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

atomic_load_add returns the value before addition, but sets EFLAGS based on the
result of the addition. That means it's setting the flags based on effectively
subtracting C from the value at x, which is also what the outer cmp does.

This targets a pattern that occurs 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@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@Z+0x14>
  14:   c3                      ret

and with this patch it becomes:

00000000 <?decrement@@YAXPCJ@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@Z+0xd>
   d:   c3                      ret

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

Differential Revision: https://reviews.llvm.org/D27781

llvm-svn: 289955

1 files changed, 20 insertions, 9 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index c9aa9e2eb7d..82a7f8c8274 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -28879,11 +28879,19 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
   return SDValue();
 }
 
-/// Combine:
+/// Combine brcond/cmov/setcc/.. based on comparing the result of
+/// atomic_load_add to use EFLAGS produced by the addition
+/// directly if possible. For example:
+///
+///   (setcc (cmp (atomic_load_add x, -C) C), COND_E)
+/// becomes:
+///   (setcc (LADD x, -C), COND_E)
+///
+/// and
 ///   (brcond/cmov/setcc .., (cmp (atomic_load_add x, 1), 0), COND_S)
-/// to:
+/// becomes:
 ///   (brcond/cmov/setcc .., (LADD x, 1), COND_LE)
-/// i.e., reusing the EFLAGS produced by the LOCKed instruction.
+///
 /// Note that this is only legal for some op/cc combinations.
 static SDValue combineSetCCAtomicArith(SDValue Cmp, X86::CondCode &CC,
                                        SelectionDAG &DAG) {
@@ -28892,7 +28900,7 @@ static SDValue combineSetCCAtomicArith(SDValue Cmp, X86::CondCode &CC,
         (Cmp.getOpcode() == X86ISD::SUB && !Cmp->hasAnyUseOfValue(0))))
     return SDValue();
 
-  // This only applies to variations of the common case:
+  // This applies to variations of the common case:
   //   (icmp slt x, 0) -> (icmp sle (add x, 1), 0)
   //   (icmp sge x, 0) -> (icmp sgt (add x, 1), 0)
   //   (icmp sle x, 0) -> (icmp slt (sub x, 1), 0)
@@ -28911,8 +28919,9 @@ static SDValue combineSetCCAtomicArith(SDValue Cmp, X86::CondCode &CC,
     return SDValue();
 
   auto *CmpRHSC = dyn_cast<ConstantSDNode>(CmpRHS);
-  if (!CmpRHSC || CmpRHSC->getZExtValue() != 0)
+  if (!CmpRHSC)
     return SDValue();
+  APInt Comparand = CmpRHSC->getAPIntValue();
 
   const unsigned Opc = CmpLHS.getOpcode();
 
@@ -28928,13 +28937,15 @@ static SDValue combineSetCCAtomicArith(SDValue Cmp, X86::CondCode &CC,
   if (Opc == ISD::ATOMIC_LOAD_SUB)
     Addend = -Addend;
 
-  if (CC == X86::COND_S && Addend == 1)
+  if (Comparand == -Addend)
+    CC = CC; // No change.
+  else if (CC == X86::COND_S && Comparand == 0 && Addend == 1)
     CC = X86::COND_LE;
-  else if (CC == X86::COND_NS && Addend == 1)
+  else if (CC == X86::COND_NS && Comparand == 0 && Addend == 1)
     CC = X86::COND_G;
-  else if (CC == X86::COND_G && Addend == -1)
+  else if (CC == X86::COND_G && Comparand == 0 && Addend == -1)
     CC = X86::COND_GE;
-  else if (CC == X86::COND_LE && Addend == -1)
+  else if (CC == X86::COND_LE && Comparand == 0 && Addend == -1)
     CC = X86::COND_L;
   else
     return SDValue();