<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <title>p0528R2: The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange</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,
	table.index tbody {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
	 vertical-align: baseline;
	 text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Reverse color scheme */
		color: black;
		border-color: #3980B5;
		border-bottom-width: 3px !important;
		margin-bottom: 0px !important;
	}
	.toc a:visited {
		border-color: #054572;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
		line-height: 1.1rem; /* consistent spacing */
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li             { font-weight: bold;   }
	.toc > li li          { font-weight: normal; }
	.toc > li li li       { font-size:   95%;    }
	.toc > li li li li    { font-size:   90%;    }
	.toc > li li li li li { font-size:   85%;    }

	.toc > li             { margin: 1.5rem 0;    }
	.toc > li li          { margin: 0.3rem 0;    }
	.toc > li li li       { margin-left: 2rem;   }

	/* Section numbers in a column of their own */
	.toc .secno {
		float: left;
		width: 4rem;
		white-space: nowrap;
	}
	.toc > li li li li .secno {
		font-size: 85%;
	}
	.toc > li li li li li .secno {
		font-size: 100%;
	}

	:not(li) > .toc              { margin-left:  5rem; }
	.toc .secno                  { margin-left: -5rem; }
	.toc > li li li .secno       { margin-left: -7rem; }
	.toc > li li li li .secno    { margin-left: -9rem; }
	.toc > li li li li li .secno { margin-left: -11rem; }

	/* Tighten up indentation in narrow ToCs */
	@media (max-width: 30em) {
		:not(li) > .toc              { margin-left:  4rem; }
		.toc .secno                  { margin-left: -4rem; }
		.toc > li li li              { margin-left:  1rem; }
		.toc > li li li .secno       { margin-left: -5rem; }
		.toc > li li li li .secno    { margin-left: -6rem; }
		.toc > li li li li li .secno { margin-left: -7rem; }
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) :not(li) > .toc              { margin-left:  4rem; }
		body:not(.toc-inline) .toc .secno                  { margin-left: -4rem; }
		body:not(.toc-inline) .toc > li li li              { margin-left:  1rem; }
		body:not(.toc-inline) .toc > li li li .secno       { margin-left: -5rem; }
		body:not(.toc-inline) .toc > li li li li .secno    { margin-left: -6rem; }
		body:not(.toc-inline) .toc > li li li li li .secno { margin-left: -7rem; }
	}
	body.toc-sidebar #toc :not(li) > .toc              { margin-left:  4rem; }
	body.toc-sidebar #toc .toc .secno                  { margin-left: -4rem; }
	body.toc-sidebar #toc .toc > li li li              { margin-left:  1rem; }
	body.toc-sidebar #toc .toc > li li li .secno       { margin-left: -5rem; }
	body.toc-sidebar #toc .toc > li li li li .secno    { margin-left: -6rem; }
	body.toc-sidebar #toc .toc > li li li li li .secno { margin-left: -7rem; }

	.toc li {
		clear: both;
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index       { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li    { margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em }
	ul.index dl    { margin-top: 0; }
	ul.index dt    { margin: .2em 0 .2em 20px;}
	ul.index dd    { margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		background: #f7f8f9;
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}

/******************************************************************************/
/*                                    Legacy                                  */
/******************************************************************************/

	/* This rule is inherited from past style sheets. No idea what it's for. */
	.hide { display: none }



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-left: calc(13px + 26.5rem - 50vw);
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-left: calc(40em - 50vw) !important;
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-left: 0 !important;
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			 * http://lea.verou.me/2012/04/background-attachment-local/
			 *
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
			            top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
			            top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
<style type="text/css">
    table, th, td {
      border: 1px solid black;
      border-collapse: collapse;
      vertical-align: top;
    }
    th, td {
      border-left: none;
      border-right: none;
      padding: 0px 10px;
    }
    th {
      text-align: center;
    }
  </style>
  <meta content="Bikeshed version 4a25cc37f1c8d18c48090861a99f58d746a27f8f" name="generator">
  <link href="http://wg21.link/P0528r2" rel="canonical">
  <meta content="84750e4e42873594e82490b9226e9aa5fc14e579" name="document-revision">
<style>/* style-md-lists */

/* This is a weird hack for me not yet following the commonmark spec
   regarding paragraph and lists. */
[data-md] > :first-child {
    margin-top: 0;
}
[data-md] > :last-child {
    margin-bottom: 0;
}</style>
<style>/* style-counters */

body {
    counter-reset: example figure issue;
}
.issue {
    counter-increment: issue;
}
.issue:not(.no-marker)::before {
    content: "Issue " counter(issue);
}

.example {
    counter-increment: example;
}
.example:not(.no-marker)::before {
    content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
    content: "Invalid Example" counter(example);
}

figcaption {
    counter-increment: figure;
}
figcaption:not(.no-marker)::before {
    content: "Figure " counter(figure) " ";
}</style>
<style>/* style-syntax-highlighting */

.highlight:not(.idl) { background: hsl(24, 20%, 95%); }
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }
.highlight .c { color: #708090 } /* Comment */
.highlight .k { color: #990055 } /* Keyword */
.highlight .l { color: #000000 } /* Literal */
.highlight .n { color: #0077aa } /* Name */
.highlight .o { color: #999999 } /* Operator */
.highlight .p { color: #999999 } /* Punctuation */
.highlight .cm { color: #708090 } /* Comment.Multiline */
.highlight .cp { color: #708090 } /* Comment.Preproc */
.highlight .c1 { color: #708090 } /* Comment.Single */
.highlight .cs { color: #708090 } /* Comment.Special */
.highlight .kc { color: #990055 } /* Keyword.Constant */
.highlight .kd { color: #990055 } /* Keyword.Declaration */
.highlight .kn { color: #990055 } /* Keyword.Namespace */
.highlight .kp { color: #990055 } /* Keyword.Pseudo */
.highlight .kr { color: #990055 } /* Keyword.Reserved */
.highlight .kt { color: #990055 } /* Keyword.Type */
.highlight .ld { color: #000000 } /* Literal.Date */
.highlight .m { color: #000000 } /* Literal.Number */
.highlight .s { color: #a67f59 } /* Literal.String */
.highlight .na { color: #0077aa } /* Name.Attribute */
.highlight .nc { color: #0077aa } /* Name.Class */
.highlight .no { color: #0077aa } /* Name.Constant */
.highlight .nd { color: #0077aa } /* Name.Decorator */
.highlight .ni { color: #0077aa } /* Name.Entity */
.highlight .ne { color: #0077aa } /* Name.Exception */
.highlight .nf { color: #0077aa } /* Name.Function */
.highlight .nl { color: #0077aa } /* Name.Label */
.highlight .nn { color: #0077aa } /* Name.Namespace */
.highlight .py { color: #0077aa } /* Name.Property */
.highlight .nt { color: #669900 } /* Name.Tag */
.highlight .nv { color: #222222 } /* Name.Variable */
.highlight .ow { color: #999999 } /* Operator.Word */
.highlight .mb { color: #000000 } /* Literal.Number.Bin */
.highlight .mf { color: #000000 } /* Literal.Number.Float */
.highlight .mh { color: #000000 } /* Literal.Number.Hex */
.highlight .mi { color: #000000 } /* Literal.Number.Integer */
.highlight .mo { color: #000000 } /* Literal.Number.Oct */
.highlight .sb { color: #a67f59 } /* Literal.String.Backtick */
.highlight .sc { color: #a67f59 } /* Literal.String.Char */
.highlight .sd { color: #a67f59 } /* Literal.String.Doc */
.highlight .s2 { color: #a67f59 } /* Literal.String.Double */
.highlight .se { color: #a67f59 } /* Literal.String.Escape */
.highlight .sh { color: #a67f59 } /* Literal.String.Heredoc */
.highlight .si { color: #a67f59 } /* Literal.String.Interpol */
.highlight .sx { color: #a67f59 } /* Literal.String.Other */
.highlight .sr { color: #a67f59 } /* Literal.String.Regex */
.highlight .s1 { color: #a67f59 } /* Literal.String.Single */
.highlight .ss { color: #a67f59 } /* Literal.String.Symbol */
.highlight .vc { color: #0077aa } /* Name.Variable.Class */
.highlight .vg { color: #0077aa } /* Name.Variable.Global */
.highlight .vi { color: #0077aa } /* Name.Variable.Instance */
.highlight .il { color: #000000 } /* Literal.Number.Integer.Long */
</style>
<style>/* style-selflinks */

.heading, .issue, .note, .example, li, dt {
    position: relative;
}
a.self-link {
    position: absolute;
    top: 0;
    left: calc(-1 * (3.5rem - 26px));
    width: calc(3.5rem - 26px);
    height: 2em;
    text-align: center;
    border: none;
    transition: opacity .2s;
    opacity: .5;
}
a.self-link:hover {
    opacity: 1;
}
.heading > a.self-link {
    font-size: 83%;
}
li > a.self-link {
    left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
    top: auto;
    left: auto;
    opacity: 0;
    width: 1.5em;
    height: 1.5em;
    background: gray;
    color: white;
    font-style: normal;
    transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
    opacity: 1;
}
dfn > a.self-link:hover {
    color: black;
}

a.self-link::before            { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before      { content: "#"; }</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: #005a9c;
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}</style>
 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">p0528R2<br>The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-03-16">16 March 2018</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/P0528r2">http://wg21.link/P0528r2</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jfbastien@apple.com">JF Bastien</a> (<span class="p-org org">Apple</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:bigcheesegs@gmail.com">Michael Spencer</a> (<span class="p-org org">Sony Playstation</span>)
     <dt>Audience:
     <dd>CWG
     <dt>Project:
     <dd>ISO JTC1/SC22/WG21: Programming Language C++
     <dt>Source:
     <dd><a href="https://github.com/jfbastien/papers/blob/master/source/P0528r2.bs">github.com/jfbastien/papers/blob/master/source/P0528r2.bs</a>
    </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>Compare-and-exchange on a struct with padding bits should Just Work.</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="#edit"><span class="secno">1</span> <span class="content">Edit History</span></a>
     <ol class="toc">
      <li><a href="#r1r2"><span class="secno">1.1</span> <span class="content">r1 → r2</span></a>
      <li><a href="#r0r1"><span class="secno">1.2</span> <span class="content">r0 → r1</span></a>
     </ol>
    <li><a href="#word"><span class="secno">2</span> <span class="content">Proposed Wording</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>
   <p>This issue has been discussed by the authors at every recent Standards meetings,
yet a full solution has been elusive despite helpful proposals. We believe that
this proposal can fix this oft-encountered problem once and for all.</p>
   <p><a data-link-type="biblio" href="#biblio-p0528r0">[P0528r0]</a> details extensive background on this problem (not repeated here),
and proposed standardizing a trait, <code class="highlight"><span class="n">has_padding_bits</span></code>, and using it on <code class="highlight"><span class="n">compare_and_exchange_</span><span class="o">*</span></code>. <a data-link-type="biblio" href="#biblio-p0528r1">[P0528r1]</a> applied EWG guidance and simply added
wording directing implementations to ensure that the desired behavior occur. At
SG1’s request this paper follows EWG’s guidance but uses different wording.</p>
   <h2 class="heading settled" data-level="1" id="edit"><span class="secno">1. </span><span class="content">Edit History</span><a class="self-link" href="#edit"></a></h2>
   <h3 class="heading settled" data-level="1.1" id="r1r2"><span class="secno">1.1. </span><span class="content">r1 → r2</span><a class="self-link" href="#r1r2"></a></h3>
   <p>In Jacksonville, SG1 supported the paper but suggested an alternate way to
approach the wording than the one EWG proposed in Albuquerque: don’t talk about
contents of the memory, but rather discuss the value representation to describe
compare-and-exchange. This paper follows SG1’s guidance and offers different
wording, with the intent that the semantics be equivalent. EWG reviewed the
updated wording an voted to support it and forward to Core.</p>
   <h3 class="heading settled" data-level="1.2" id="r0r1"><span class="secno">1.2. </span><span class="content">r0 → r1</span><a class="self-link" href="#r0r1"></a></h3>
   <p>In Albuquerque, EWG voted to make the padding bits of <code class="highlight"><span class="n">atomic</span></code> and the incoming
value of <code class="highlight"><span class="n">T</span></code> have a consistent value for the purposes of read/modify/write
atomic operations?</p>
   <p>Purposefully not addressed in this paper:</p>
   <ul>
    <li data-md="">
     <p><code class="highlight"><span class="k">union</span></code> with padding bits</p>
    <li data-md="">
     <p>Types with trap representations</p>
   </ul>
   <h2 class="heading settled" data-level="2" id="word"><span class="secno">2. </span><span class="content">Proposed Wording</span><a class="self-link" href="#word"></a></h2>
   <p>Edit ❡17 and onwards as follows:</p>
   <blockquote>
    <p><em>Requires:</em> The <code class="highlight"><span class="n">failure</span></code> argument shall not be <code class="highlight"><span class="n">memory_order</span><span class="o">::</span><span class="n">release</span></code> nor <code class="highlight"><span class="n">memory_order</span><span class="o">::</span><span class="n">acq_rel</span></code>.</p>
    <p>
     <em>Effects:</em> Retrieves the value in <code class="highlight"><span class="n">expected</span></code>. It then atomically compares
the 
     <del>contents of the memory pointed to by <code class="highlight"><span class="k">this</span></code></del>
     <ins>value representation
of <code class="highlight"><span class="o">*</span><span class="k">this</span></code></ins>
      for equality with that previously retrieved from <code class="highlight"><span class="n">expected</span></code>,
and if true, replaces the 
     <del>contents of the memory pointed to
by <code class="highlight"><span class="k">this</span></code></del>
     <ins>value representation of <code class="highlight"><span class="o">*</span><span class="k">this</span></code></ins>
      with that in <code class="highlight"><span class="n">desired</span></code>.  If
and only if the comparison is true, memory is affected according to the value of <code class="highlight"><span class="n">success</span></code>, and if the comparison is false, memory is affected according to the
value of <code class="highlight"><span class="n">failure</span></code>. When only one <code class="highlight"><span class="n">memory_order</span></code> argument is supplied, the value
of <code class="highlight"><span class="n">success</span></code> is <code class="highlight"><span class="n">order</span></code>, and the value of <code class="highlight"><span class="n">failure</span></code> is <code class="highlight"><span class="n">order</span></code> except that a
value of <code class="highlight"><span class="n">memory_order</span><span class="o">::</span><span class="n">acq_rel</span></code> shall be replaced by the value <code class="highlight"><span class="n">memory_order</span><span class="o">::</span><span class="n">acquire</span></code> and a value of <code class="highlight"><span class="n">memory_order</span><span class="o">::</span><span class="n">release</span></code> shall be replaced
by the value <code class="highlight"><span class="n">memory_order</span><span class="o">::</span><span class="n">relaxed</span></code>. If and only if the comparison is false
then, after the atomic operation, the 
     <del>contents of the memory</del>
     <ins>the
value representation</ins>
      in <code class="highlight"><span class="n">expected</span></code> are replaced by the value
     <ins> representation</ins>
      read from the memory pointed to by <code class="highlight"><span class="k">this</span></code> during the atomic
comparison. If the operation returns <code class="highlight">true</code>, these operations are atomic
read-modify-write operations on the memory pointed to by <code class="highlight"><span class="k">this</span></code>. Otherwise,
these operations are atomic load operations on that memory.
    </p>
    <p><em>Returns:</em> The result of the comparison.</p>
    <p>[<em>Note:</em></p>
    <p>
     For example, the effect of <code class="highlight"><span class="n">compare_exchange_strong</span></code> 
     <ins>on objects without padding bits </ins>
     is
    </p>
<pre class="highlight">  
<span class="k">if</span> <span class="p">(</span><span class="n">memcmp</span><span class="p">(</span><span class="k">this</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">expected</span><span class="p">,</span> <span class="k">sizeof</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">))</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span>
  <span class="n">memcpy</span><span class="p">(</span><span class="k">this</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">desired</span><span class="p">,</span> <span class="k">sizeof</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">));</span>
<span class="k">else</span>
   <span class="nf">memcpy</span><span class="p">(</span><span class="n">expected</span><span class="p">,</span> <span class="k">this</span><span class="p">,</span> <span class="k">sizeof</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">));</span>
</pre>
    <p>—<em>end note</em>]</p>
    <p>[<em>Example:</em></p>
    <p>The expected use of the compare-and-exchange operations is as follows. The
  compare-and-exchange operations will update <code class="highlight"><span class="n">expected</span></code> when another iteration
  of the loop is needed.</p>
<pre class="highlight"><span class="n">expected</span> <span class="o">=</span> <span class="n">current</span><span class="p">.</span><span class="n">load</span><span class="p">();</span>
<span class="k">do</span> <span class="p">{</span>
  <span class="n">desired</span> <span class="o">=</span> <span class="n">function</span><span class="p">(</span><span class="n">expected</span><span class="p">);</span>
<span class="p">}</span> <span class="k">while</span> <span class="p">(</span><span class="o">!</span><span class="n">current</span><span class="p">.</span><span class="n">compare_exchange_weak</span><span class="p">(</span><span class="n">expected</span><span class="p">,</span> <span class="n">desired</span><span class="p">));</span>
</pre>
    <p>—<em>end example</em>]</p>
    <p>[<em>Example:</em></p>
    <p>Because the expected value is updated only on failure, code releasing the
  memory containing the <code class="highlight"><span class="n">expected</span></code> value on success will work. E.g. list head
  insertion will act atomically and would not introduce a data race in the
  following code:</p>
<pre class="highlight"><span class="k">do</span> <span class="p">{</span>
  <span class="n">p</span><span class="o">-></span><span class="n">next</span> <span class="o">=</span> <span class="n">head</span><span class="p">;</span> <span class="c1">// make new list node point to the current head</span>
<span class="c1"></span><span class="p">}</span> <span class="k">while</span> <span class="p">(</span><span class="o">!</span><span class="n">head</span><span class="p">.</span><span class="n">compare_exchange_weak</span><span class="p">(</span><span class="n">p</span><span class="o">-></span><span class="n">next</span><span class="p">,</span> <span class="n">p</span><span class="p">));</span> <span class="c1">// try to insert</span>
<span class="c1"></span></pre>
    <p>—<em>end example</em>]</p>
    <p>Implementations should ensure that weak compare-and-exchange operations do not
consistently return <code class="highlight">false</code> unless either the atomic object has value different
from <code class="highlight"><span class="n">expected</span></code> or there are concurrent modifications to the atomic object.</p>
    <p><em>Remarks:</em> A weak compare-and-exchange operation may fail spuriously. That is,
even when the contents of memory referred to by <code class="highlight"><span class="n">expected</span></code> and <code class="highlight"><span class="k">this</span></code> are equal,
it may return <code class="highlight">false</code> and store back to <code class="highlight"><span class="n">expected</span></code> the same memory contents that
were originally there.</p>
    <p>[<em>Note:</em></p>
    <p>This spurious failure enables implementation of compare-and-exchange on a
  broader class of machines, e.g., load-locked store-conditional machines. A
  consequence of spurious failure is that nearly all uses of weak
  compare-and-exchange will be in a loop. When a compare-and-exchange is in a
  loop, the weak version will yield better performance on some platforms. When a
  weak compare-and-exchange would require a loop and a strong one would not, the
  strong one is preferable.</p>
    <p>—<em>end note</em>]</p>
    <p>[<em>Note:</em></p>
    <p>
     The <code class="highlight"><span class="n">memcpy</span></code> and <code class="highlight"><span class="n">memcmp</span></code> semantics of the compare-and-exchange operations
  may result in failed comparisons for values that compare equal with <code class="highlight"><span class="k">operator</span><span class="o">==</span></code> if the underlying type has padding bits
     <ins> which sometimes
  participate in the object’s representation</ins>
     , trap bits, or
  alternate representations of the same value
     <ins> other than those caused by
  padding bits which never participate in the object’s representation</ins>
     .
  Notably, on implementations conforming to ISO/IEC/IEEE 60559, floating-point <code class="highlight"><span class="o">-</span><span class="mf">0.0</span></code> and <code class="highlight"><span class="o">+</span><span class="mf">0.0</span></code> will not compare equal with <code class="highlight"><span class="n">memcmp</span></code> but will compare equal
  with <code class="highlight"><span class="k">operator</span><span class="o">==</span></code>, and NaNs with the same payload will compare equal with <code class="highlight"><span class="n">memcmp</span></code> but will not compare equal with <code class="highlight"><span class="k">operator</span><span class="o">==</span></code>.
    </p>
    <p>—<em>end note</em>]</p>
    <ins>
     <p>[<em>Note:</em></p>
     <p>Compare-and-exchange acts on an object’s value representation, ensuring that
  padding bits which never participate in the object’s representation are ignored.</p>
     <p>As a consequence, the following code is guaranteed to avoid spurious failure:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">padded</span> <span class="p">{</span>
  <span class="kt">char</span> <span class="n">clank</span> <span class="o">=</span> <span class="mh">0x42</span><span class="p">;</span>
  <span class="c1">// Padding here.</span>
<span class="c1"></span>  <span class="kt">unsigned</span> <span class="n">biff</span> <span class="o">=</span> <span class="mh">0xC0DEFEFE</span><span class="p">;</span>
<span class="p">};</span>
<span class="n">atomic</span><span class="o">&lt;</span><span class="n">padded</span><span class="o">></span> <span class="n">pad</span> <span class="o">=</span> <span class="n">ATOMIC_VAR_INIT</span><span class="p">({});</span>

<span class="kt">bool</span> <span class="nf">zap</span><span class="p">()</span> <span class="p">{</span>
  <span class="n">padded</span> <span class="n">expected</span><span class="p">,</span> <span class="n">desired</span> <span class="p">{</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span> <span class="p">};</span>
  <span class="k">return</span> <span class="n">pad</span><span class="p">.</span><span class="n">compare_exchange_strong</span><span class="p">(</span><span class="n">expected</span><span class="p">,</span> <span class="n">desired</span><span class="p">);</span>
<span class="p">}</span>
</pre>
     <p>—<em>end note</em>]</p>
     <p>[<em>Note:</em></p>
     <p>Types which contain bits that sometimes participate in the object’s
  representation, such as a <code class="highlight"><span class="k">union</span></code> containing a type with padding bits and a
  type without, may always fail compare-and-exchange when these bits are not
  participating in the object’s representation because they have an
  indeterminate value.</p>
     <p>—<em>end note</em>]</p>
    </ins>
   </blockquote>
  </main>
<script>
(function() {
  "use strict";
  var collapseSidebarText = '<span aria-hidden="true">←</span> '
                          + '<span>Collapse Sidebar</span>';
  var expandSidebarText   = '<span aria-hidden="true">→</span> '
                          + '<span>Pop Out Sidebar</span>';
  var tocJumpText         = '<span aria-hidden="true">↑</span> '
                          + '<span>Jump to Table of Contents</span>';

  var sidebarMedia = window.matchMedia('screen and (min-width: 78em)');
  var autoToggle   = function(e){ toggleSidebar(e.matches) };
  if(sidebarMedia.addListener) {
    sidebarMedia.addListener(autoToggle);
  }

  function toggleSidebar(on) {
    if (on == undefined) {
      on = !document.body.classList.contains('toc-sidebar');
    }

    /* Don’t scroll to compensate for the ToC if we’re above it already. */
    var headY = 0;
    var head = document.querySelector('.head');
    if (head) {
      // terrible approx of "top of ToC"
      headY += head.offsetTop + head.offsetHeight;
    }
    var skipScroll = window.scrollY < headY;

    var toggle = document.getElementById('toc-toggle');
    var tocNav = document.getElementById('toc');
    if (on) {
      var tocHeight = tocNav.offsetHeight;
      document.body.classList.add('toc-sidebar');
      document.body.classList.remove('toc-inline');
      toggle.innerHTML = collapseSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, 0 - tocHeight);
      }
      tocNav.focus();
      sidebarMedia.addListener(autoToggle); // auto-collapse when out of room
    }
    else {
      document.body.classList.add('toc-inline');
      document.body.classList.remove('toc-sidebar');
      toggle.innerHTML = expandSidebarText;
      if (!skipScroll) {
        window.scrollBy(0, tocNav.offsetHeight);
      }
      if (toggle.matches(':hover')) {
        /* Unfocus button when not using keyboard navigation,
           because I don’t know where else to send the focus. */
        toggle.blur();
      }
    }
  }

  function createSidebarToggle() {
    /* Create the sidebar toggle in JS; it shouldn’t exist when JS is off. */
    var toggle = document.createElement('a');
      /* This should probably be a button, but appearance isn’t standards-track.*/
    toggle.id = 'toc-toggle';
    toggle.class = 'toc-toggle';
    toggle.href = '#toc';
    toggle.innerHTML = collapseSidebarText;

    sidebarMedia.addListener(autoToggle);
    var toggler = function(e) {
      e.preventDefault();
      sidebarMedia.removeListener(autoToggle); // persist explicit off states
      toggleSidebar();
      return false;
    }
    toggle.addEventListener('click', toggler, false);


    /* Get <nav id=toc-nav>, or make it if we don’t have one. */
    var tocNav = document.getElementById('toc-nav');
    if (!tocNav) {
      tocNav = document.createElement('p');
      tocNav.id = 'toc-nav';
      /* Prepend for better keyboard navigation */
      document.body.insertBefore(tocNav, document.body.firstChild);
    }
    /* While we’re at it, make sure we have a Jump to Toc link. */
    var tocJump = document.getElementById('toc-jump');
    if (!tocJump) {
      tocJump = document.createElement('a');
      tocJump.id = 'toc-jump';
      tocJump.href = '#toc';
      tocJump.innerHTML = tocJumpText;
      tocNav.appendChild(tocJump);
    }

    tocNav.appendChild(toggle);
  }

  var toc = document.getElementById('toc');
  if (toc) {
    createSidebarToggle();
    toggleSidebar(sidebarMedia.matches);

    /* If the sidebar has been manually opened and is currently overlaying the text
       (window too small for the MQ to add the margin to body),
       then auto-close the sidebar once you click on something in there. */
    toc.addEventListener('click', function(e) {
      if(e.target.tagName.toLowerCase() == "a" && document.body.classList.contains('toc-sidebar') && !sidebarMedia.matches) {
        toggleSidebar(false);
      }
    }, false);
  }
  else {
    console.warn("Can’t find Table of Contents. Please use <nav id='toc'> around the ToC.");
  }

  /* Wrap tables in case they overflow */
  var tables = document.querySelectorAll(':not(.overlarge) > table.data, :not(.overlarge) > table.index');
  var numTables = tables.length;
  for (var i = 0; i < numTables; i++) {
    var table = tables[i];
    var wrapper = document.createElement('div');
    wrapper.className = 'overlarge';
    table.parentNode.insertBefore(wrapper, table);
    wrapper.appendChild(table);
  }

})();
</script>
  <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-p0528r0">[P0528r0]
   <dd>JF Bastien, Michael Spencer. <a href="https://wg21.link/p0528r0">The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange</a>. 12 November 2016. URL: <a href="https://wg21.link/p0528r0">https://wg21.link/p0528r0</a>
   <dt id="biblio-p0528r1">[P0528r1]
   <dd>JF Bastien, Michael Spencer. <a href="https://wg21.link/p0528r1">The Curious Case of Padding Bits, Featuring Atomic Compare-and-Exchange</a>. URL: <a href="https://wg21.link/p0528r1">https://wg21.link/p0528r1</a>
  </dl>