NaCl documentation: update vector documentation

native_client_sdk/doc_generated/reference/pnacl-c-cpp-language-support.html

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 <section id="pnacl-c-c-language-support">
 <h1 id="pnacl-c-c-language-support">PNaCl C/C++ Language Support</h1>
 <div class="contents local" id="contents" style="display: none">
 <ul class="small-gap">
 <li><p class="first"><a class="reference internal" href="#source-language-support" id="id3">Source language support</a></p>
 <ul class="small-gap">
 <li><a class="reference internal" href="#versions" id="id4">Versions</a></li>
 <li><a class="reference internal" href="#preprocessor-definitions" id="id5">Preprocessor definitions</a></li>
@@ skipping 185 matching lines @@
 of Chrome, and more features are expected to be added in subsequent
 releases.</p>
 <section id="hand-coding-vector-extensions">
 <h3 id="hand-coding-vector-extensions">Hand-Coding Vector Extensions</h3>
 <p>The initial vector support in Portable Native Client adds <a class="reference external" href="http://clang.llvm.org/docs/LanguageExtensions.html#vectors-and-extended-vectors">LLVM vectors</a>
 and <a class="reference external" href="http://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html">GCC vectors</a> since these
 are well supported by different hardware platforms and don&#8217;t require any
 new compiler intrinsics.</p>
 <p>Vector types can be used through the <code>vector_size</code> attribute:</p>
 <pre class="prettyprint">
-typedef int v4s __attribute__((vector_size(16)));
+#define VECTOR_BYTES 16
+typedef int v4s __attribute__((vector_size(VECTOR_BYTES)));
 v4s a = {1,2,3,4};
 v4s b = {5,6,7,8};
 v4s c, d, e;
 c = b + 1;  /* c = b + {1,1,1,1}; */
 d = 2 * b;  /* d = {2,2,2,2} * b; */
 e = c + d;
 </pre>
 <p>Vector comparisons are represented as a bitmask as wide as the compared
 elements of all <code>0</code> or all <code>1</code>:</p>
 <pre class="prettyprint">
@@ skipping 88 matching lines @@
 individual elements using <code>[]</code>.</p>
 <pre class="prettyprint">
 typedef unsigned v4u __attribute__((vector_size(16)));
 template&lt;typename T&gt;
 void print(const T v) {
   for (size_t i = 0; i != sizeof(v) / sizeof(v[0]); ++i)
     std::cout &lt;&lt; v[i] &lt;&lt; ' ';
   std::cout &lt;&lt; std::endl;
 }
 </pre>
-<p>Vector shuffles are currently unsupported but will be added soon.</p>
+<p>Vector shuffles (often called permutation or swizzle) operations are
+supported through <code>__builtin_shufflevector</code>. The builtin has two
+vector arguments of the same element type, followed by a list of
+constant integers that specify the elements indices of the first two
+vectors that should be extracted and returned in a new vector. These
+element indices are numbered sequentially starting with the first
+vector, continuing into the second vector. Thus, if <code>vec1</code> is a
+4-element vector, index <code>5</code> would refer to the second element of
+<code>vec2</code>. An index of <code>-1</code> can be used to indicate that the
+corresponding element in the returned vector is a don’t care and can be
+optimized by the backend.</p>
+<p>The result of <code>__builtin_shufflevector</code> is a vector with the same
+element type as <code>vec1</code> / <code>vec2</code> but that has an element count equal
+to the number of indices specified.</p>
+<pre class="prettyprint">
+// identity operation - return 4-element vector v1.
+__builtin_shufflevector(v1, v1, 0, 1, 2, 3)
+
+// &quot;Splat&quot; element 0 of V1 into a 4-element result.
+__builtin_shufflevector(V1, V1, 0, 0, 0, 0)
+
+// Reverse 4-element vector V1.
+__builtin_shufflevector(V1, V1, 3, 2, 1, 0)
+
+// Concatenate every other element of 4-element vectors V1 and V2.
+__builtin_shufflevector(V1, V2, 0, 2, 4, 6)
+
+// Concatenate every other element of 8-element vectors V1 and V2.
+__builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)
+
+// Shuffle v1 with some elements being undefined
+__builtin_shufflevector(v1, v1, 3, -1, 1, -1)
+</pre>
 </section><section id="auto-vectorization">
 <h3 id="auto-vectorization">Auto-Vectorization</h3>
 <p>Auto-vectorization is currently not enabled for Portable Native Client,
 but will be in a future release.</p>
 </section></section><section id="undefined-behavior">
 <h2 id="undefined-behavior">Undefined Behavior</h2>
 <p>The C and C++ languages expose some undefined behavior which is
 discussed in <a class="reference internal" href="/native-client/reference/pnacl-undefined-behavior.html#undefined-behavior"><em>PNaCl Undefined Behavior</em></a>.</p>
 </section><section id="floating-point">
 <h2 id="floating-point">Floating-Point</h2>
@@ skipping 64 matching lines @@
 <p>Neither PNaCl nor NaCl currently support asynchronous interruption
 or suspension of threads.</p>
 </li>
 </ul>
 <p>If PNaCl were to support either of these, the interaction of
 <code>volatile</code> and atomics with same-thread signal handling would need
 to be carefully detailed.</p>
 </section></section></section>
 
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