[NaClDocs] Add note about errno in PNaCl ABI.

native_client_sdk/doc_generated/reference/pnacl-bitcode-abi.html

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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" id="contents" style="display: none">
 <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>
@@ skipping 18 matching lines @@
 <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>
+<li><a class="reference internal" href="#errno-and-errors-in-arithmetic-instructions" id="id24">Errno and errors in arithmetic instructions</a></li>
+<li><p class="first"><a class="reference internal" href="#instruction-reference" id="id25">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>
+<li><a class="reference internal" href="#list-of-allowed-instructions" id="id26">List of allowed instructions</a></li>
+<li><a class="reference internal" href="#alloca" id="id27"><code>alloca</code></a></li>
 </ul>
 </li>
-<li><p class="first"><a class="reference internal" href="#intrinsic-functions" id="id27">Intrinsic Functions</a></p>
+<li><p class="first"><a class="reference internal" href="#intrinsic-functions" id="id28">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>
+<li><a class="reference internal" href="#list-of-allowed-intrinsics" id="id29">List of allowed intrinsics</a></li>
+<li><a class="reference internal" href="#thread-pointer-related-intrinsics" id="id30">Thread pointer related intrinsics</a></li>
+<li><a class="reference internal" href="#setjmp-and-longjmp" id="id31">Setjmp and Longjmp</a></li>
+<li><a class="reference internal" href="#atomic-intrinsics" id="id32">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>
@@ skipping 155 matching lines @@
 <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="errno-and-errors-in-arithmetic-instructions">
+<span id="ir-and-errno"></span><h2 id="errno-and-errors-in-arithmetic-instructions"><span id="ir-and-errno"></span>Errno and errors in arithmetic instructions</h2>
+<p>Some arithmetic instructions and intrinsics have the similar semantics to
+libc math functions, but differ in the treatment of <code>errno</code>. While the
+libc functions may set <code>errno</code> for domain errors, the instructions and
+intrinsics do not. This is because the variable <code>errno</code> is not special
+and is not required to be part of the program.</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>
@@ skipping 11 matching lines @@
 <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>frem</code></p>
+<p>The frem instruction has the semantics of the libc fmod function for
+computing the floating point remainder. If the numerator is infinity, or
+denominator is zero, or either are NaN, then the result is NaN.
+Unlike the libc fmod function, this does not set <code>errno</code> when the
+result is NaN (see the <a class="reference internal" href="#ir-and-errno"><em>instructions and errno</em></a>
+section).</p>
+</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>
@@ skipping 50 matching lines @@
 </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>
+and double arguments types. This has the same semantics as the libc
+sqrt function, returning NaN for values less than -0.0. However, this
+does not set <code>errno</code> when the result is NaN (see the
+<a class="reference internal" href="#ir-and-errno"><em>instructions and errno</em></a> section).</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>
@@ skipping 89 matching lines @@
 <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>
 
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