[Bug c/61939] warn when __attribute__((aligned(x))) is ignored

[daniel.santos at pobox dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61939

--- Comment #2 from Daniel Santos <daniel.santos at pobox dot com> ---
(In reply to Vedran Miletic from comment #1)
> #include <numeric>
> #include <vector>
> float f(std::vector<float>& A, std::vector<float>& B)
> {
>   __builtin_assume_aligned(A.data(), 64);
>   __builtin_assume_aligned(B.data(), 64);
>   return std::inner_product(A.begin(), A.end(), B.begin(), 0.f);
> }

You are doing it wrong. __builtin_assume_aligned() returns void* and you must
use it's return value for it to be effective. So your code should be something
like this:

float f(std::vector<float>& A, std::vector<float>& B)
{
  float *a_data = __builtin_assume_aligned(A.data(), 64);
  float *b_data = __builtin_assume_aligned(B.data(), 64);
  return std::inner_product(a_data, b_data, B.begin(), 0.f);
}

Of course, this assumes that the buffer that your vector<> implementation
supplies is 64 byte aligned.