[Bug c/79518] __builtin_assume_aligned should mark argument as aligned

[evan@coeus-group.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79518

--- Comment #4 from Evan Nemerson <evan@coeus-group.com> ---
I agree that GCC's implementation makes more sense, but unfortunately it makes
it hard to write portable code.

I'm not suggesting the current behavior be abandoned, only that
__builtin_assume_aligned be enhanced so the argument is marked as aligned when
possible.

Think of it as an optimization opportunity; given something like

  void* foo = __builtin_assume_aligned(bar, 16);
  /* Use bar for something instead of foo */

GCC could see that bar is 16-byte aligned and generate better code accordingly,
even though the programmer did something dumb.

I won't reopen this, but I hope you will…  AFAICT there aren't any
disadvantages, and it would be very helpful for people with code already
optimized using MSVC, ICC, etc.