PNaCl documentation: add SIMD vectors.

native_client_sdk/doc_generated/reference/pnacl-undefined-behavior.html

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 <section id="pnacl-undefined-behavior">
 <h1 id="pnacl-undefined-behavior">PNaCl Undefined Behavior</h1>
 <div class="contents local" id="contents" style="display: none">
 <ul class="small-gap">
 <li><a class="reference internal" href="#overview" id="id2">Overview</a></li>
 <li><a class="reference internal" href="#specification" id="id3">Specification</a></li>
 <li><p class="first"><a class="reference internal" href="#behavior-in-pnacl-bitcode" id="id4">Behavior in PNaCl Bitcode</a></p>
 <ul class="small-gap">
 <li><a class="reference internal" href="#well-defined" id="id5">Well-Defined</a></li>
 <li><p class="first"><a class="reference internal" href="#not-well-defined" id="id6">Not Well-Defined</a></p>
 <ul class="small-gap">
 <li><a class="reference internal" href="#potentially-fixable" id="id7">Potentially Fixable</a></li>
 <li><a class="reference internal" href="#floating-point" id="id8">Floating-Point</a></li>
-<li><a class="reference internal" href="#hard-to-fix" id="id9">Hard to Fix</a></li>
+<li><a class="reference internal" href="#simd-vectors" id="id9">SIMD Vectors</a></li>
+<li><a class="reference internal" href="#hard-to-fix" id="id10">Hard to Fix</a></li>
 </ul>
 </li>
 </ul>
 </li>
 </ul>
 
 </div><section id="overview">
 <span id="undefined-behavior"></span><h2 id="overview"><span id="undefined-behavior"></span>Overview</h2>
 <p>C and C++ undefined behavior allows efficient mapping of the source
 language onto hardware, but leads to different behavior on different
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 could mandate flush-to-zero, and may give an API to enable denormals
 in a future release. The latter is problematic for SIMD and
 vectorization support, where some platforms do not support denormal
 SIMD operations.</li>
 <li><code>NaN</code> values are currently not guaranteed to be canonical; see <a class="reference external" href="https://code.google.com/p/nativeclient/issues/detail?id=3536">bug
 3536</a>.</li>
 <li>Passing <code>NaN</code> to STL functions (the math is defined, but the
 function implementation isn&#8217;t, e.g. <code>std::min</code> and <code>std::max</code>), is
 well-defined in the <em>pexe</em>.</li>
 </ul>
+</section><section id="simd-vectors">
+<h4 id="simd-vectors">SIMD Vectors</h4>
+<p>SIMD vector instructions aren&#8217;t part of the C/C++ standards and as such
+their behavior isn&#8217;t specified at all in C/C++; it is usually left up to
+the target architecture to specify behavior. Portable Native Client
+instead exposed <a class="reference internal" href="/native-client/reference/pnacl-c-cpp-language-support.html#portable-simd-vectors"><em>Portable SIMD Vectors</em></a> and
+offers the same guarantees on these vectors as the guarantees offered by
+the contained elements. Of notable interest amongst these guarantees are
+those of alignment for load/store instructions on vectors: they have the
+same alignment restriction as the contained elements.</p>
 </section><section id="hard-to-fix">
 <h4 id="hard-to-fix">Hard to Fix</h4>
 <ul class="small-gap">
 <li><p class="first">Null pointer/reference has behavior determined by the NaCl sandbox:</p>
 <ul class="small-gap">
 <li>Raises a segmentation fault in the bottom <code>64KiB</code> bytes on all
 platforms, and on some sandboxes there are further non-writable
 pages after the initial <code>64KiB</code>.</li>
 <li>Negative offsets aren&#8217;t handled consistently on all platforms:
 x86-64 and ARM will wrap around to the stack (because they mask the
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 dependent on stack and memory allocation.</li>
 <li>Data races: any two operations that conflict (target overlapping
 memory), at least one of which is a store or atomic read-modify-write,
 and at least one of which is not atomic: this will be very dependent
 on processor and execution sequence, see <a class="reference internal" href="/native-client/reference/pnacl-c-cpp-language-support.html#memory-model-and-atomics"><em>Memory Model and
 Atomics</em></a>.</li>
 </ul>
 </section></section></section></section>
 
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