokkos tasking backend:</p>
<pre class="language-c++ highlight">  <c- n>using</c-> <c- n>future_type</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>BasicFuture</c-><c- o>&lt;</c-><c- b>void</c-><c- p>,</c-> <c- n>Scheduler</c-><c- o>></c-><c- p>;</c->
</pre>
   <p>The coroutine scheduler provides <code class="highlight"><c- n>Awaitable</c-></code> nested types to be returned by <code class="highlight"><c- n>scheduler</c-><c- p>.</c-><c- n>spawn</c-><c- p>()</c-></code> and <code class="highlight"><c- n>scheduler</c-><c- p>.</c-><c- n>when_all</c-><c- p>()</c-></code>:</p>
<pre class="language-c++ highlight">  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>CoroutineFunctor</c-><c- o>></c->
  <c- k>struct</c-> <c- n>SpawnedAwaitable</c-><c- p>;</c->
  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-><c- p>...</c-><c- o>></c->
  <c- k>struct</c-> <c- n>WhenAllAwaitable</c-><c- p>;</c->
</pre>
   <p>The coroutine scheduler also provides the return type for the user’s coroutine, through which all of the necessary plumbing is communicated to the compiler:</p>
<pre class="language-c++ highlight">  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c->
  <c- k>struct</c-> <c- n>coroutine_return_type</c-> <c- p>{</c->
    <c- c1>// assume value_type is not void for brevity</c->
    <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- n>T</c-><c- p>;</c->
    <c- c1>// forward declaration:</c->
    <c- k>struct</c-> <c- n>promise_type</c-><c- p>;</c->
    <c- c1>// for brevity:</c->
    <c- n>using</c-> <c- n>coroutine_handle</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>coroutine_handle</c-><c- o>&lt;</c-><c- n>promise_type</c-><c- o>></c-><c- p>;</c->
</pre>
   <p>We store the <code class="highlight"><c- n>coroutine_handle</c-></code> in a data member of the return object:</p>
<pre class="language-c++ highlight">    <c- n>coroutine_handle</c-> <c- n>handle</c-><c- p>;</c->
</pre>
   <p>The coroutine promise type holds the storage for the result as well as the future representing the dependence at the current suspend point:</p>
<pre class="language-c++ highlight">    <c- k>struct</c-> <c- n>promise_type</c-> <c- p>{</c->

      <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-> <c- n>result</c-><c- p>;</c->
      <c- n>future_type</c-><c- o>*</c-> <c- n>m_current_dep</c-> <c- o>=</c-> <c- n>nullptr</c-><c- p>;</c->
</pre>
   <p>Promise creation must be followed by an initial suspend in order to set up <code class="highlight"><c- n>m_current_dep</c-></code>:</p>
<pre class="language-c++ highlight">      <c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>suspend_always</c-> <c- n>initial_suspend</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- p>{</c-> <c- p>};</c-> <c- p>}</c->
</pre>
   <p>The coroutine return must be followed by a suspend in order to extract the result before the promise is destroyed:</p>
<pre class="language-c++ highlight">      <c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>suspend_always</c-> <c- n>final_suspend</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- p>{</c-> <c- p>};</c-> <c- p>}</c->
</pre>
   <p>When the user’s coroutine returns, we simply store the result:</p>
<pre class="language-c++ highlight">      <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>Value</c-><c- o>></c->
      <c- b>void</c-> <c- n>return_value</c-><c- p>(</c-><c- n>Value</c-><c- o>&amp;&amp;</c-> <c- n>value</c-><c- p>)</c-> <c- p>{</c->
        <c- n>result</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Value</c-><c- o>></c-><c- p>(</c-><c- n>value</c-><c- p>);</c->
      <c- p>}</c->
</pre>
   <p>The returned object holds the coroutine handle, which is constructed from the promise directly:</p>
<pre class="language-c++ highlight">      <c- n>coroutine_return_type</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c->
      <c- n>get_return_object</c-><c- p>()</c-> <c- p>{</c->
        <c- k>return</c-> <c- p>{</c-> <c- n>coroutine_handle</c-><c- o>::</c-><c- n>from_promise</c-><c- p>(</c-><c- o>*</c-><c- n>this</c-><c- p>)</c-> <c- p>};</c->
      <c- p>}</c->
</pre>
   <p>Now comes the critical part.  When the user <code class="highlight"><c- n>co_await</c-></code>s on the result of a <code class="highlight"><c- n>spawn</c-></code> or <code class="highlight"><c- n>when_all</c-></code>, we need to point the future that communicates the suspension of the current coroutine to the dependence that the suspension needs to wait on.  Fortunately, <code class="highlight"><c- n>await_transform</c-></code> allows us to do that:</p>
<pre class="language-c++ highlight">      <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>ValueType</c-><c- o>></c->
      <c- kr>typename</c-> <c- n>SpawnedAwaitable</c-><c- o>&lt;</c-><c- n>ValueType</c-><c- o>>::</c-><c- n>template</c-> <c- n>SpawnedPromise</c-><c- o>&lt;</c-><c- n>promise_type</c-><c- o>></c->
      <c- n>await_transform</c-><c- p>(</c-><c- n>SpawnedAwaitable</c-><c- o>&lt;</c-><c- n>ValueType</c-><c- o>>&amp;</c-> <c- n>awaitable</c-><c- p>)</c-> <c- p>{</c->
        <c- k>return</c-> <c- p>{</c-> <c- n>awaitable</c-><c- p>.</c-><c- n>m_done_future</c-><c- p>,</c-> <c- o>*</c-><c- n>this</c-> <c- p>};</c->
      <c- p>}</c->
</pre>
   <p>By storing a reference to the parent promise in the awaitable (<code class="highlight"><c- o>*</c-><c- k>this</c-></code> in the above code), the transformed awaitable is able to communicate its dependence through to the Kokkos backend when <code class="highlight"><c- n>await_suspend</c-></code> is called.  (We don’t do this now because <code class="highlight"><c- n>await_ready</c-></code> may return true, obviating the need to communicate the dependence to the backend.)  A similar thing happens for the <code class="highlight"><c- n>when_all</c-></code> case:</p>
<pre class="language-c++ highlight">      <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-><c- p>...</c-> <c- n>Awaitables</c-><c- o>></c->
      <c- kr>typename</c-> <c- n>WhenAllAwaitable</c-><c- o>&lt;</c-><c- n>Awaitables</c-><c- p>...</c-><c- o>>::</c-><c- n>template</c-> <c- n>SpawnedPromise</c-><c- o>&lt;</c-><c- n>promise_type</c-><c- o>></c->
      <c- n>await_transform</c-><c- p>(</c-><c- n>WhenAllAwaitable</c-><c- o>&lt;</c-><c- n>Awaitables</c-><c- p>...</c-><c- o>></c-><c- k>const</c-> <c- o>&amp;</c-> <c- n>awaitable</c-><c- p>)</c-> <c- p>{</c->
        <c- k>return</c-> <c- p>{</c->
          <c- n>awaitable</c-><c- p>.</c-><c- n>m_done_future</c-><c- p>,</c->
          <c- n>awaitable</c-><c- p>.</c-><c- n>m_value_futures</c-><c- p>,</c->
          <c- o>*</c-><c- n>this</c->
        <c- p>};</c->
      <c- p>}</c->
</pre>
   <p>And that’s it for the <code class="highlight"><c- n>promise_type</c-></code>.  We include a couple of convenience methods in the <code class="highlight"><c- n>coroutine_return_type</c-></code> to make the rest of the code more readable, and then close out that class also:</p>
<pre class="language-c++ highlight">    <c- p>};</c->


    <c- b>bool</c-> <c- nf>is_done</c-><c- p>()</c-> <c- p>{</c->
      <c- k>return</c-> <c- b>bool</c-><c- p>(</c-><c- n>handle</c-><c- p>.</c-><c- n>promise</c-><c- p>().</c-><c- n>result</c-><c- p>);</c->
    <c- p>}</c->

    <c- n>value_type</c-><c- o>&amp;</c-> <c- n>get_result</c-><c- p>()</c-> <c- p>{</c->
      <c- k>return</c-> <c- o>*</c-><c- n>handle</c-><c- p>.</c-><c- n>promise</c-><c- p>().</c-><c- n>result</c-><c- p>;</c->
    <c- p>}</c->

    <c- n>coroutine_handle</c-> <c- n>handle</c-><c- p>;</c->

  <c- p>};</c->
</pre>
   <p>To communicate the dependence structure of the user’s coroutine to the backend, we use a Kokkos task functor, just like the one in the old version of the Fibonacci example above:</p>
<pre class="language-c++ highlight">  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>CoroutineFunctor</c-><c- o>></c->
  <c- k>struct</c-> <c- n>TaskFunctor</c-> <c- p>{</c->
    <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- kr>typename</c-> <c- n>CoroutineFunctor</c-><c- o>::</c-><c- n>value_type</c-><c- p>;</c->
</pre>
   <p>We store the user’s functor itself, the coroutine return object, and the suspension dependence as members of this <code class="highlight"><c- n>TaskFunctor</c-></code>, so that we can find them when Kokkos respawns us:</p>
<pre class="language-c++ highlight">    <c- n>CoroutineFunctor</c-> <c- n>m_functor</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>coroutine_return_type</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>>></c-> <c- n>m_coroutine_return</c-><c- p>;</c->
    <c- n>future_type</c-> <c- n>m_current_dep</c-><c- p>;</c->
</pre>
   <p>Just like in the Fibonacci example above, we provide a call operator that Kokkos will invoke when all of the task’s dependences are ready:</p>
<pre class="language-c++ highlight">    <c- b>void</c-> <c- nf>operator</c-><c- p>()(</c-><c- kr>typename</c-> <c- n>Scheduler</c-><c- o>::</c-><c- n>member_type</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>member</c-><c- p>,</c-> <c- n>value_type</c-><c- o>&amp;</c-> <c- n>value</c-><c- p>)</c-> <c- p>{</c->
</pre>
   <p>We create an instance of the coroutine scheduler (the "wrapper" that we’re building) to pass to the user’s coroutine functor:</p>
<pre class="language-c++ highlight">      <c- k>auto</c-> <c- n>coro_scheduler</c-> <c- o>=</c-> <c- n>coroutine_scheduler_type</c-><c- p>{</c-><c- n>member</c-><c- p>.</c-><c- n>scheduler</c-><c- p>()};</c->
</pre>
   <p>Keeping in mind that we are going to respawn this functor, we need to check which respawn we’re on in the body of the call operator.  If the coroutine return object hasn’t been created yet, we’re on our first time through, and we should create it:</p>
<pre class="language-c++ highlight">      <c- k>if</c-><c- p>(</c-><c- n>not</c-> <c- n>m_coroutine_return</c-><c- p>)</c-> <c- p>{</c->
        <c- c1>// initial invocation</c->
        <c- n>m_coroutine_return</c-> <c- o>=</c-> <c- n>m_functor</c-><c- p>(</c-><c- n>coro_scheduler</c-><c- p>);</c->
      <c- p>}</c->
</pre>
   <p>Any time Kokkos calls in to this functor, all of the prerequisites of the task it represents will be ready.  In this case, this means that the dependence we suspended for (if any) is ready, so we should resume the coroutine.  First, we store a (non-owning) pointer to our future that regulates suspension in the promise, so that <code class="highlight"><c- n>await_transform</c-></code> (and the <code class="highlight"><c- n>await_suspend</c-></code> of the awaitable it returns) can find it:</p>
<pre class="language-c++ highlight">        <c- n>m_coroutine_return</c-><c- o>-></c-><c- n>handle</c-><c- p>.</c-><c- n>promise</c-><c- p>().</c-><c- n>m_current_dep</c-> <c- o>=</c-> <c- o>&amp;</c-><c- n>m_current_dep</c-><c- p>;</c->
</pre>
   <p>Then we resume the coroutine:</p>
<pre class="language-c++ highlight">        <c- n>m_coroutine_return</c-><c- o>-></c-><c- n>handle</c-><c- p>.</c-><c- n>resume</c-><c- p>();</c->
</pre>
   <p>Now if the coroutine is done, we need to communicate the return value back to Kokkos (which is done by assigning to a reference passed in as a parameter) and destroy the coroutine handle:</p>
<pre class="language-c++ highlight">      <c- k>if</c-><c- p>(</c-><c- n>m_coroutine_return</c-><c- o>-></c-><c- n>is_done</c-><c- p>())</c-> <c- p>{</c->
        <c- n>value</c-> <c- o>=</c-> <c- n>m_coroutine_return</c-><c- o>-></c-><c- n>get_result</c-><c- p>();</c->
        <c- n>m_coroutine_return</c-><c- o>-></c-><c- n>handle</c-><c- p>.</c-><c- n>destroy</c-><c- p>();</c->
      <c- p>}</c->
</pre>
   <p>Otherwise, we need to respawn with the suspension dependence as our prerequisite (this doesn’t actually respawn the task in place, but rather marks the task for respawning and handles the respawn when the task returns):</p>
<pre class="language-c++ highlight">      <c- k>else</c-> <c- p>{</c->
        <c- n>Kokkos</c-><c- o>::</c-><c- n>respawn</c-><c- p>(</c-><c- n>this</c-><c- p>,</c-> <c- n>m_current_dep</c-><c- p>,</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>High</c-><c- p>);</c->
</pre>
   <p>Futures in Kokkos are reference counted and cannot be reused once they’re made ready, so we need to replace the future held by <code class="highlight"><c- k>this</c-></code> with an empty one:</p>
<pre class="language-c++ highlight">        <c- n>m_current_dep</c-> <c- o>=</c-> <c- n>future_type</c-><c- p>{};</c->
      <c- p>}</c->
</pre>
   <p>And that’s it for the task functor:</p>
<pre class="language-c++ highlight">    <c- p>}</c->

  <c- p>};</c->
</pre>
   <p>We now need to implement the type returned by <code class="highlight"><c- n>scheduler</c-><c- p>.</c-><c- n>spawn</c-><c- p>()</c-></code>, which stores the value returned by <code class="highlight"><c- n>Kokkos</c-><c- o>::</c-><c- n>task_spawn</c-><c- p>()</c-></code> in a data member:</p>
<pre class="language-c++ highlight">  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>ValueType</c-><c- o>></c->
  <c- k>struct</c-> <c- n>SpawnedAwaitable</c-> <c- p>{</c->
    <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- n>ValueType</c-><c- p>;</c->
    <c- n>using</c-> <c- n>scheduler_type</c-> <c- o>=</c-> <c- n>Scheduler</c-><c- p>;</c->
    <c- n>using</c-> <c- n>value_future_type</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>BasicFuture</c-><c- o>&lt;</c-><c- n>value_type</c-><c- p>,</c-> <c- n>scheduler_type</c-><c- o>></c-><c- p>;</c->

    <c- n>value_future_type</c-> <c- n>m_done_future</c-><c- p>;</c->
</pre>
   <p>We provide an accessor for the user to interface with existing Kokkos tasking code:</p>
<pre class="language-c++ highlight">    <c- n>value_future_type</c-> <c- nf>get_future</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>m_done_future</c-><c- p>;</c-> <c- p>}</c->
</pre>
   <p>And then we need to implement the nested promise type, which is returned by <code class="highlight"><c- n>await_transform</c-></code> when the user applies operator <code class="highlight"><c- n>co_await</c-></code>.  It stores a copy of the future from the <code class="highlight"><c- n>SpawnedAwaitable</c-></code> (futures in Kokkos have reference semantics with shared ownership) and a reference to the <code class="highlight"><c- n>promise_type</c-></code> instance from the enclosing coroutine:</p>
<pre class="language-c++ highlight">    <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>ParentPromise</c-><c- o>></c->
    <c- k>struct</c-> <c- n>SpawnedPromise</c-> <c- p>{</c->

      <c- n>value_future_type</c-> <c- n>m_done_future</c-><c- p>;</c->
      <c- n>ParentPromise</c-><c- o>&amp;</c-> <c- n>m_parent_promise</c-><c- p>;</c->
</pre>
   <p>The <code class="highlight"><c- n>await_ready</c-></code> hook simply checks if the future is ready:</p>
<pre class="language-c++ highlight">      <c- b>bool</c-> <c- nf>await_ready</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> 
        <c- k>return</c-> <c- n>m_done_future</c-><c- p>.</c-><c- n>is_ready</c-><c- p>();</c->
      <c- p>}</c->
</pre>
   <p>And the <code class="highlight"><c- n>await_suspend</c-></code> hook assigns the future to the one pointed to by the parent promise, which should be set to the <code class="highlight"><c- n>TaskFunctor</c-></code>'s future that controls suspension.  (Note that non-<code class="highlight"><c- b>void</c-></code> futures can be assigned to <code class="highlight"><c- b>void</c-></code> futures in Kokkos.)</p>
<pre class="language-c++ highlight">      <c- b>void</c->
      <c- nf>await_suspend</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>coroutine_handle</c-><c- o>&lt;</c-><c- n>ParentPromise</c-><c- o>></c-> <c- n>handle</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
        <c- o>*</c-><c- n>m_parent_promise</c-><c- p>.</c-><c- n>m_current_dep</c-> <c- o>=</c-> <c- n>m_done_future</c-><c- p>;</c->
      <c- p>}</c->
</pre>
   <p>Finally, since the <code class="highlight"><c- n>TaskFunctor</c-></code> ensures that <code class="highlight"><c- n>resume</c-><c- p>()</c-></code> is only called on the coroutine handle when the future is ready, <code class="highlight"><c- n>await_resume</c-><c- p>()</c-></code> is trivial:</p>
<pre class="language-c++ highlight">      <c- n>value_type</c-><c- o>&amp;</c->
      <c- n>await_resume</c-><c- p>()</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>m_done_future</c-><c- p>.</c-><c- n>get</c-><c- p>();</c->
      <c- p>}</c->
    <c- p>};</c->
  <c- p>};</c->
</pre>
   <p>The implementation of <code class="highlight"><c- n>WhenAllAwaitable</c-></code> is similar, albeit messier:</p>
<pre class="language-c++ highlight">  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-><c- p>...</c-> <c- n>Awaitables</c-><c- o>></c->
  <c- k>struct</c-> <c- n>WhenAllAwaitable</c-> <c- p>{</c->
    <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- kr>typename</c-> <c- n>Awaitables</c-><c- o>::</c-><c- n>value_type</c-><c- p>...</c-><c- o>></c-><c- p>;</c->
    <c- n>using</c-> <c- n>scheduler_type</c-> <c- o>=</c-> <c- n>Scheduler</c-><c- p>;</c->
    <c- n>using</c-> <c- n>aggregate_future_type</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>BasicFuture</c-><c- o>&lt;</c-><c- b>void</c-><c- p>,</c-> <c- n>scheduler_type</c-><c- o>></c-><c- p>;</c->
    <c- n>using</c-> <c- n>value_future_tuple</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c->
      <c- n>Kokkos</c-><c- o>::</c-><c- n>BasicFuture</c-><c- o>&lt;</c-><c- kr>typename</c-> <c- n>Awaitables</c-><c- o>::</c-><c- n>value_type</c-><c- p>,</c-> <c- n>scheduler_type</c-><c- o>></c-><c- p>...</c->
    <c- o>></c-><c- p>;</c->

    <c- n>aggregate_future_type</c-> <c- n>m_done_future</c-><c- p>;</c->
    <c- n>value_future_tuple</c-> <c- n>m_value_futures</c-><c- p>;</c->
    
    <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>ParentPromise</c-><c- o>></c->
    <c- k>struct</c-> <c- n>SpawnedPromise</c-> <c- p>{</c->

      <c- n>aggregate_future_type</c-> <c- n>m_done_future</c-><c- p>;</c->
      <c- n>value_future_tuple</c-> <c- n>m_value_futures</c-><c- p>;</c->
      <c- n>ParentPromise</c-><c- o>&amp;</c-> <c- n>m_parent_promise</c-><c- p>;</c->

      <c- n>SpawnedPromise</c-><c- p>(</c->
        <c- n>aggregate_future_type</c-> <c- n>arg_done_future</c-><c- p>,</c->
        <c- n>value_future_tuple</c-> <c- n>arg_value_futures</c-><c- p>,</c->
        <c- n>ParentPromise</c-><c- o>&amp;</c-> <c- n>arg_parent_promise</c->
      <c- p>)</c-> <c- o>:</c-> <c- n>m_done_future</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>arg_done_future</c-><c- p>)),</c->
          <c- n>m_value_futures</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>arg_value_futures</c-><c- p>)),</c->
          <c- n>m_parent_promise</c-><c- p>(</c-><c- n>arg_parent_promise</c-><c- p>)</c->
      <c- p>{</c-> <c- p>}</c->

      <c- b>bool</c-> <c- n>await_ready</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> 
        <c- k>return</c-> <c- n>m_done_future</c-><c- p>.</c-><c- n>is_ready</c-><c- p>();</c->
      <c- p>}</c->

      <c- b>void</c->
      <c- n>await_suspend</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>coroutine_handle</c-><c- o>&lt;</c-><c- n>ParentPromise</c-><c- o>></c-> <c- n>handle</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
        <c- o>*</c-><c- n>m_parent_promise</c-><c- p>.</c-><c- n>m_current_dep</c-> <c- o>=</c-> <c- n>m_done_future</c-><c- p>;</c->
      <c- p>}</c->

      <c- n>template</c-> <c- o>&lt;</c-><c- b>size_t</c-><c- p>...</c-> <c- n>Idxs</c-><c- o>></c->
      <c- n>value_type</c->
      <c- n>_await_resume_impl</c-><c- p>(</c->
        <c- n>std</c-><c- o>::</c-><c- n>integer_sequence</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>Idxs</c-><c- p>...</c-><c- o>></c->
      <c- p>)</c->
      <c- p>{</c->
        <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>make_tuple</c-><c- p>(</c->
          <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- n>Idxs</c-><c- o>></c-><c- p>(</c-><c- n>m_value_futures</c-><c- p>).</c-><c- n>get</c-><c- p>())...</c->
        <c- p>);</c->
      <c- p>}</c->

      <c- n>value_type</c->
      <c- n>await_resume</c-><c- p>()</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>_await_resume_impl</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>index_sequence_for</c-><c- o>&lt;</c-><c- n>Awaitables</c-><c- p>...</c-><c- o>></c-><c- p>{});</c->
      <c- p>}</c->
    <c- p>};</c->
  <c- p>};</c->
</pre>
   <p>Finally, the user-facing <code class="highlight"><c- n>spawn</c-><c- p>()</c-></code> and <code class="highlight"><c- n>when_all</c-><c- p>()</c-></code> methods merely delegate to their corresponding implementations in Kokkos:</p>
<pre class="language-c++ highlight">  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>CoroutineFunctor</c-><c- o>></c->
  <c- n>SpawnedAwaitable</c-><c- o>&lt;</c-><c- kr>typename</c-> <c- n>CoroutineFunctor</c-><c- o>::</c-><c- n>value_type</c-><c- o>></c->
  <c- n>spawn</c-><c- p>(</c-><c- n>CoroutineFunctor</c-> <c- n>functor</c-><c- p>,</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-> <c- n>priority</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>Regular</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
    <c- k>return</c-> <c- p>{</c->
      <c- n>Kokkos</c-><c- o>::</c-><c- n>task_spawn</c-><c- p>(</c->
        <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskSingle</c-><c- p>(</c-><c- n>m_scheduler</c-><c- p>,</c-> <c- n>priority</c-><c- p>),</c-> <c- n>TaskFunctor</c-><c- o>&lt;</c-><c- n>CoroutineFunctor</c-><c- o>></c-><c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>functor</c-><c- p>)}</c->
      <c- p>)</c->
    <c- p>};</c->
  <c- p>}</c->

  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-><c- p>...</c-> <c- n>Awaitables</c-><c- o>></c->
  <c- n>WhenAllAwaitable</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>decay_t</c-><c- o>&lt;</c-><c- n>Awaitables</c-><c- o>></c-><c- p>...</c-><c- o>></c->
  <c- n>when_all</c-><c- p>(</c-><c- n>Awaitables</c-><c- o>&amp;&amp;</c-><c- p>...</c-> <c- n>awaitables</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
    <c- n>future_type</c-> <c- n>all_void</c-><c- p>[]</c-> <c- o>=</c-> <c- p>{</c-> <c- p>(</c-><c- n>awaitables</c-><c- p>.</c-><c- n>m_done_future</c-><c- p>)...</c-> <c- p>};</c->
    <c- k>return</c-> <c- p>{</c->
      <c- n>Kokkos</c-><c- o>::</c-><c- n>when_all</c-><c- p>(</c->
        <c- n>all_void</c-><c- p>,</c-> <c- k>sizeof</c-><c- p>...(</c-><c- n>Awaitables</c-><c- p>)</c->
      <c- p>),</c->
      <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Awaitables</c-><c- o>></c-><c- p>(</c-><c- n>awaitables</c-><c- p>)...</c->
    <c- p>};</c->
  <c- p>}</c->
<c- p>};</c->
</pre>
   <p>And that’s it.  Notice nothing internal to Kokkos needed to be touched to make this work.  We feel this is evidence that the current form of the Coroutines TS nicely complements existing practice.</p>
   <h2 class="heading settled" data-level="4" id="benchmarks"><span class="secno">4. </span><span class="content">Benchmarks</span><a class="self-link" href="#benchmarks"></a></h2>
   <p>In our informal benchmarking of the Fibonacci example given above, we found that the coroutine-wrapped version was consistently a bit <em>faster</em> than the non-coroutine version—that is, the abstraction actually has <em>negative</em> overhead.  This is attributed to the fact that <code class="highlight"><c- n>await_ready</c-></code> can check for the completion of the (eagerly spawned) task that the awaitable depends on and skip suspension altogether.</p>
   <h2 class="heading settled" data-level="5" id="appendix-source-code"><span class="secno">5. </span><span class="content">Appendix: Source code</span><a class="self-link" href="#appendix-source-code"></a></h2>
<pre class="language-c++ highlight"><c- cp>#include</c-> &lt;Kokkos_Core.hpp>

<c- cp>#include</c-> &lt;impl/Kokkos_Timer.hpp>

<c- cp>#include</c-> &lt;cstring>
<c- cp>#include</c-> &lt;cstdlib>
<c- cp>#include</c-> &lt;limits>
<c- cp>#include</c-> &lt;optional>
<c- cp>#include</c-> &lt;algorithm>
<c- cp>#include</c-> &lt;experimental/coroutine>
<c- cp>#include</c-> &lt;tuple>

<c- c1>//----------------------------------------------------------------------------—&lt;wbr>// uncomment this to get something that’s more analogous to what the non-coroutine</c->
<c- c1>// version has to do because there’s no way to short circuit the respawn dependent</c->
<c- c1>// on a ready future without coroutines</c->
<c- c1>//#define DISABLE_CHECK_IN_AWAIT_READY 1</c->

<c- c1>//----------------------------------------------------------------------------—&lt;wbr>template &lt;class Scheduler></c->
<c- k>struct</c-> <c- n>BasicCoroutineScheduler</c-> <c- p>{</c->

  <c- n>using</c-> <c- n>future_type</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>BasicFuture</c-><c- o>&lt;</c-><c- b>void</c-><c- p>,</c-> <c- n>Scheduler</c-><c- o>></c-><c- p>;</c->

  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>CoroutineFunctor</c-><c- o>></c->
  <c- k>struct</c-> <c- n>SpawnedAwaitable</c-><c- p>;</c->
  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-><c- p>...</c-><c- o>></c->
  <c- k>struct</c-> <c- n>WhenAllAwaitable</c-><c- p>;</c->

  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>T</c-><c- o>></c->
  <c- k>struct</c-> <c- n>coroutine_return_type</c-> <c- p>{</c->
    <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- n>T</c-><c- p>;</c->
    <c- c1>// assume value_type is not void for now</c->

    <c- k>struct</c-> <c- n>promise_type</c-><c- p>;</c->

    <c- n>using</c-> <c- n>coroutine_handle</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>coroutine_handle</c-><c- o>&lt;</c-><c- n>promise_type</c-><c- o>></c-><c- p>;</c->
  
    <c- k>struct</c-> <c- n>promise_type</c-> <c- p>{</c->

      <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-> <c- n>result</c-><c- p>;</c->
      <c- n>future_type</c-><c- o>*</c-> <c- n>m_current_dep</c-> <c- o>=</c-> <c- n>nullptr</c-><c- p>;</c->
      
      <c- c1>// promise creation must be followed by a suspend in order to set up m_current_dep</c->
      <c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>suspend_always</c-> <c- n>initial_suspend</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- p>{</c-> <c- p>};</c-> <c- p>}</c->
      <c- c1>// co_return must be followed by a suspend in order to use the result before it is destroyed</c->
      <c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>suspend_always</c-> <c- n>final_suspend</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- p>{</c-> <c- p>};</c-> <c- p>}</c->

      <c- n>coroutine_return_type</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c->
      <c- n>get_return_object</c-><c- p>()</c-> <c- p>{</c->
        <c- k>return</c-> <c- p>{</c-> <c- n>coroutine_handle</c-><c- o>::</c-><c- n>from_promise</c-><c- p>(</c-><c- o>*</c-><c- n>this</c-><c- p>)</c-> <c- p>};</c->
      <c- p>}</c->

      <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>Value</c-><c- o>></c->
      <c- b>void</c-> <c- n>return_value</c-><c- p>(</c-><c- n>Value</c-><c- o>&amp;&amp;</c-> <c- n>value</c-><c- p>)</c-> <c- p>{</c->
        <c- n>result</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>value</c-><c- p>);</c->
      <c- p>}</c->

      <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>ValueType</c-><c- o>></c->
      <c- kr>typename</c-> <c- n>SpawnedAwaitable</c-><c- o>&lt;</c-><c- n>ValueType</c-><c- o>>::</c-><c- n>template</c-> <c- n>SpawnedPromise</c-><c- o>&lt;</c-><c- n>promise_type</c-><c- o>></c->
      <c- n>await_transform</c-><c- p>(</c-><c- n>SpawnedAwaitable</c-><c- o>&lt;</c-><c- n>ValueType</c-><c- o>>&amp;</c-> <c- n>awaitable</c-><c- p>)</c-> <c- p>{</c->
        <c- k>return</c-> <c- p>{</c->
          <c- n>awaitable</c-><c- p>.</c-><c- n>m_done_future</c-><c- p>,</c->
          <c- o>*</c-><c- n>this</c->
        <c- p>};</c->
      <c- p>}</c->

      <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-><c- p>...</c-> <c- n>Awaitables</c-><c- o>></c->
      <c- kr>typename</c-> <c- n>WhenAllAwaitable</c-><c- o>&lt;</c-><c- n>Awaitables</c-><c- p>...</c-><c- o>>::</c-><c- n>template</c-> <c- n>SpawnedPromise</c-><c- o>&lt;</c-><c- n>promise_type</c-><c- o>></c->
      <c- n>await_transform</c-><c- p>(</c-><c- n>WhenAllAwaitable</c-><c- o>&lt;</c-><c- n>Awaitables</c-><c- p>...</c-><c- o>></c-><c- k>const</c-> <c- o>&amp;</c-> <c- n>awaitable</c-><c- p>)</c-> <c- p>{</c->
        <c- k>return</c-> <c- p>{</c->
          <c- n>awaitable</c-><c- p>.</c-><c- n>m_done_future</c-><c- p>,</c->
          <c- n>awaitable</c-><c- p>.</c-><c- n>m_value_futures</c-><c- p>,</c->
          <c- o>*</c-><c- n>this</c->
        <c- p>};</c->
      <c- p>}</c->

      <c- b>void</c-> <c- n>unhandled_exception</c-><c- p>()</c-> <c- p>{</c-> <c- n>std</c-><c- o>::</c-><c- n>abort</c-><c- p>();</c-> <c- p>}</c->

    <c- p>};</c->


    <c- b>bool</c-> <c- nf>is_done</c-><c- p>()</c-> <c- p>{</c->
      <c- k>return</c-> <c- b>bool</c-><c- p>(</c-><c- n>handle</c-><c- p>.</c-><c- n>promise</c-><c- p>().</c-><c- n>result</c-><c- p>);</c->
    <c- p>}</c->

    <c- n>value_type</c-><c- o>&amp;</c-> <c- n>get_result</c-><c- p>()</c-> <c- p>{</c->
      <c- k>return</c-> <c- o>*</c-><c- n>handle</c-><c- p>.</c-><c- n>promise</c-><c- p>().</c-><c- n>result</c-><c- p>;</c->
    <c- p>}</c->

    <c- n>coroutine_handle</c-> <c- n>handle</c-><c- p>;</c->

  <c- p>};</c->
  

  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>CoroutineFunctor</c-><c- o>></c->
  <c- k>struct</c-> <c- n>TaskFunctor</c-> <c- p>{</c->

    <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- kr>typename</c-> <c- n>CoroutineFunctor</c-><c- o>::</c-><c- n>value_type</c-><c- p>;</c->
    <c- n>using</c-> <c- n>coroutine_scheduler_type</c-> <c- o>=</c-> <c- n>BasicCoroutineScheduler</c-><c- p>;</c->

    <c- n>CoroutineFunctor</c-> <c- n>m_functor</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>optional</c-><c- o>&lt;</c-><c- n>coroutine_return_type</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>>></c-> <c- n>m_coroutine_return</c-><c- p>;</c->
    <c- n>future_type</c-> <c- n>m_current_dep</c-><c- p>;</c->

    <c- b>void</c-> <c- nf>operator</c-><c- p>()(</c-><c- kr>typename</c-> <c- n>Scheduler</c-><c- o>::</c-><c- n>member_type</c-> <c- k>const</c-><c- o>&amp;</c-> <c- n>member</c-><c- p>,</c-> <c- n>value_type</c-><c- o>&amp;</c-> <c- n>value</c-><c- p>)</c-> <c- p>{</c->
      
      <c- k>auto</c-> <c- n>coro_scheduler</c-> <c- o>=</c-> <c- n>coroutine_scheduler_type</c-><c- p>{</c-><c- n>member</c-><c- p>.</c-><c- n>scheduler</c-><c- p>()};</c->

      <c- k>if</c-><c- p>(</c-><c- n>not</c-> <c- n>m_coroutine_return</c-><c- p>)</c-> <c- p>{</c->
        <c- c1>// initial invocation</c->
        <c- n>m_coroutine_return</c-> <c- o>=</c-> <c- n>m_functor</c-><c- p>(</c-><c- n>coro_scheduler</c-><c- p>);</c->
      <c- p>}</c->

      <c- c1>// no dependency once we get here</c->
      <c- n>assert</c-><c- p>(</c-><c- n>m_current_dep</c-><c- p>.</c-><c- n>is_null</c-><c- p>()</c-> <c- o>==</c-> true <c- o>||</c-> <c- n>m_current_dep</c-><c- p>.</c-><c- n>is_ready</c-><c- p>());</c->

      <c- n>assert</c-><c- p>(</c-><c- n>m_coroutine_return</c-><c- o>-></c-><c- n>handle</c-><c- p>.</c-><c- n>promise</c-><c- p>().</c-><c- n>m_current_dep</c-> <c- o>==</c-> <c- n>nullptr</c-> <c- o>&amp;&amp;</c-> <c- s>"dependence already set"</c-><c- p>);</c->
      <c- n>assert</c-><c- p>(</c-><c- n>m_current_dep</c-><c- p>.</c-><c- n>is_null</c-><c- p>());</c->

      <c- c1>// put a pointer to our dep in the promise so that any co_await calls inside handle.resume()</c->
      <c- c1>// will know what dependence to set for the respawn</c->
      <c- n>m_coroutine_return</c-><c- o>-></c-><c- n>handle</c-><c- p>.</c-><c- n>promise</c-><c- p>().</c-><c- n>m_current_dep</c-> <c- o>=</c-> <c- o>&amp;</c-><c- n>m_current_dep</c-><c- p>;</c->

      <c- c1>// resume the coroutine</c->
      <c- n>m_coroutine_return</c-><c- o>-></c-><c- n>handle</c-><c- p>.</c-><c- n>resume</c-><c- p>();</c->

      <c- c1>// Reset the promises parent dep</c->
      <c- n>m_coroutine_return</c-><c- o>-></c-><c- n>handle</c-><c- p>.</c-><c- n>promise</c-><c- p>().</c-><c- n>m_current_dep</c-> <c- o>=</c-> <c- n>nullptr</c-><c- p>;</c->

      <c- c1>// Either it was done to begin with, or done after we resumed above:</c->
      <c- k>if</c-><c- p>(</c-><c- n>m_coroutine_return</c-><c- o>-></c-><c- n>is_done</c-><c- p>())</c-> <c- p>{</c->
        <c- c1>// destroy the coroutine handle</c->
        <c- n>assert</c-><c- p>(</c-><c- n>m_current_dep</c-><c- p>.</c-><c- n>is_null</c-><c- p>());</c->
        <c- n>value</c-> <c- o>=</c-> <c- n>m_coroutine_return</c-><c- o>-></c-><c- n>get_result</c-><c- p>();</c->
        <c- n>m_coroutine_return</c-><c- o>-></c-><c- n>handle</c-><c- p>.</c-><c- n>destroy</c-><c- p>();</c->
      <c- p>}</c->
      <c- k>else</c-> <c- p>{</c->
        <c- c1>// Respawn dependent on whatever caused the resume to not reach the co_return</c->
        <c- n>Kokkos</c-><c- o>::</c-><c- n>respawn</c-><c- p>(</c-><c- n>this</c-><c- p>,</c-> <c- n>m_current_dep</c-><c- p>,</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>High</c-><c- p>);</c->

        <c- c1>// Reset our dependence, since we’ve handled the respawn</c->
        <c- n>m_current_dep</c-> <c- o>=</c-> <c- n>future_type</c-><c- p>{};</c->
      <c- p>}</c->

    <c- p>}</c->

  <c- p>};</c->

  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>ValueType</c-><c- o>></c->
  <c- k>struct</c-> <c- n>SpawnedAwaitable</c-> <c- p>{</c->
    <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- n>ValueType</c-><c- p>;</c->
    <c- n>using</c-> <c- n>scheduler_type</c-> <c- o>=</c-> <c- n>Scheduler</c-><c- p>;</c->
    <c- n>using</c-> <c- n>value_future_type</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>BasicFuture</c-><c- o>&lt;</c-><c- n>value_type</c-><c- p>,</c-> <c- n>scheduler_type</c-><c- o>></c-><c- p>;</c->

    <c- n>value_future_type</c-> <c- n>m_done_future</c-><c- p>;</c->

    <c- n>value_future_type</c-> <c- nf>get_future</c-><c- p>()</c-> <c- p>{</c-> <c- k>return</c-> <c- n>m_done_future</c-><c- p>;</c-> <c- p>}</c->

    <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>ParentPromise</c-><c- o>></c->
    <c- k>struct</c-> <c- n>SpawnedPromise</c-> <c- p>{</c->

      <c- n>value_future_type</c-> <c- n>m_done_future</c-><c- p>;</c->
      <c- n>ParentPromise</c-><c- o>&amp;</c-> <c- n>m_parent_promise</c-><c- p>;</c->

      <c- n>SpawnedPromise</c-><c- p>(</c->
        <c- n>value_future_type</c-> <c- n>arg_done_future</c-><c- p>,</c->
        <c- n>ParentPromise</c-><c- o>&amp;</c-> <c- n>arg_parent_promise</c->
      <c- p>)</c-> <c- o>:</c-> <c- n>m_done_future</c-><c- p>(</c-><c- n>arg_done_future</c-><c- p>),</c->
          <c- n>m_parent_promise</c-><c- p>(</c-><c- n>arg_parent_promise</c-><c- p>)</c->
      <c- p>{</c-> <c- p>}</c->

      <c- b>bool</c-> <c- n>await_ready</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> 
<c- cp>#ifdef DISABLE_CHECK_IN_AWAIT_READY        </c->
        <c- k>return</c-> false<c- p>;</c->
<c- cp>#else</c->
        <c- k>return</c-> <c- n>m_done_future</c-><c- p>.</c-><c- n>is_ready</c-><c- p>();</c->
<c- cp>#endif</c->
      <c- p>}</c->

      <c- b>void</c->
      <c- n>await_suspend</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>coroutine_handle</c-><c- o>&lt;</c-><c- n>ParentPromise</c-><c- o>></c-> <c- n>handle</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
        <c- c1>// for some value_type T, handle.promise() is of type coroutine_return_type&lt;T>::promise_type;        </c->
        <c- c1>// We now have something to resume when the future is ready</c->
        <c- n>assert</c-><c- p>(</c-><c- n>m_parent_promise</c-><c- p>.</c-><c- n>m_current_dep</c-> <c- o>!=</c-> <c- n>nullptr</c-><c- p>);</c->
        <c- c1>// Tell the parent that it needs to respawn with this as a dependence</c->
        <c- o>*</c-><c- n>m_parent_promise</c-><c- p>.</c-><c- n>m_current_dep</c-> <c- o>=</c-> <c- n>m_done_future</c-><c- p>;</c->
      <c- p>}</c->

      <c- n>value_type</c-><c- o>&amp;</c->
      <c- n>await_resume</c-><c- p>()</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>m_done_future</c-><c- p>.</c-><c- n>get</c-><c- p>();</c->
      <c- p>}</c->

    <c- p>};</c->

  <c- p>};</c->

  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-><c- p>...</c-> <c- n>Awaitables</c-><c- o>></c->
  <c- k>struct</c-> <c- n>WhenAllAwaitable</c-> <c- p>{</c->
    <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c-><c- kr>typename</c-> <c- n>Awaitables</c-><c- o>::</c-><c- n>value_type</c-><c- p>...</c-><c- o>></c-><c- p>;</c->
    <c- n>using</c-> <c- n>scheduler_type</c-> <c- o>=</c-> <c- n>Scheduler</c-><c- p>;</c->
    <c- n>using</c-> <c- n>aggregate_future_type</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>BasicFuture</c-><c- o>&lt;</c-><c- b>void</c-><c- p>,</c-> <c- n>scheduler_type</c-><c- o>></c-><c- p>;</c->
    <c- n>using</c-> <c- n>value_future_tuple</c-> <c- o>=</c-> <c- n>std</c-><c- o>::</c-><c- n>tuple</c-><c- o>&lt;</c->
      <c- n>Kokkos</c-><c- o>::</c-><c- n>BasicFuture</c-><c- o>&lt;</c-><c- kr>typename</c-> <c- n>Awaitables</c-><c- o>::</c-><c- n>value_type</c-><c- p>,</c-> <c- n>scheduler_type</c-><c- o>></c-><c- p>...</c->
    <c- o>></c-><c- p>;</c->

    <c- n>aggregate_future_type</c-> <c- n>m_done_future</c-><c- p>;</c->
    <c- n>value_future_tuple</c-> <c- n>m_value_futures</c-><c- p>;</c->
    
    <c- n>WhenAllAwaitable</c-><c- p>(</c->
      <c- n>aggregate_future_type</c-><c- o>&amp;&amp;</c-> <c- n>done_future</c-><c- p>,</c->
      <c- n>Awaitables</c-> <c- k>const</c-><c- o>&amp;</c-><c- p>...</c-> <c- n>awaitables</c->
    <c- p>)</c-> <c- o>:</c-> <c- n>m_done_future</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>done_future</c-><c- p>)),</c->
        <c- n>m_value_futures</c-><c- p>(</c-><c- n>awaitables</c-><c- p>.</c-><c- n>m_done_future</c-><c- p>...)</c->
    <c- p>{</c-> <c- p>}</c->

    <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>ParentPromise</c-><c- o>></c->
    <c- k>struct</c-> <c- n>SpawnedPromise</c-> <c- p>{</c->

      <c- n>aggregate_future_type</c-> <c- n>m_done_future</c-><c- p>;</c->
      <c- n>value_future_tuple</c-> <c- n>m_value_futures</c-><c- p>;</c->
      <c- n>ParentPromise</c-><c- o>&amp;</c-> <c- n>m_parent_promise</c-><c- p>;</c->

      <c- n>SpawnedPromise</c-><c- p>(</c->
        <c- n>aggregate_future_type</c-> <c- n>arg_done_future</c-><c- p>,</c->
        <c- n>value_future_tuple</c-> <c- n>arg_value_futures</c-><c- p>,</c->
        <c- n>ParentPromise</c-><c- o>&amp;</c-> <c- n>arg_parent_promise</c->
      <c- p>)</c-> <c- o>:</c-> <c- n>m_done_future</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>arg_done_future</c-><c- p>)),</c->
          <c- n>m_value_futures</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>arg_value_futures</c-><c- p>)),</c->
          <c- n>m_parent_promise</c-><c- p>(</c-><c- n>arg_parent_promise</c-><c- p>)</c->
      <c- p>{</c-> <c- p>}</c->

      <c- b>bool</c-> <c- n>await_ready</c-><c- p>()</c-> <c- k>const</c-> <c- p>{</c-> 
<c- cp>#ifdef DISABLE_CHECK_IN_AWAIT_READY        </c->
        <c- k>return</c-> false<c- p>;</c-> 
<c- cp>#else</c->
        <c- k>return</c-> <c- n>m_done_future</c-><c- p>.</c-><c- n>is_ready</c-><c- p>();</c->
<c- cp>#endif</c->
      <c- p>}</c->

      <c- b>void</c->
      <c- n>await_suspend</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>experimental</c-><c- o>::</c-><c- n>coroutine_handle</c-><c- o>&lt;</c-><c- n>ParentPromise</c-><c- o>></c-> <c- n>handle</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->
        <c- c1>// for some value_type T, handle.promise() is of type coroutine_return_type&lt;T>::promise_type;        </c->
        <c- c1>// We now have something to resume when the future is ready</c->
        <c- n>assert</c-><c- p>(</c-><c- n>m_parent_promise</c-><c- p>.</c-><c- n>m_current_dep</c-> <c- o>!=</c-> <c- n>nullptr</c-><c- p>);</c->
        <c- c1>// Tell the parent that it needs to respawn with this as a dependence</c->
        <c- o>*</c-><c- n>m_parent_promise</c-><c- p>.</c-><c- n>m_current_dep</c-> <c- o>=</c-> <c- n>m_done_future</c-><c- p>;</c->
      <c- p>}</c->

      <c- n>template</c-> <c- o>&lt;</c-><c- b>size_t</c-><c- p>...</c-> <c- n>Idxs</c-><c- o>></c->
      <c- n>value_type</c->
      <c- n>_await_resume_impl</c-><c- p>(</c->
        <c- n>std</c-><c- o>::</c-><c- n>integer_sequence</c-><c- o>&lt;</c-><c- b>size_t</c-><c- p>,</c-> <c- n>Idxs</c-><c- p>...</c-><c- o>></c->
      <c- p>)</c->
      <c- p>{</c->
        <c- k>return</c-> <c- n>std</c-><c- o>::</c-><c- n>make_tuple</c-><c- p>(</c->
          <c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>get</c-><c- o>&lt;</c-><c- n>Idxs</c-><c- o>></c-><c- p>(</c-><c- n>m_value_futures</c-><c- p>).</c-><c- n>get</c-><c- p>())...</c->
        <c- p>);</c->
      <c- p>}</c->

      <c- n>value_type</c->
      <c- n>await_resume</c-><c- p>()</c-> <c- p>{</c->
        <c- k>return</c-> <c- n>_await_resume_impl</c-><c- p>(</c-><c- n>std</c-><c- o>::</c-><c- n>index_sequence_for</c-><c- o>&lt;</c-><c- n>Awaitables</c-><c- p>...</c-><c- o>></c-><c- p>{});</c->
      <c- p>}</c->

    <c- p>};</c->

  <c- p>};</c->

  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>CoroutineFunctor</c-><c- o>></c->
  <c- n>SpawnedAwaitable</c-><c- o>&lt;</c-><c- kr>typename</c-> <c- n>CoroutineFunctor</c-><c- o>::</c-><c- n>value_type</c-><c- o>></c->
  <c- n>spawn</c-><c- p>(</c-><c- n>CoroutineFunctor</c-> <c- n>functor</c-><c- p>,</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-> <c- n>priority</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>Regular</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->

    <c- k>return</c-> <c- p>{</c->
      <c- n>Kokkos</c-><c- o>::</c-><c- n>task_spawn</c-><c- p>(</c->
        <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskSingle</c-><c- p>(</c-><c- n>m_scheduler</c-><c- p>,</c-> <c- n>priority</c-><c- p>),</c-> <c- n>TaskFunctor</c-><c- o>&lt;</c-><c- n>CoroutineFunctor</c-><c- o>></c-><c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>functor</c-><c- p>)}</c->
      <c- p>)</c->
    <c- p>};</c->

  <c- p>}</c->

  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-><c- p>...</c-> <c- n>Awaitables</c-><c- o>></c->
  <c- n>WhenAllAwaitable</c-><c- o>&lt;</c-><c- n>std</c-><c- o>::</c-><c- n>decay_t</c-><c- o>&lt;</c-><c- n>Awaitables</c-><c- o>></c-><c- p>...</c-><c- o>></c->
  <c- n>when_all</c-><c- p>(</c-><c- n>Awaitables</c-><c- o>&amp;&amp;</c-><c- p>...</c-> <c- n>awaitables</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->

    <c- n>future_type</c-> <c- n>all_void</c-><c- p>[]</c-> <c- o>=</c-> <c- p>{</c-> <c- p>(</c-><c- n>awaitables</c-><c- p>.</c-><c- n>m_done_future</c-><c- p>)...</c-> <c- p>};</c->
    <c- k>return</c-> <c- p>{</c->
      <c- n>Kokkos</c-><c- o>::</c-><c- n>when_all</c-><c- p>(</c->
        <c- n>all_void</c-><c- p>,</c-> <c- k>sizeof</c-><c- p>...(</c-><c- n>Awaitables</c-><c- p>)</c->
      <c- p>),</c->
      <c- n>std</c-><c- o>::</c-><c- n>forward</c-><c- o>&lt;</c-><c- n>Awaitables</c-><c- o>></c-><c- p>(</c-><c- n>awaitables</c-><c- p>)...</c->
    <c- p>};</c->

  <c- p>}</c->

  <c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>CoroutineFunctor</c-><c- o>></c->
  <c- n>SpawnedAwaitable</c-><c- o>&lt;</c-><c- n>CoroutineFunctor</c-><c- o>></c->
  <c- n>spawn_team</c-><c- p>(</c-><c- n>CoroutineFunctor</c-> <c- n>functor</c-><c- p>,</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-> <c- n>priority</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>Regular</c-><c- p>)</c-> <c- k>const</c-> <c- p>{</c->

    <c- k>return</c-> <c- p>{</c->
      <c- n>m_scheduler</c-><c- p>,</c->
      <c- n>Kokkos</c-><c- o>::</c-><c- n>task_spawn</c-><c- p>(</c->
        <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskTeam</c-><c- p>(</c-><c- n>m_scheduler</c-><c- p>,</c-> <c- n>priority</c-><c- p>),</c-> <c- n>TaskFunctor</c-><c- o>&lt;</c-><c- n>CoroutineFunctor</c-><c- o>></c-><c- p>{</c-><c- n>std</c-><c- o>::</c-><c- n>move</c-><c- p>(</c-><c- n>functor</c-><c- p>)}</c->
      <c- p>)</c->
    <c- p>};</c->

  <c- p>}</c->

  <c- n>Scheduler</c-> <c- n>m_scheduler</c-><c- p>;</c->

<c- p>};</c->

<c- c1>//----------------------------------------------------------------------------—&lt;wbr>// Simple version, without the when_all</c->
<c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>Scheduler</c-><c- o>></c->
<c- k>struct</c-> <c- n>TestFibCoroutine</c-> <c- p>{</c->

  <c- n>using</c-> <c- n>scheduler_type</c-> <c- o>=</c-> <c- n>Scheduler</c-><c- p>;</c->
  <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- b>long</c-><c- p>;</c->
  <c- n>using</c-> <c- n>coroutine_scheduler_type</c-> <c- o>=</c-> <c- n>BasicCoroutineScheduler</c-><c- o>&lt;</c-><c- n>scheduler_type</c-><c- o>></c-><c- p>;</c->
  <c- n>using</c-> <c- n>coroutine_return_type</c-> <c- o>=</c-> <c- kr>typename</c-> <c- n>coroutine_scheduler_type</c-><c- o>::</c-><c- n>template</c-> <c- n>coroutine_return_type</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-><c- p>;</c->

  <c- n>value_type</c-> <c- n>n</c-><c- p>;</c->

  <c- n>coroutine_return_type</c->
  <c- nf>operator</c-><c- p>()(</c-><c- n>coroutine_scheduler_type</c-><c- o>&amp;</c-> <c- n>sched</c-><c- p>)</c-> <c- p>{</c->
    <c- k>if</c-><c- p>(</c-><c- n>n</c-> <c- o>&lt;</c-> <c- mi>2</c-><c- p>)</c-> <c- p>{</c->
      <c- n>co_return</c-> <c- n>n</c-><c- p>;</c->
    <c- p>}</c->
    <c- k>else</c-> <c- p>{</c->
      <c- k>auto</c-> <c- n>f_2</c-> <c- o>=</c-> <c- n>sched</c-><c- p>.</c-><c- n>spawn</c-><c- p>(</c-><c- n>TestFibCoroutine</c-><c- p>{</c-><c- n>n</c-><c- o>-</c-><c- mi>2</c-><c- p>},</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>High</c-><c- p>);</c->
      <c- k>auto</c-> <c- n>f_1</c-> <c- o>=</c-> <c- n>sched</c-><c- p>.</c-><c- n>spawn</c-><c- p>(</c-><c- n>TestFibCoroutine</c-><c- p>{</c-><c- n>n</c-><c- o>-</c-><c- mi>1</c-><c- p>});</c->
      <c- n>co_return</c-> <c- n>co_await</c-> <c- n>f_1</c-> <c- o>+</c-> <c- n>co_await</c-> <c- n>f_2</c-><c- p>;</c->
    <c- p>}</c->
  <c- p>}</c->

<c- p>};</c->

<c- c1>//----------------------------------------------------------------------------—&lt;wbr>// Coroutines using the when_all</c->
<c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>Scheduler</c-><c- o>></c->
<c- k>struct</c-> <c- n>TestWhenAllFibCoroutine</c-> <c- p>{</c->

  <c- n>using</c-> <c- n>scheduler_type</c-> <c- o>=</c-> <c- n>Scheduler</c-><c- p>;</c->
  <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- b>long</c-><c- p>;</c->
  <c- n>using</c-> <c- n>coroutine_scheduler_type</c-> <c- o>=</c-> <c- n>BasicCoroutineScheduler</c-><c- o>&lt;</c-><c- n>scheduler_type</c-><c- o>></c-><c- p>;</c->
  <c- n>using</c-> <c- n>coroutine_return_type</c-> <c- o>=</c-> <c- kr>typename</c-> <c- n>coroutine_scheduler_type</c-><c- o>::</c-><c- n>template</c-> <c- n>coroutine_return_type</c-><c- o>&lt;</c-><c- n>value_type</c-><c- o>></c-><c- p>;</c->

  <c- n>value_type</c-> <c- n>n</c-><c- p>;</c->

  <c- n>coroutine_return_type</c->
  <c- nf>operator</c-><c- p>()(</c-><c- n>coroutine_scheduler_type</c-><c- o>&amp;</c-> <c- n>sched</c-><c- p>)</c-> <c- p>{</c->
    <c- k>if</c-><c- p>(</c-><c- n>n</c-> <c- o>&lt;</c-> <c- mi>2</c-><c- p>)</c-> <c- p>{</c->
      <c- n>co_return</c-> <c- n>n</c-><c- p>;</c->
    <c- p>}</c->
    <c- k>else</c-> <c- p>{</c->
      <c- k>auto</c-> <c- n>f_2</c-> <c- o>=</c-> <c- n>sched</c-><c- p>.</c-><c- n>spawn</c-><c- p>(</c-><c- n>TestWhenAllFibCoroutine</c-><c- p>{</c-><c- n>n</c-><c- o>-</c-><c- mi>2</c-><c- p>},</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>High</c-><c- p>);</c->
      <c- k>auto</c-> <c- n>f_1</c-> <c- o>=</c-> <c- n>sched</c-><c- p>.</c-><c- n>spawn</c-><c- p>(</c-><c- n>TestWhenAllFibCoroutine</c-><c- p>{</c-><c- n>n</c-><c- o>-</c-><c- mi>1</c-><c- p>});</c->
      <c- k>auto</c-> <c- p>[</c-><c- n>v1</c-><c- p>,</c-> <c- n>v2</c-><c- p>]</c-> <c- o>=</c-> <c- n>co_await</c-> <c- n>sched</c-><c- p>.</c-><c- n>when_all</c-><c- p>(</c-><c- n>f_1</c-><c- p>,</c-> <c- n>f_2</c-><c- p>);</c->
      <c- n>co_return</c-> <c- n>v1</c-> <c- o>+</c-> <c- n>v2</c-><c- p>;</c->
    <c- p>}</c->
  <c- p>}</c->

<c- p>};</c->

<c- c1>//----------------------------------------------------------------------------—&lt;wbr>// Old version</c->
<c- n>template</c-> <c- o>&lt;</c-><c- n>class</c-> <c- n>Scheduler</c-><c- o>></c->
<c- k>struct</c-> <c- n>TestFib</c-> <c- p>{</c->

  <c- n>using</c-> <c- n>MemorySpace</c-> <c- o>=</c-> <c- kr>typename</c-> <c- n>Scheduler</c-><c- o>::</c-><c- n>memory_space</c-><c- p>;</c->
  <c- n>using</c-> <c- n>MemberType</c-> <c- o>=</c-> <c- kr>typename</c-> <c- n>Scheduler</c-><c- o>::</c-><c- n>member_type</c-><c- p>;</c->
  <c- n>using</c-> <c- n>FutureType</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>BasicFuture</c-><c- o>&lt;</c-><c- b>long</c-><c- p>,</c-> <c- n>Scheduler</c-><c- o>></c-><c- p>;</c->

  <c- n>using</c-> <c- n>value_type</c-> <c- o>=</c-> <c- b>long</c-><c- p>;</c->

  <c- n>FutureType</c-> <c- n>dep</c-><c- p>[</c-><c- mi>2</c-><c- p>];</c->
  <c- k>const</c-> <c- n>value_type</c-> <c- n>n</c-><c- p>;</c->

  <c- n>KOKKOS_INLINE_FUNCTION</c->
  <c- nf>TestFib</c-><c- p>(</c-><c- k>const</c-> <c- n>value_type</c-> <c- n>arg_n</c-><c- p>)</c->
    <c- o>:</c-> <c- n>dep</c-><c- p>{},</c-> <c- n>n</c-><c- p>(</c-><c- n>arg_n</c-><c- p>)</c->
  <c- p>{</c-> <c- p>}</c->

  <c- n>KOKKOS_INLINE_FUNCTION</c->
  <c- b>void</c-> <c- n>operator</c-><c- p>()(</c-> <c- k>const</c-> <c- n>MemberType</c-> <c- o>&amp;</c-> <c- n>member</c-><c- p>,</c-> <c- n>value_type</c-> <c- o>&amp;</c-> <c- n>result</c-> <c- p>)</c-> <c- n>noexcept</c->
    <c- p>{</c->
      <c- k>auto</c-> <c- n>sched</c-> <c- o>=</c-> <c- n>member</c-><c- p>.</c-><c- n>scheduler</c-><c- p>();</c->
      <c- k>if</c-> <c- p>(</c-><c- n>n</c-> <c- o>&lt;</c-> <c- mi>2</c-><c- p>)</c-> <c- p>{</c->
        <c- n>result</c-> <c- o>=</c-> <c- n>n</c-><c- p>;</c->
      <c- p>}</c->
      <c- k>else</c-> <c- k>if</c-><c- p>(</c-><c- o>!</c-><c- n>dep</c-><c- p>[</c-><c- mi>0</c-><c- p>].</c-><c- n>is_null</c-><c- p>()</c-> <c- o>&amp;&amp;</c-> <c- o>!</c-><c- n>dep</c-><c- p>[</c-><c- mi>1</c-><c- p>].</c-><c- n>is_null</c-><c- p>())</c-> <c- p>{</c->
        <c- n>result</c-> <c- o>=</c-> <c- n>dep</c-><c- p>[</c-><c- mi>0</c-><c- p>].</c-><c- n>get</c-><c- p>()</c-> <c- o>+</c-> <c- n>dep</c-><c- p>[</c-><c- mi>1</c-><c- p>].</c-><c- n>get</c-><c- p>();</c->
      <c- p>}</c->
      <c- k>else</c-> <c- p>{</c->
        <c- c1>// Spawn new children and respawn myself to sum their results.</c->
        <c- c1>// Spawn lower value at higher priority as it has a shorter</c->
        <c- c1>// path to completion.</c->

        <c- n>dep</c-><c- p>[</c-><c- mi>1</c-><c- p>]</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>task_spawn</c-><c- p>(</c->
          <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskSingle</c-><c- p>(</c-><c- n>sched</c-><c- p>,</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>High</c-><c- p>),</c->
          <c- n>TestFib</c-><c- p>(</c-><c- n>n</c-> <c- o>-</c-> <c- mi>2</c-><c- p>)</c->
        <c- p>);</c->

        <c- n>dep</c-><c- p>[</c-><c- mi>0</c-><c- p>]</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>task_spawn</c-><c- p>(</c->
          <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskSingle</c-><c- p>(</c-><c- n>sched</c-><c- p>),</c->
          <c- n>TestFib</c-><c- p>(</c-><c- n>n</c-> <c- o>-</c-> <c- mi>1</c-><c- p>)</c->
        <c- p>);</c->

        <c- k>auto</c-> <c- n>fib_all</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>when_all</c-><c- p>(</c-><c- n>dep</c-><c- p>,</c-> <c- mi>2</c-><c- p>);</c->

        <c- c1>// High priority to retire this branch.</c->
        <c- n>Kokkos</c-><c- o>::</c-><c- n>respawn</c-><c- p>(</c-><c- n>this</c-><c- p>,</c-> <c- n>fib_all</c-><c- p>,</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>TaskPriority</c-><c- o>::</c-><c- n>High</c-><c- p>);</c->
      <c- p>}</c->
    <c- p>}</c->
<c- p>};</c->

<c- c1>//----------------------------------------------------------------------------—&lt;wbr>int main(int argc , char* argv[]) {</c->
  <c- n>Kokkos</c-><c- o>::</c-><c- n>initialize</c-><c- p>(</c-><c- n>argc</c-><c- p>,</c-><c- n>argv</c-><c- p>);</c->

  <c- p>{</c->
    <c- k>static</c-> <c- n>constexpr</c-> <c- k>auto</c-> <c- n>N</c-> <c- o>=</c-> <c- mi>30</c-><c- p>;</c->
    <c- k>static</c-> <c- n>constexpr</c-> <c- k>auto</c-> <c- n>repeats</c-> <c- o>=</c-> <c- mi>3</c-><c- p>;</c->

    <c- n>using</c-> <c- n>scheduler_type</c-> <c- o>=</c-> <c- n>Kokkos</c-><c- o>::</c-><c- n>NewTaskSchedulerMultiple</c-><c- o>&lt;</c-><c- n>Kokkos</c-><c- o>::</c-><c- n>OpenMP</c-><c- o>></c-><c- p>;</c->
    <c- n>using</c-> <c- n>coroutine_scheduler_type</c-> <c- o>=</c-> <c- n>BasicCoroutineScheduler</c-><c- o>&lt;</c-><c- n>scheduler_type</c-><c- o>></c-><c- p>;</c->
    <c- n>using</c-> <c- n>memory_space</c-> <c- o>=</c-> <c- n>scheduler_type</c-><c- o>::</c-><c- n>memory_space</c-><c- p>;</c->

    <c- k>static</c-> <c- n>constexpr</c-> <c- b>size_t</c-> <c- n>MinBlockSize</c-> <c- o>=</c-> <c- mi>64</c-><c- p>;</c->
    <c- k>static</c-> <c- n>constexpr</c-> <c- b>size_t</c-> <c- n>MemoryCapacity</c-> <c- o>=</c-> <c- p>(</c-><c- n>N</c-><c- o>+</c-><c- mi>1</c-><c- p>)</c-> <c- o>*</c-> <c- p>(</c-><c- n>N</c-><c- o>+</c-><c- mi>1</c-><c- p>)</c-> <c- o>*</c-> <c- mi>2000</c-><c- p>;</c->
    <c- k>static</c-> <c- n>constexpr</c-> <c- b>size_t</c-> <c- n>MaxBlockSize</c-> <c- o>=</c-> <c- mi>1024</c-><c- p>;</c->
    <c- k>static</c-> <c- n>constexpr</c-> <c- b>size_t</c-> <c- n>SuperBlockSize</c-> <c- o>=</c-> <c- mi>4096</c-><c- p>;</c->

    <c- n>scheduler_type</c-> <c- nf>scheduler</c-><c- p>(</c->
      <c- n>memory_space</c-><c- p>(),</c-> <c- n>MemoryCapacity</c-><c- p>,</c-> <c- n>MinBlockSize</c-><c- p>,</c->
      <c- n>std</c-><c- o>::</c-><c- n>min</c-><c- p>(</c-><c- b>size_t</c-><c- p>(</c-><c- n>MaxBlockSize</c-><c- p>),</c-> <c- n>MemoryCapacity</c-><c- p>),</c->
      <c- n>std</c-><c- o>::</c-><c- n>min</c-><c- p>(</c-><c- b>size_t</c-><c- p>(</c-><c- n>SuperBlockSize</c-><c- p>),</c-> <c- n>MemoryCapacity</c-><c- p>)</c->
    <c- p>);</c->
    <c- k>auto</c-> <c- n>coroutine_scheduler</c-> <c- o>=</c-> <c- n>coroutine_scheduler_type</c-><c- p>{</c-> <c- n>scheduler</c-> <c- p>};</c->

    <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"Running benchmark Fib(n) for 0 &lt;= n &lt; "</c-> <c- o>&lt;&lt;</c-> <c- n>N</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
    <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"----------------------------------------"</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
    <c- k>for</c-><c- p>(</c-><c- b>int</c-> <c- n>irepeat</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>irepeat</c-> <c- o>&lt;</c-> <c- n>repeats</c-><c- p>;</c-> <c- o>++</c-><c- n>irepeat</c-><c- p>)</c-> <c- p>{</c->
      <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"Benchmarking repeat #"</c-> <c- o>&lt;&lt;</c-> <c- p>(</c-><c- n>irepeat</c-> <c- o>+</c-> <c- mi>1</c-><c- p>)</c-> <c- o>&lt;&lt;</c-> <c- s>" of "</c-> <c- o>&lt;&lt;</c-> <c- n>repeats</c-> <c- o>&lt;&lt;</c-> <c- s>":"</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->

      <c- p>{</c->
        <c- n>Kokkos</c-><c- o>::</c-><c- n>Impl</c-><c- o>::</c-><c- n>Timer</c-> <c- n>timer</c-><c- p>;</c->
        <c- k>for</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>;</c-> <c- o>++</c-><c- n>i</c-><c- p>)</c-> <c- p>{</c->
          <c- k>auto</c-> <c- n>result</c-> <c- o>=</c-> <c- n>coroutine_scheduler</c-><c- p>.</c-><c- n>spawn</c-><c- p>(</c-><c- n>TestFibCoroutine</c-><c- o>&lt;</c-><c- n>scheduler_type</c-><c- o>></c-><c- p>{</c-><c- n>i</c-><c- p>});</c->
          <c- n>Kokkos</c-><c- o>::</c-><c- n>wait</c-><c- p>(</c-><c- n>scheduler</c-><c- p>);</c->
        <c- p>}</c->

        <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"  Simple coroutine version took "</c-> <c- o>&lt;&lt;</c-> <c- n>timer</c-><c- p>.</c-><c- n>seconds</c-><c- p>()</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
      <c- p>}</c->

      <c- p>{</c->
        <c- n>Kokkos</c-><c- o>::</c-><c- n>Impl</c-><c- o>::</c-><c- n>Timer</c-> <c- n>timer</c-><c- p>;</c->
        <c- k>for</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>;</c-> <c- o>++</c-><c- n>i</c-><c- p>)</c-> <c- p>{</c->
          <c- k>auto</c-> <c- n>result</c-> <c- o>=</c-> <c- n>coroutine_scheduler</c-><c- p>.</c-><c- n>spawn</c-><c- p>(</c-><c- n>TestWhenAllFibCoroutine</c-><c- o>&lt;</c-><c- n>scheduler_type</c-><c- o>></c-><c- p>{</c-><c- n>i</c-><c- p>});</c->
          <c- n>Kokkos</c-><c- o>::</c-><c- n>wait</c-><c- p>(</c-><c- n>scheduler</c-><c- p>);</c->
        <c- p>}</c->

        <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"  Coroutine version with when_all took "</c-> <c- o>&lt;&lt;</c-> <c- n>timer</c-><c- p>.</c-><c- n>seconds</c-><c- p>()</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
      <c- p>}</c->

      <c- p>{</c->
        <c- n>Kokkos</c-><c- o>::</c-><c- n>Impl</c-><c- o>::</c-><c- n>Timer</c-> <c- n>timer</c-><c- p>;</c->
        <c- k>for</c-><c- p>(</c-><c- b>int</c-> <c- n>i</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c-> <c- n>i</c-> <c- o>&lt;</c-> <c- n>N</c-><c- p>;</c-> <c- o>++</c-><c- n>i</c-><c- p>)</c-> <c- p>{</c->
          <c- k>auto</c-> <c- n>result_future</c-> <c- o>=</c->
            <c- n>Kokkos</c-><c- o>::</c-><c- n>host_spawn</c-><c- p>(</c-><c- n>Kokkos</c-><c- o>::</c-><c- n>TaskSingle</c-><c- p>(</c-><c- n>scheduler</c-><c- p>),</c-> <c- n>TestFib</c-><c- o>&lt;</c-><c- n>scheduler_type</c-><c- o>></c-><c- p>{</c-><c- n>i</c-><c- p>});</c->
          <c- n>Kokkos</c-><c- o>::</c-><c- n>wait</c-><c- p>(</c-><c- n>scheduler</c-><c- p>);</c->
        <c- p>}</c->

        <c- n>std</c-><c- o>::</c-><c- n>cout</c-> <c- o>&lt;&lt;</c-> <c- s>"  Old version took "</c-> <c- o>&lt;&lt;</c-> <c- n>timer</c-><c- p>.</c-><c- n>seconds</c-><c- p>()</c-> <c- o>&lt;&lt;</c-> <c- n>std</c-><c- o>::</c-><c- n>endl</c-><c- p>;</c->
      <c- p>}</c->

    <c- p>}</c->

  <c- p>}</c-> <c- c1>// end scope to destroy scheduler before finalize</c->

  <c- n>Kokkos</c-><c- o>::</c-><c- n>finalize</c-><c- p>();</c->

<c- p>}</c->
</pre>
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