[PATCH] D27781: [X86] Fold (setcc (cmp (atomic_load_add x, -C) C), COND) to (setcc (LADD x, -C), COND) (PR31367)

[Hans Wennborg via Phabricator via llvm-commits]

hans created this revision.
hans added a reviewer: rnk.
hans added subscribers: llvm-commits, thakis.

This targets a pattern that occurrs frequently with reference counting pointers:

  void decrement(long volatile *ptr) {
    if (_InterlockedDecrement(ptr) == 0)
      release();
  }

Clang would previously compile it (for 32-bit at `-Os`) as:

  00000000 <?decrement@@YAXPCJ at Z>:
     0:   8b 44 24 04             mov    0x4(%esp),%eax
     4:   31 c9                   xor    %ecx,%ecx
     6:   49                      dec    %ecx
     7:   f0 0f c1 08             lock xadd %ecx,(%eax)
     b:   83 f9 01                cmp    $0x1,%ecx
     e:   0f 84 00 00 00 00       je     14 <?decrement@@YAXPCJ at Z+0x14>
    14:   c3                      ret

and with this patch it becomes:

  00000000 <?decrement@@YAXPCJ at Z>:
     0:   8b 44 24 04             mov    0x4(%esp),%eax
     4:   f0 ff 08                lock decl (%eax)
     7:   0f 84 00 00 00 00       je     d <?decrement@@YAXPCJ at Z+0xd>
     d:   c3                      ret

(Equivalent variants with _InterlockedExchangeAdd, std::atomic<>'s fetch_add or pre-decrement operator generate the same code.)


https://reviews.llvm.org/D27781

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/atomic-eflags-reuse.ll

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