Request to hook up `-fno-builtin-aligned_alloc`

[clin111]

Bugzilla Link	45655
Version	trunk
OS	All
CC	@DougGregor,@efriedma-quic,@zygoloid

Extended Description

For users that have a different aligned_alloc implementation such as jemalloc: http://jemalloc.net/

it might be desirable to turn off TLI recognition of aligned_alloc (commit D76970) without having to turn off all builtins with -fno-builtin or -ffreestanding. -fno-builtin-aligned_alloc doesn't look like it's recognized by the front end (and doesn't result in a nobuiltin attribute on the call).

As an example of a difference, the above jemalloc library is strict POSIX and will set errno if a non-power-of-2 alignment is given. When the code below is compiled with clang -O2 and jemalloc, the printf will print (nil) 0 because jemalloc will produce NULL, but InstCombine assumes the call will succeed and replaces errno with 0. Having a way to turn off recognition just for this function, would be very helpful to fix the inconsistency.

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>

int main() {
  void *p;

  errno = 0;
  p = (int *)aligned_alloc(0x81,1);
  printf("%p %d\n", p, errno);

  return 0;
}

[efriedma-quic]

I think we just need to add aligned_alloc to clang/include/clang/Basic/Builtins.def

[bondhugula]

It's not immediately clear to me what has to be done here. I've never worked on Clang, but this looks pretty straightforward to fix for someone who has. It's also not clear if InstCombine has to be fixed to do the right thing whenever the alignment isn't known to be a power of two. When the alignment isn't a power of two, aligned_alloc returns NULL and errno should be EINVAL.