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+
+<section id="pnacl-bitcode-reference-manual">
+<h1 id="pnacl-bitcode-reference-manual">PNaCl Bitcode Reference Manual</h1>
+<div class="contents local topic" id="contents">
+<ul class="small-gap">
+<li><a class="reference internal" href="#introduction" id="id1">Introduction</a></li>
+<li><p class="first"><a class="reference internal" href="#high-level-structure" id="id2">High Level Structure</a></p>
+<ul class="small-gap">
+<li><a class="reference internal" href="#data-model" id="id3">Data Model</a></li>
+<li><a class="reference internal" href="#linkage-types" id="id4">Linkage Types</a></li>
+<li><a class="reference internal" href="#calling-conventions" id="id5">Calling Conventions</a></li>
+<li><a class="reference internal" href="#visibility-styles" id="id6">Visibility Styles</a></li>
+<li><a class="reference internal" href="#global-variables" id="id7">Global Variables</a></li>
+<li><a class="reference internal" href="#functions" id="id8">Functions</a></li>
+<li><a class="reference internal" href="#aliases" id="id9">Aliases</a></li>
+<li><a class="reference internal" href="#named-metadata" id="id10">Named Metadata</a></li>
+<li><a class="reference internal" href="#module-level-inline-assembly" id="id11">Module-Level Inline Assembly</a></li>
+<li><a class="reference internal" href="#volatile-memory-accesses" id="id12">Volatile Memory Accesses</a></li>
+<li><a class="reference internal" href="#memory-model-for-concurrent-operations" id="id13">Memory Model for Concurrent Operations</a></li>
+<li><a class="reference internal" href="#fast-math-flags" id="id14">Fast-Math Flags</a></li>
+</ul>
+</li>
+<li><p class="first"><a class="reference internal" href="#type-system" id="id15">Type System</a></p>
+<ul class="small-gap">
+<li><a class="reference internal" href="#scalar-types" id="id16">Scalar types</a></li>
+<li><a class="reference internal" href="#array-and-struct-types" id="id17">Array and struct types</a></li>
+<li><a class="reference internal" href="#pointer-types" id="id18">Pointer types</a></li>
+<li><a class="reference internal" href="#undefined-values" id="id19">Undefined Values</a></li>
+<li><a class="reference internal" href="#constant-expressions" id="id20">Constant Expressions</a></li>
+</ul>
+</li>
+<li><p class="first"><a class="reference internal" href="#other-values" id="id21">Other Values</a></p>
+<ul class="small-gap">
+<li><a class="reference internal" href="#metadata-nodes-and-metadata-strings" id="id22">Metadata Nodes and Metadata Strings</a></li>
+</ul>
+</li>
+<li><a class="reference internal" href="#intrinsic-global-variables" id="id23">Intrinsic Global Variables</a></li>
+<li><p class="first"><a class="reference internal" href="#instruction-reference" id="id24">Instruction Reference</a></p>
+<ul class="small-gap">
+<li><a class="reference internal" href="#list-of-allowed-instructions" id="id25">List of allowed instructions</a></li>
+<li><a class="reference internal" href="#alloca" id="id26"><code>alloca</code></a></li>
+</ul>
+</li>
+<li><p class="first"><a class="reference internal" href="#intrinsic-functions" id="id27">Intrinsic Functions</a></p>
+<ul class="small-gap">
+<li><a class="reference internal" href="#list-of-allowed-intrinsics" id="id28">List of allowed intrinsics</a></li>
+<li><a class="reference internal" href="#thread-pointer-related-intrinsics" id="id29">Thread pointer related intrinsics</a></li>
+<li><a class="reference internal" href="#setjmp-and-longjmp" id="id30">Setjmp and Longjmp</a></li>
+<li><a class="reference internal" href="#atomic-intrinsics" id="id31">Atomic intrinsics</a></li>
+</ul>
+</li>
+</ul>
+</div>
+<section id="introduction">
+<h2 id="introduction">Introduction</h2>
+<p>This document is a reference manual for the PNaCl bitcode format. It describes
+the bitcode on a <em>semantic</em> level; the physical encoding level will be described
+elsewhere. For the purpose of this document, the textual form of LLVM IR is
+used to describe instructions and other bitcode constructs.</p>
+<p>Since the PNaCl bitcode is based to a large extent on LLVM IR, many sections
+in this document point to a relevant section of the LLVM language reference
+manual. Only the changes, restrictions and variations specific to PNaCl are
+described&#8212;full semantic descriptions are not duplicated from the LLVM
+reference manual.</p>
+</section><section id="high-level-structure">
+<h2 id="high-level-structure">High Level Structure</h2>
+<p>A PNaCl portable executable (<strong>pexe</strong> in short) is a single LLVM IR module.</p>
+<section id="data-model">
+<h3 id="data-model">Data Model</h3>
+<p>The data model for PNaCl bitcode is fixed at little-endian ILP32: pointers are
+32 bits in size. 64-bit integer types are also supported natively via the i64
+type (for example, a front-end can generate these from the C/C++ type
+<code>long long</code>).</p>
+<p>Floating point support is fixed at IEEE 754 32-bit and 64-bit values (f32 and
+f64, respectively).</p>
+</section><section id="linkage-types">
+<span id="bitcode-linkagetypes"></span><h3 id="linkage-types"><span id="bitcode-linkagetypes"></span>Linkage Types</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#linkage">LLVM LangRef: Linkage Types</a></p>
+<p>The linkage types supported by PNaCl bitcode are <code>internal</code> and <code>external</code>.
+A single function in the pexe, named <code>_start</code>, has the linkage type
+<code>external</code>. All the other functions and globals have the linkage type
+<code>internal</code>.</p>
+</section><section id="calling-conventions">
+<h3 id="calling-conventions">Calling Conventions</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#callingconv">LLVM LangRef: Calling Conventions</a></p>
+<p>The only calling convention supported by PNaCl bitcode is <code>ccc</code> - the C
+calling convention.</p>
+</section><section id="visibility-styles">
+<h3 id="visibility-styles">Visibility Styles</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#visibility-styles">LLVM LangRef: Visibility Styles</a></p>
+<p>PNaCl bitcode does not support visibility styles.</p>
+</section><section id="global-variables">
+<span id="bitcode-globalvariables"></span><h3 id="global-variables"><span id="bitcode-globalvariables"></span>Global Variables</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#globalvars">LLVM LangRef: Global Variables</a></p>
+<p>Restrictions on global variables:</p>
+<ul class="small-gap">
+<li>PNaCl bitcode does not support LLVM IR TLS models. See
+<a class="reference internal" href="/native-client/reference/pnacl-c-cpp-language-support.html#language-support-threading"><em>Threading</em></a> for more details.</li>
+<li>Restrictions on <a class="reference internal" href="#bitcode-linkagetypes"><em>linkage types</em></a>.</li>
+<li>The <code>addrspace</code>, <code>section</code>, <code>unnamed_addr</code> and
+<code>externally_initialized</code> attributes are not supported.</li>
+</ul>
+<p>Every global variable must have an initializer. Each initializer must be
+either a <em>SimpleElement</em> or a <em>CompoundElement</em>, defined as follows.</p>
+<p>A <em>SimpleElement</em> is one of the following:</p>
+<ol class="arabic simple">
+<li>An i8 array literal or <code>zeroinitializer</code>:</li>
+</ol>
+<pre>
+   [SIZE x i8] c&quot;DATA&quot;
+   [SIZE x i8] zeroinitializer
+</pre>
+<ol class="arabic simple" start="2">
+<li>A reference to a <em>GlobalValue</em> (a function or global variable) with an
+optional 32-bit byte offset added to it (the addend, which may be
+negative):</li>
+</ol>
+<pre>
+   ptrtoint (TYPE* &#64;GLOBAL to i32)
+   add (i32 ptrtoint (TYPE* &#64;GLOBAL to i32), i32 ADDEND)
+</pre>
+<p>A <em>CompoundElement</em> is a unnamed, packed struct containing more than one
+<em>SimpleElement</em>.</p>
+</section><section id="functions">
+<h3 id="functions">Functions</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#functionstructure">LLVM LangRef: Functions</a></p>
+<p>The restrictions on <a class="reference internal" href="#bitcode-linkagetypes"><em>linkage types</em></a>, calling
+conventions and visibility styles apply to functions. In addition, the following
+are not supported for functions:</p>
+<ul class="small-gap">
+<li>Function attributes (either for the the function itself, its parameters or its
+return type).</li>
+<li>Garbage collector name (<code>gc</code>).</li>
+<li>Functions with a variable number of arguments (<em>vararg</em>).</li>
+<li>Alignment (<code>align</code>).</li>
+</ul>
+</section><section id="aliases">
+<h3 id="aliases">Aliases</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#aliases">LLVM LangRef: Aliases</a></p>
+<p>PNaCl bitcode does not support aliases.</p>
+</section><section id="named-metadata">
+<h3 id="named-metadata">Named Metadata</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#namedmetadatastructure">LLVM LangRef: Named Metadata</a></p>
+<p>While PNaCl bitcode has provisions for debugging metadata, it is not considered
+part of the stable ABI. It exists for tool support and should not appear in
+distributed pexes.</p>
+<p>Other kinds of LLVM metadata are not supported.</p>
+</section><section id="module-level-inline-assembly">
+<h3 id="module-level-inline-assembly">Module-Level Inline Assembly</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#moduleasm">LLVM LangRef: Module-Level Inline Assembly</a></p>
+<p>PNaCl bitcode does not support inline assembly.</p>
+</section><section id="volatile-memory-accesses">
+<h3 id="volatile-memory-accesses">Volatile Memory Accesses</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#volatile">LLVM LangRef: Volatile Memory Accesses</a></p>
+<p>PNaCl bitcode does not support volatile memory accesses. The
+<code>volatile</code> attribute on loads and stores is not supported. See the
+<a class="reference internal" href="/native-client/reference/pnacl-c-cpp-language-support.html"><em>PNaCl C/C++ Language Support</em></a> for more details.</p>
+</section><section id="memory-model-for-concurrent-operations">
+<h3 id="memory-model-for-concurrent-operations">Memory Model for Concurrent Operations</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#memmodel">LLVM LangRef: Memory Model for Concurrent Operations</a></p>
+<p>See the <a class="reference external" href="PNaClDeveloperGuide.html">PNaCl Developer&#8217;s Guide</a> for more
+details.</p>
+</section><section id="fast-math-flags">
+<h3 id="fast-math-flags">Fast-Math Flags</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#fastmath">LLVM LangRef: Fast-Math Flags</a></p>
+<p>Fast-math mode is not currently supported by the PNaCl bitcode.</p>
+</section></section><section id="type-system">
+<h2 id="type-system">Type System</h2>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#typesystem">LLVM LangRef: Type System</a></p>
+<p>The LLVM types allowed in PNaCl bitcode are restricted, as follows:</p>
+<section id="scalar-types">
+<h3 id="scalar-types">Scalar types</h3>
+<ul class="small-gap">
+<li><p class="first">The only scalar types allowed are integer, float (32-bit floating point),
+double (64-bit floating point) and void.</p>
+<ul class="small-gap">
+<li>The only integer sizes allowed are i1, i8, i16, i32 and i64.</li>
+<li>The only integer sizes allowed for function arguments and function return
+values are i32 and i64.</li>
+</ul>
+</li>
+</ul>
+</section><section id="array-and-struct-types">
+<h3 id="array-and-struct-types">Array and struct types</h3>
+<p>Array and struct types are only allowed in
+<a class="reference internal" href="#bitcode-globalvariables"><em>global variable initializers</em></a>.</p>
+</section><section id="pointer-types">
+<span id="bitcode-pointertypes"></span><h3 id="pointer-types"><span id="bitcode-pointertypes"></span>Pointer types</h3>
+<p>Only the following pointer types are allowed:</p>
+<ul class="small-gap">
+<li>Pointers to valid PNaCl bitcode scalar types, as specified above.</li>
+<li>Pointers to functions.</li>
+</ul>
+<p>In addition, the address space for all pointers must be 0.</p>
+<p>A pointer is <em>inherent</em> when it represents the return value of an <code>alloca</code>
+instruction, or is an address of a global value.</p>
+<p>A pointer is <em>normalized</em> if it&#8217;s either:</p>
+<ul class="small-gap">
+<li><em>inherent</em></li>
+<li>Is the return value of a <code>bitcast</code> instruction.</li>
+<li>Is the return value of a <code>inttoptr</code> instruction.</li>
+</ul>
+</section><section id="undefined-values">
+<h3 id="undefined-values">Undefined Values</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#undefvalues">LLVM LangRef: Undefined Values</a></p>
+<p><code>undef</code> is only allowed within functions, not in global variable initializers.</p>
+</section><section id="constant-expressions">
+<h3 id="constant-expressions">Constant Expressions</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#constant-expressions">LLVM LangRef: Constant Expressions</a></p>
+<p>Constant expressions are only allowed in
+<a class="reference internal" href="#bitcode-globalvariables"><em>global variable initializers</em></a>.</p>
+</section></section><section id="other-values">
+<h2 id="other-values">Other Values</h2>
+<section id="metadata-nodes-and-metadata-strings">
+<h3 id="metadata-nodes-and-metadata-strings">Metadata Nodes and Metadata Strings</h3>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#metadata">LLVM LangRef: Metadata Nodes and Metadata Strings</a></p>
+<p>While PNaCl bitcode has provisions for debugging metadata, it is not considered
+part of the stable ABI. It exists for tool support and should not appear in
+distributed pexes.</p>
+<p>Other kinds of LLVM metadata are not supported.</p>
+</section></section><section id="intrinsic-global-variables">
+<h2 id="intrinsic-global-variables">Intrinsic Global Variables</h2>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#intrinsic-global-variables">LLVM LangRef: Intrinsic Global Variables</a></p>
+<p>PNaCl bitcode does not support intrinsic global variables.</p>
+</section><section id="instruction-reference">
+<h2 id="instruction-reference">Instruction Reference</h2>
+<section id="list-of-allowed-instructions">
+<h3 id="list-of-allowed-instructions">List of allowed instructions</h3>
+<p>This is a list of LLVM instructions supported by PNaCl bitcode. Where
+applicable, PNaCl-specific restrictions are provided.</p>
+<p>The following attributes are disallowed for all instructions:</p>
+<ul class="small-gap">
+<li><code>nsw</code> and <code>nuw</code></li>
+<li><code>exact</code></li>
+</ul>
+<p>Only the LLVM instructions listed here are supported by PNaCl bitcode.</p>
+<ul class="small-gap">
+<li><code>ret</code></li>
+<li><code>br</code></li>
+<li><p class="first"><code>switch</code></p>
+<p>i1 values are disallowed for <code>switch</code>.</p>
+</li>
+<li><p class="first"><code>add</code>, <code>sub</code>, <code>mul</code>, <code>shl</code>,  <code>udiv</code>, <code>sdiv</code>, <code>urem</code>, <code>srem</code>,
+<code>lshr</code>, <code>ashr</code></p>
+<p>These arithmetic operations are disallowed on values of type <code>i1</code>.</p>
+<p>Integer division (<code>udiv</code>, <code>sdiv</code>, <code>urem</code>, <code>srem</code>) by zero is
+guaranteed to trap in PNaCl bitcode.</p>
+</li>
+<li><code>and</code></li>
+<li><code>or</code></li>
+<li><code>xor</code></li>
+<li><code>fadd</code></li>
+<li><code>fsub</code></li>
+<li><code>fmul</code></li>
+<li><code>fdiv</code></li>
+<li><code>frem</code></li>
+<li><p class="first"><code>alloca</code></p>
+<p>See <a class="reference internal" href="#bitcode-allocainst"><em>alloca instructions</em></a>.</p>
+</li>
+<li><p class="first"><code>load</code>, <code>store</code></p>
+<p>The pointer argument of these instructions must be a <em>normalized</em> pointer (see
+<a class="reference internal" href="#bitcode-pointertypes"><em>pointer types</em></a>). The <code>volatile</code> and <code>atomic</code>
+attributes are not supported. Loads and stores of the type <code>i1</code> are not
+supported.</p>
+<p>These instructions must use <code>align 1</code> on integer memory accesses, <code>align 4</code>
+for <code>float</code> accesses and <code>align 8</code> for <code>double</code> accesses.</p>
+</li>
+<li><code>trunc</code></li>
+<li><code>zext</code></li>
+<li><code>sext</code></li>
+<li><code>fptrunc</code></li>
+<li><code>fpext</code></li>
+<li><code>fptoui</code></li>
+<li><code>fptosi</code></li>
+<li><code>uitofp</code></li>
+<li><code>sitofp</code></li>
+<li><p class="first"><code>ptrtoint</code></p>
+<p>The pointer argument of a <code>ptrtoint</code> instruction must be a <em>normalized</em>
+pointer (see <a class="reference internal" href="#bitcode-pointertypes"><em>pointer types</em></a>) and the integer
+argument must be an i32.</p>
+</li>
+<li><p class="first"><code>inttoptr</code></p>
+<p>The integer argument of a <code>inttoptr</code> instruction must be an i32.</p>
+</li>
+<li><p class="first"><code>bitcast</code></p>
+<p>The pointer argument of a <code>bitcast</code> instruction must be a <em>inherent</em> pointer
+(see <a class="reference internal" href="#bitcode-pointertypes"><em>pointer types</em></a>).</p>
+</li>
+<li><code>icmp</code></li>
+<li><code>fcmp</code></li>
+<li><code>phi</code></li>
+<li><code>select</code></li>
+<li><code>call</code></li>
+</ul>
+</section><section id="alloca">
+<span id="bitcode-allocainst"></span><h3 id="alloca"><span id="bitcode-allocainst"></span><code>alloca</code></h3>
+<p>The only allowed type for <code>alloca</code> instructions in PNaCl bitcode is i8. The
+size argument must be an i32. For example:</p>
+<pre>
+  %buf = alloca i8, i32 8, align 4
+</pre>
+</section></section><section id="intrinsic-functions">
+<h2 id="intrinsic-functions">Intrinsic Functions</h2>
+<p><a class="reference external" href="http://llvm.org/releases/3.3/docs/LangRef.html#intrinsics">LLVM LangRef: Intrinsic Functions</a></p>
+<section id="list-of-allowed-intrinsics">
+<h3 id="list-of-allowed-intrinsics">List of allowed intrinsics</h3>
+<p>The only intrinsics supported by PNaCl bitcode are the following.</p>
+<ul class="small-gap">
+<li><code>llvm.memcpy</code></li>
+<li><code>llvm.memmove</code></li>
+<li><p class="first"><code>llvm.memset</code></p>
+<p>These intrinsics are only supported with an i32 <code>len</code> argument.</p>
+</li>
+<li><p class="first"><code>llvm.bswap</code></p>
+<p>The overloaded <code>llvm.bswap</code> intrinsic is only supported with the following
+argument types: i16, i32, i64 (the types supported by C-style GCC builtins).</p>
+</li>
+<li><code>llvm.ctlz</code></li>
+<li><code>llvm.cttz</code></li>
+<li><p class="first"><code>llvm.ctpop</code></p>
+<p>The overloaded llvm.ctlz, llvm.cttz, and llvm.ctpop intrinsics are only
+supported with the i32 and i64 argument types (the types supported by
+C-style GCC builtins).</p>
+</li>
+<li><p class="first"><code>llvm.sqrt</code></p>
+<p>The overloaded <code>llvm.sqrt</code> intrinsic is only supported for float
+and double arguments types. Unlike the standard LLVM intrinsic,
+PNaCl guarantees that llvm.sqrt returns a QNaN for values less than -0.0.</p>
+</li>
+<li><code>llvm.stacksave</code></li>
+<li><p class="first"><code>llvm.stackrestore</code></p>
+<p>These intrinsics are used to implement language features like scoped automatic
+variable sized arrays in C99. <code>llvm.stacksave</code> returns a value that
+represents the current state of the stack. This value may only be used as the
+argument to <code>llvm.stackrestore</code>, which restores the stack to the given
+state.</p>
+</li>
+<li><p class="first"><code>llvm.trap</code></p>
+<p>This intrinsic is lowered to a target dependent trap instruction, which aborts
+execution.</p>
+</li>
+<li><p class="first"><code>llvm.nacl.read.tp</code></p>
+<p>See <a class="reference internal" href="#bitcode-threadpointerintrinsics"><em>thread pointer related intrinsics</em></a>.</p>
+</li>
+<li><code>llvm.nacl.longjmp</code></li>
+<li><p class="first"><code>llvm.nacl.setjmp</code></p>
+<p>See <a class="reference internal" href="#bitcode-setjmplongjmp"><em>Setjmp and Longjmp</em></a>.</p>
+</li>
+<li><code>llvm.nacl.atomic.store</code></li>
+<li><code>llvm.nacl.atomic.load</code></li>
+<li><code>llvm.nacl.atomic.rmw</code></li>
+<li><code>llvm.nacl.atomic.cmpxchg</code></li>
+<li><code>llvm.nacl.atomic.fence</code></li>
+<li><code>llvm.nacl.atomic.fence.all</code></li>
+<li><p class="first"><code>llvm.nacl.atomic.is.lock.free</code></p>
+<p>See <a class="reference internal" href="#bitcode-atomicintrinsics"><em>atomic intrinsics</em></a>.</p>
+</li>
+</ul>
+</section><section id="thread-pointer-related-intrinsics">
+<span id="bitcode-threadpointerintrinsics"></span><h3 id="thread-pointer-related-intrinsics"><span id="bitcode-threadpointerintrinsics"></span>Thread pointer related intrinsics</h3>
+<pre>
+  declare i8* &#64;llvm.nacl.read.tp()
+</pre>
+<p>Returns a read-only thread pointer. The value is controlled by the embedding
+sandbox&#8217;s runtime.</p>
+</section><section id="setjmp-and-longjmp">
+<span id="bitcode-setjmplongjmp"></span><h3 id="setjmp-and-longjmp"><span id="bitcode-setjmplongjmp"></span>Setjmp and Longjmp</h3>
+<pre>
+  declare void &#64;llvm.nacl.longjmp(i8* %jmpbuf, i32)
+  declare i32 &#64;llvm.nacl.setjmp(i8* %jmpbuf)
+</pre>
+<p>These intrinsics implement the semantics of C11 <code>setjmp</code> and <code>longjmp</code>. The
+<code>jmpbuf</code> pointer must be 64-bit aligned and point to at least 1024 bytes of
+allocated memory.</p>
+</section><section id="atomic-intrinsics">
+<span id="bitcode-atomicintrinsics"></span><h3 id="atomic-intrinsics"><span id="bitcode-atomicintrinsics"></span>Atomic intrinsics</h3>
+<pre>
+  declare iN &#64;llvm.nacl.atomic.load.&lt;size&gt;(
+          iN* &lt;source&gt;, i32 &lt;memory_order&gt;)
+  declare void &#64;llvm.nacl.atomic.store.&lt;size&gt;(
+          iN &lt;operand&gt;, iN* &lt;destination&gt;, i32 &lt;memory_order&gt;)
+  declare iN &#64;llvm.nacl.atomic.rmw.&lt;size&gt;(
+          i32 &lt;computation&gt;, iN* &lt;object&gt;, iN &lt;operand&gt;, i32 &lt;memory_order&gt;)
+  declare iN &#64;llvm.nacl.atomic.cmpxchg.&lt;size&gt;(
+          iN* &lt;object&gt;, iN &lt;expected&gt;, iN &lt;desired&gt;,
+          i32 &lt;memory_order_success&gt;, i32 &lt;memory_order_failure&gt;)
+  declare void &#64;llvm.nacl.atomic.fence(i32 &lt;memory_order&gt;)
+  declare void &#64;llvm.nacl.atomic.fence.all()
+</pre>
+<p>Each of these intrinsics is overloaded on the <code>iN</code> argument, which is
+reflected through <code>&lt;size&gt;</code> in the overload&#8217;s name. Integral types of
+8, 16, 32 and 64-bit width are supported for these arguments.</p>
+<p>The <code>&#64;llvm.nacl.atomic.rmw</code> intrinsic implements the following
+read-modify-write operations, from the general and arithmetic sections
+of the C11/C++11 standards:</p>
+<blockquote>
+<div><ul class="small-gap">
+<li><code>add</code></li>
+<li><code>sub</code></li>
+<li><code>or</code></li>
+<li><code>and</code></li>
+<li><code>xor</code></li>
+<li><code>exchange</code></li>
+</ul>
+</div></blockquote>
+<p>For all of these read-modify-write operations, the returned value is
+that at <code>object</code> before the computation. The <code>computation</code> argument
+must be a compile-time constant.</p>
+<p>All atomic intrinsics also support C11/C++11 memory orderings, which
+must be compile-time constants.</p>
+<p>Integer values for these computations and memory orderings are defined
+in <code>&quot;llvm/IR/NaClAtomicIntrinsics.h&quot;</code>.</p>
+<p>The <code>&#64;llvm.nacl.atomic.fence.all</code> intrinsic is equivalent to the
+<code>&#64;llvm.nacl.atomic.fence</code> intrinsic with sequentially consistent
+ordering and compiler barriers preventing most non-atomic memory
+accesses from reordering around it.</p>
+<aside class="note">
+<blockquote>
+<div>These intrinsics allow PNaCl to support C11/C++11 style atomic
+operations as well as some legacy GCC-style <code>__sync_*</code> builtins
+while remaining stable as the LLVM codebase changes. The user isn&#8217;t
+expected to use these intrinsics directly.</div></blockquote>
+
+</aside>
+<pre>
+  declare i1 &#64;llvm.nacl.atomic.is.lock.free(i32 &lt;byte_size&gt;, i8* &lt;address&gt;)
+</pre>
+<p>The <code>llvm.nacl.atomic.is.lock.free</code> intrinsic is designed to
+determine at translation time whether atomic operations of a certain
+<code>byte_size</code> (a compile-time constant), at a particular <code>address</code>,
+are lock-free or not. This reflects the C11 <code>atomic_is_lock_free</code>
+function from header <code>&lt;stdatomic.h&gt;</code> and the C++11 <code>is_lock_free</code>
+member function in header <code>&lt;atomic&gt;</code>. It can be used through the
+<code>__nacl_atomic_is_lock_free</code> builtin.</p>
+</section></section></section>
+
+{{/partials.standard_nacl_article}}