Improve c++ example

examples/c++/sum_over_array_opt.cpp

@@ -1,15 +1,18 @@
 // Compile with -O3 -march=native to see autovectorization
+// assumes input is aligned on 64-byte boundary and that
+// length is a multiple of 64.
 int testFunction(int* input, int length) {
-    // Alignment hints supported on GCC 4.7+ and any compiler
-    // supporting the appropriate builtin (clang 3.6+).
+  // Alignment hints supported on GCC 4.7+ and any compiler
+  // supporting the appropriate builtin (clang 3.6+).
 #ifndef __has_builtin
 #define __has_builtin(x) 0
 #endif
 #if __GNUC__ > 4 \
-        || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7) \
-        || __has_builtin(__builtin_assume_aligned)
-  input = static_cast<int*>(__builtin_assume_aligned(input, 16));
+    || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7) \
+    || __has_builtin(__builtin_assume_aligned)
+  input = static_cast<int*>(__builtin_assume_aligned(input, 64));
 #endif
+  if (length & 63) __builtin_unreachable();
   int sum = 0;
   for (int i = 0; i < length; ++i) {
     sum += input[i];