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

native_client_sdk/src/doc/reference/pnacl-c-cpp-language-support.rst

 ============================
 PNaCl C/C++ Language Support
 ============================
 
 .. contents::
    :local:
    :backlinks: none
    :depth: 3
 
 Source language support
@@ skipping 214 matching lines @@
 
 Vector types can be used through the ``vector_size`` attribute:
 
 .. naclcode::
 
   #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 >> a; /* d = {2,1,0,0} */
 
 Vector comparisons are represented as a bitmask as wide as the compared
 elements of all ``0`` or all ``1``:
 
 .. naclcode::
 
   typedef int v4s __attribute__((vector_size(16)));
   v4s snip(v4s in) {
     v4s limit = {32,64,128,256};
     v4s mask = in > limit;
@@ skipping 80 matching lines @@
 
 The result of ``__builtin_shufflevector`` is a vector with the same
 element type as ``vec1`` / ``vec2`` but that has an element count equal
 to the number of indices specified.
 
 .. naclcode::
 
   // identity operation - return 4-element vector v1.
   __builtin_shufflevector(v1, v1, 0, 1, 2, 3)
 
-  // "Splat" element 0 of V1 into a 4-element result.
-  __builtin_shufflevector(V1, V1, 0, 0, 0, 0)
+  // "Splat" 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)
 
+One common use of ``__builtin_shufflevector`` is to perform
+vector-scalar operations:
+
+.. naclcode::
+
+  typedef int v4s __attribute__((vector_size(16)));
+  v4s shift_right_by(v4s shift_me, int shift_amount) {
+    v4s tmp = {shift_amount};
+    return shift_me >> __builtin_shuffle_vector(tmp, tmp, 0, 0, 0, 0);
+  }
+
 Auto-Vectorization
 ------------------
 
 Auto-vectorization is currently not enabled for Portable Native Client,
 but will be in a future release.
 
 Undefined Behavior
 ==================
 
 The C and C++ languages expose some undefined behavior which is
@@ skipping 75 matching lines @@
   A similar feature is **thread suspension**: The ability to
   asynchronously suspend and resume a thread and inspect or modify its
   execution state (such as register state).
 
   Neither PNaCl nor NaCl currently support asynchronous interruption
   or suspension of threads.
 
 If PNaCl were to support either of these, the interaction of
 ``volatile`` and atomics with same-thread signal handling would need
 to be carefully detailed.