[PATCH] D45321: [atomics] Fix runtime calls for misaligned atomics

[Tim Northover via Phabricator via llvm-commits]

t.p.northover added a comment.
Herald added a reviewer: javed.absar.

> The Linux libatomic __atomic_is_lock_free returns false for unaligned pointers, even on x86. clang must generate code which is compatible with that, so it *cannot* inline misaligned atomic operations.

I agree we need to be compatible, but I don't see that:

  #include <stdlib.h>
  #include <stdio.h>
  
  int main() {
    char *mem = malloc(16);
  
    for (int i = 1; i <= 64; i <<= 1)
      printf("__atomic_is_lock_free(%d, %p) = %d\n", i, mem+1, __atomic_is_lock_free(i, mem+1));
     free(mem);
  }

tells me that everything <= 4 is lock-free on x86_64 Linux (I have no idea why 4). GCC also does inline them (and I see no special misaligned libatomic implementation except for `__atomic_load` and `__atomic_store` which take a lock).

Basically, I think GCC is in as bad shape as we are but in practice lock-free implementations are what gets emitted. At least we can change our implementation in the future since we do emit a libcall for misaligned operations.

> I don't see any compelling reason for compiler-rt to try to lower misaligned atomics using lock-free operations.

My best argument is still that I think it's the maximally compatible way to proceed. It is ugly though.


Repository:
  rCRT Compiler Runtime

https://reviews.llvm.org/D45321