PNaCl SIMD documentation: fix vector-scalar examples, currently unsupported by LLVM

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 190 matching lines @@
 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">
 #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;
+c = a + b;  /* c = {6,8,10,12} */
+d = b &gt;&gt; a; /* d = {2,1,0,0} */
 </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">
 typedef int v4s __attribute__((vector_size(16)));
 v4s snip(v4s in) {
   v4s limit = {32,64,128,256};
   v4s mask = in &gt; limit;
   v4s ret = in &amp; mask;
   return ret;
@@ skipping 100 matching lines @@
 <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)
+// &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)
+// 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 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)
+// 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>
+<p>One common use of <code>__builtin_shufflevector</code> is to perform
+vector-scalar operations:</p>
+<pre class="prettyprint">
+typedef int v4s __attribute__((vector_size(16)));
+v4s shift_right_by(v4s shift_me, int shift_amount) {
+  v4s tmp = {shift_amount};
+  return shift_me &gt;&gt; __builtin_shuffle_vector(tmp, tmp, 0, 0, 0, 0);
+}
+</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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