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 <head>
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  <title>P1220R0: Controlling When Inline Functions are Emitted</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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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1220R0<br>Controlling When Inline Functions are Emitted</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-09-28">2018-09-28</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P1220">http://wg21.link/P1220</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:ckennelly@google.com">Chris Kennelly</a> (<span class="p-org org">Google</span>)
     <dt>Audience:
     <dd>EWG
     <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>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>Provide a means to avoid emitting all inline function bodies</p>
  </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="#motivation"><span class="secno">1</span> <span class="content">Motivation</span></a>
    <li><a href="#proposal"><span class="secno">2</span> <span class="content">Proposal</span></a>
    <li><a href="#bikeshed"><span class="secno">3</span> <span class="content">Bikeshedding</span></a>
    <li><a href="#related"><span class="secno">4</span> <span class="content">Related Work</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="motivation"><span class="secno">1. </span><span class="content">Motivation</span><a class="self-link" href="#motivation"></a></h2>
   <p>Consider the fast allocation path for <a data-link-type="biblio" href="#biblio-gperftools">[gperftools]</a> <a href="https://github.com/gperftools/gperftools/blob/master/src/tcmalloc.cc">allocation
path</a>:</p>
<pre class="highlight"><c- k>template</c-><c- o>&lt;</c-><c- n>OOMHandler</c-><c- o>></c->
<c- b>void</c-><c- o>*</c-> <c- n>malloc_fast_path</c-><c- p>(</c-><c- b>size_t</c-> <c- n>size</c-><c- p>)</c-> <c- p>{</c->
  <c- n>uint32</c-> <c- n>cl</c-><c- p>;</c->
  <c- k>if</c-> <c- p>(</c-><c- n>PREDICT_FALSE</c-><c- p>(</c-><c- o>!</c-><c- n>Static</c-><c- o>::</c-><c- n>sizemap</c-><c- p>()</c-><c- o>-></c-><c- n>GetSizeClass</c-><c- p>(</c-><c- n>size</c-><c- p>,</c-> <c- o>&amp;</c-><c- n>cl</c-><c- p>)))</c-> <c- p>{</c->
    <c- k>return</c-> <c- n>tcmalloc</c-><c- o>::</c-><c- n>dispatch_allocate_full</c-><c- o>&lt;</c-><c- n>OOMHandler</c-><c- o>></c-><c- p>(</c-><c- n>size</c-><c- p>);</c->
  <c- p>}</c->

  <c- c1>// Allocate an object from sizeclass "cl" and return elided...</c->
<c- p>}</c->
</pre>
   <p>With <code class="highlight"><c- n>GetSizeClass</c-></code> providing a <a href="https://github.com/gperftools/gperftools/blob/master/src/common.h#L257">size
to size class computation</a>.</p>
   <p class="note" role="note"><span>Note:</span> As of this writing, <code class="highlight"><c- n>malloc_fast_path</c-></code> is structured with other checks
first.  This is unimportant for correctness, but does represent the most
profitable opportunity for compile-time optimization.</p>
   <p>For many allocations, we can optimize much of this lookup cost were inlining
possible:</p>
   <ul>
    <li data-md>
     <p>Constant-sized allocations can have the size class fully calculated at
compile-time, allowing us to jump directly to the rest of the fast path
(for small sizes) or to the large allocation fallback path.</p>
    <li data-md>
     <p>With dynamic-sized allocations, we can partially compute the lookup based
on knowledge we have at the callsite (we’re allocating an array of <code class="highlight"><c- n>T</c-></code>,
whose size gives us guarantees that our request is a multiple of 8).  For
overaligned allocations, the size calculation is more complicated, but <em>alignment</em> values are frequently known at compile time (making this an
even more profitable optimization).</p>
   </ul>
   <p>However, we can’t actually apply this to <code class="highlight"><c- k>operator</c-> <c- k>new</c-></code>:</p>
   <ul>
    <li data-md>
     <p>Under [dcl.inline], "an inline function...shall be defined in every
translation unit in which it is odr-used."  Ensuring that we can make an
inlined <code class="highlight"><c- k>operator</c-> <c- k>new</c-></code> definition available in every translation unit may
be impossible.  Even with modules, the existence of any legacy, precompiled
library (which almost certainly uses <code class="highlight"><c- k>operator</c-> <c- k>new</c-></code>) means we cannot ensure
the inline function is defined in <em>every</em> translation unit.</p>
    <li data-md>
     <p>Under 15.5.4.6 [replacement.functions], the ability to inline the
replaceable functions used for allocation is forbidden ("The program’s
declarations shall not be specified as inline. No diagnostic is
required.").  This was the resolution of <a data-link-type="biblio" href="#biblio-lwg404">[LWG404]</a>.  Allowing this is
addressed in a separate paper <a data-link-type="biblio" href="#biblio-p1284">[P1284]</a>.</p>
   </ul>
   <p>This paper specifically focuses on the inlining problem that is not unique to <code class="highlight"><c- k>operator</c-> <c- k>new</c-></code>.  Any code that has a valuable fast path but needs to avoid the
overhead of a <em>second</em> function call when inlining does not happen can benefit
from this technique.</p>
   <p>Consider the parsing functions of <a data-link-type="biblio" href="#biblio-protobuf">[protobuf]</a>.  A <a href="https://github.com/protocolbuffers/protobuf/blob/8d6f8df1ec57faf2c354e6bf1cdcd1f1d1e8ee68/src/google/protobuf/io/coded_stream.h#L917">fast-path
is provided for parsing single-byte varints</a> with a fallback for longer
values or exceptional cases (the buffer being exhausted):</p>
<pre class="highlight"><c- kr>inline</c-> <c- b>bool</c-> <c- n>CodedInputStream</c-><c- o>::</c-><c- n>ReadVarint32</c-><c- p>(</c-><c- n>uint32</c-><c- o>*</c-> <c- n>value</c-><c- p>)</c-> <c- p>{</c->
  <c- n>uint32</c-> <c- n>v</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
  <c- k>if</c-> <c- p>(</c-><c- n>PROTOBUF_PREDICT_TRUE</c-><c- p>(</c-><c- n>buffer_</c-> <c- o>&lt;</c-> <c- n>buffer_end_</c-><c- p>))</c-> <c- p>{</c->
    <c- n>v</c-> <c- o>=</c-> <c- o>*</c-><c- n>buffer_</c-><c- p>;</c->
    <c- k>if</c-> <c- p>(</c-><c- n>v</c-> <c- o>&lt;</c-> <c- mh>0x80</c-><c- p>)</c-> <c- p>{</c->
      <c- o>*</c-><c- n>value</c-> <c- o>=</c-> <c- n>v</c-><c- p>;</c->
      <c- n>Advance</c-><c- p>(</c-><c- mi>1</c-><c- p>);</c->
      <c- k>return</c-> true<c- p>;</c->
    <c- p>}</c->
  <c- p>}</c->
  <c- n>int64</c-> <c- n>result</c-> <c- o>=</c-> <c- n>ReadVarint32Fallback</c-><c- p>(</c-><c- n>v</c-><c- p>);</c->
  <c- o>*</c-><c- n>value</c-> <c- o>=</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- n>uint32</c-><c- o>></c-><c- p>(</c-><c- n>result</c-><c- p>);</c->
  <c- k>return</c-> <c- n>result</c-> <c- o>>=</c-> <c- mi>0</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>When we fail to inline <code class="highlight"><c- n>ReadVarint32</c-></code>, we are also penalized by the second
function call to <code class="highlight"><c- n>ReadVarint32Fallback</c-></code>, which is placed out-of-line, not in
the header.  (We only see <code class="highlight"><c- n>ReadVarint32</c-></code> and <code class="highlight"><c- n>ReadVarint32Fallback</c-></code> in one
translation unit, and there’s no guarantee the linker picks that definition.)</p>
   <p>We propose a <code class="highlight"><c- p>[[</c-><c- n>noemit</c-><c- p>]]</c-></code> attribute to indicate that particular definitions
should <em>only</em> be used for inlining and discarded when inlining does not happen.
We can make an inlineable definition available such as:</p>
<pre class="highlight"><c- p>[[</c-><c- n>noemit</c-><c- p>]]</c-> <c- kr>inline</c-> <c- b>bool</c-> <c- n>CodedInputStream</c-><c- o>::</c-><c- n>ReadVarint32</c-><c- p>(</c-><c- n>uint32</c-><c- o>*</c-> <c- n>value</c-><c- p>)</c-> <c- p>{</c->
  <c- n>uint32</c-> <c- n>v</c-> <c- o>=</c-> <c- mi>0</c-><c- p>;</c->
  <c- k>if</c-> <c- p>(</c-><c- n>PROTOBUF_PREDICT_TRUE</c-><c- p>(</c-><c- n>buffer_</c-> <c- o>&lt;</c-> <c- n>buffer_end_</c-><c- p>))</c-> <c- p>{</c->
    <c- n>v</c-> <c- o>=</c-> <c- o>*</c-><c- n>buffer_</c-><c- p>;</c->
    <c- k>if</c-> <c- p>(</c-><c- n>v</c-> <c- o>&lt;</c-> <c- mh>0x80</c-><c- p>)</c-> <c- p>{</c->
      <c- o>*</c-><c- n>value</c-> <c- o>=</c-> <c- n>v</c-><c- p>;</c->
      <c- n>Advance</c-><c- p>(</c-><c- mi>1</c-><c- p>);</c->
      <c- k>return</c-> true<c- p>;</c->
    <c- p>}</c->
  <c- p>}</c->
  <c- n>int64</c-> <c- n>result</c-> <c- o>=</c-> <c- n>ReadVarint32Fallback</c-><c- p>(</c-><c- n>v</c-><c- p>);</c->
  <c- o>*</c-><c- n>value</c-> <c- o>=</c-> <c- k>static_cast</c-><c- o>&lt;</c-><c- n>uint32</c-><c- o>></c-><c- p>(</c-><c- n>result</c-><c- p>);</c->
  <c- k>return</c-> <c- n>result</c-> <c- o>>=</c-> <c- mi>0</c-><c- p>;</c->
<c- p>}</c->
</pre>
   <p>We would then explicitly emit <code class="highlight"><c- n>ReadVarint32</c-></code> in a single translation unit.
Because this definition is explicitly emitted only in this one translation
unit, we can arrange for it to be in the <em>same</em> translation unit as <code class="highlight"><c- n>ReadVarint32Fallback</c-></code> and we can be much more aggressive in inlining the
fallback code into this single, out-of-line definition.  At most, the fallback
code is emitted twice.</p>
<pre class="highlight"><c- b>bool</c-> <c- n>CodedInputStream</c-><c- o>::</c-><c- n>ReadVarint32</c-><c- p>(</c-><c- n>uint32</c-><c- o>*</c-><c- p>)</c-> <c- o>=</c-> <c- kr>inline</c-><c- p>;</c->

<c- n>int64</c-> <c- n>CodedInputStream</c-><c- o>::</c-><c- n>ReadVarint32Fallback</c-><c- p>(</c-><c- n>uint32</c-> <c- n>first_byte_or_zero</c-><c- p>)</c-> <c- p>{</c->
  <c- c1>// ... handle remaining bytes...</c->
<c- p>}</c->
</pre>
   <h2 class="heading settled" data-level="2" id="proposal"><span class="secno">2. </span><span class="content">Proposal</span><a class="self-link" href="#proposal"></a></h2>
   <p>Wording is relative to <a data-link-type="biblio" href="#biblio-n4762">[N4762]</a>.</p>
   <ul>
    <li data-md>
     <p>9.1.6 [dcl.inline]</p>
   </ul>
   <blockquote>
     A function declaration ([dcl.fct], [class.mfct], [class.friend]) with an <code class="highlight"><c- kr>inline</c-></code> specifier declares an inline function. 
    <ins>An inline function
declaration with a <code class="highlight"><c- p>[[</c-><c- n>noemit</c-><c- p>]]</c-></code> attribute ([dcl.attr]) declares an <em>noemit
inline function</em>.  (Note: The intent is that an <em>noemit inline function</em> allows
the body to be considered for inlining, but no out-of-line copy of the function
would be generated in the translation unit [dcl.attr.noemit].  As <code class="highlight"><c- k>constexpr</c-></code> functions are implicitly inline per [dcl.constexpr], <code class="highlight"><c- p>[[</c-><c- n>noemit</c-><c- p>]]</c-> <c- k>constexpr</c-></code> also declares an <em>noemit inline function</em>.) </ins>
   </blockquote>
   <blockquote>
     An inline function
    <ins>, except <em>noemit inline functions</em>,</ins>
     or variable
shall be defined in every translation unit in which it is odr-used. 
    <ins>An
inline function or variable</ins>
    <del>and</del>
     shall have exactly the same
definition in every case ([basic.def.odr]). 
   </blockquote>
   <ul>
    <li data-md>
     <p>9.11 [dcl.attr.noemit]</p>
   </ul>
   <blockquote>
    <ins> The <em>attribute-token</em> <code class="highlight"><c- n>noemit</c-></code> specifies that an inline function be considered
for inlining, but no out-of-line copy shall be generated in the translation
unit. </ins>
   </blockquote>
   <ul>
    <li data-md>
     <p>6.2 [basic.def.odr]</p>
   </ul>
   <blockquote>
     Every program shall contain exactly one definition of every non-inline function
or variable that is odr-used in that program outside of a discarded statement;
no diagnostic required. The definition can appear explicitly in the program, it
can be found in the standard or a user-defined library, or (when appropriate)
it is implicitly defined (see [class.ctor], [class.dtor], [class.copy.ctor],
and [class.copy.assign]). An inline function
    <ins>, except <em>noemit inline
functions</em>,</ins>
     or variable shall be defined in every translation unit in
which it is odr-used outside of a discarded statement. 
    <ins>Every program
shall contain exactly one explicitly-emitted definition for every <em>noemit inline
function</em> that is odr-used in that program outside of a discarded statement; no
diagnostic required.</ins>
   </blockquote>
   <ul>
    <li data-md>
     <p>9.4 [dcl.fct.def]</p>
   </ul>
   <blockquote>
    <ins> A function definition whose function-body is of the form <code class="highlight"><c- o>=</c-> <c- kr>inline</c-><c- p>;</c-></code> is called
an explicitly-emitted definition. A function that is explicitly emitted shall
have a corresponding <em>noemit inline function</em> body. </ins>
   </blockquote>
   <h2 class="heading settled" data-level="3" id="bikeshed"><span class="secno">3. </span><span class="content">Bikeshedding</span><a class="self-link" href="#bikeshed"></a></h2>
   <p>How should we invoke this feature?</p>
   <ul>
    <li data-md>
     <p><code class="highlight"><c- p>[[</c-><c- n>noemit</c-><c- p>]]</c-> <c- kr>inline</c-></code>, as presented</p>
    <li data-md>
     <p><code class="highlight"><c- k>try</c-> <c- kr>inline</c-></code></p>
    <li data-md>
     <p><code class="highlight"><c- k>extern</c-> <c- kr>inline</c-></code>:  Existing code may already declare functions as <code class="highlight"><c- k>extern</c-> <c- kr>inline</c-></code>.</p>
    <li data-md>
     <p><code class="highlight"><c- kr>inline</c-> <c- k>extern</c-></code>:  This sequence is less common in existing code.</p>
    <li data-md>
     <p><code class="highlight"><c- b>void</c-> <c- nf>f</c-><c- p>()</c-> <c- kr>inline</c-> <c- p>{</c-> <c- p>...</c-> <c- p>}</c-></code></p>
   </ul>
   <h2 class="heading settled" data-level="4" id="related"><span class="secno">4. </span><span class="content">Related Work</span><a class="self-link" href="#related"></a></h2>
   <p>The approach described here has previously been used for GCC’s <code class="highlight"><c- n>gnu_inline</c-></code> mode (<a data-link-type="biblio" href="#biblio-gnu_inline">[gnu_inline]</a>):</p>
   <blockquote> "If you specify both inline and extern in the function definition, then the
definition is used only for inlining. In no case is the function compiled on
its own, not even if you refer to its address explicitly. Such an address
becomes an external reference, as if you had only declared the function, and
had not defined it." </blockquote>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-gnu_inline">[GNU_INLINE]
   <dd><a href="https://gcc.gnu.org/onlinedocs/gcc/Inline.html">Using the GNU Compiler Collection: Inline</a>. URL: <a href="https://gcc.gnu.org/onlinedocs/gcc/Inline.html">https://gcc.gnu.org/onlinedocs/gcc/Inline.html</a>
   <dt id="biblio-gperftools">[GPERFTOOLS]
   <dd><a href="https://github.com/gperftools/gperftools">gperftools</a>. URL: <a href="https://github.com/gperftools/gperftools">https://github.com/gperftools/gperftools</a>
   <dt id="biblio-lwg404">[LWG404]
   <dd><a href="http://cplusplus.github.io/LWG/lwg-defects.html#404">May a replacement function be declared inline?</a>. 2016-12-23. URL: <a href="http://cplusplus.github.io/LWG/lwg-defects.html#404">http://cplusplus.github.io/LWG/lwg-defects.html#404</a>
   <dt id="biblio-n4762">[N4762]
   <dd><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4762.pdf">Working Draft, Standard for Programming Language C++</a>. 2018-07-07. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4762.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4762.pdf</a>
   <dt id="biblio-p1284">[P1284]
   <dd><a href="https://wg21.link/P1284">Allowing Replaceable Functions to be Inlined</a>. 2018-10-05. URL: <a href="https://wg21.link/P1284">https://wg21.link/P1284</a>
   <dt id="biblio-protobuf">[PROTOBUF]
   <dd><a href="https://github.com/protocolbuffers/protobuf">Protocol Buffers</a>. 2018-09-22. URL: <a href="https://github.com/protocolbuffers/protobuf">https://github.com/protocolbuffers/protobuf</a>
  </dl>