[AArch64] Missed CMN opportunities

[llvmbot]

Bugzilla Link	33486
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@Arnaud-de-Grandmaison-ARM

Extended Description

Tests:
bool test1(int a, int b) { return (a + b) == 0; }
bool test2(int a, int b) { return (a + b) != 0; }

Clang currently generates:
test1: // @​test1
// BB#0: // %entry
neg w8, w1
cmp w8, w0
cset w0, eq
ret
test2: // @​test2
// BB#0: // %entry
neg w8, w1
cmp w8, w0
cset w0, ne
ret

However, we should generate:
test1: // @​test1
// BB#0: // %entry
cmn w0, w1
cset w0, eq
ret
test2: // @​test2
// BB#0: // %entry
cmn w0, w1
cset w0, ne
ret

Reassociation will canonicalizes to these tests to 0-x == y and 0-x != y. And then for X86 a DAG combine will covert these back to x+y == 0 and x+y != 0, respectively.

Here's a patch for AArch64 that is a straight cut and paste from X86 to AArch64:
diff --git a/lib/Target/AArch64/AArch64ISelLowering.cpp b/lib/Target/AArch64/AArch64ISelLowering.cpp
index 083ca21..8286838 100644
--- a/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -545,6 +545,7 @@ AArch64TargetLowering::AArch64TargetLowering(const TargetMachine &TM,

setTargetDAGCombine(ISD::MUL);

• setTargetDAGCombine(ISD::SETCC);
  setTargetDAGCombine(ISD::SELECT);
  setTargetDAGCombine(ISD::VSELECT);

@@ -10204,6 +10205,34 @@ static SDValue performNVCASTCombine(SDNode *N) {
return SDValue();
}

+static SDValue performSetCCCombine(SDNode *N, SelectionDAG &DAG) {

• ISD::CondCode CC = cast(N->getOperand(2))->get();
• if (CC != ISD::SETNE && CC != ISD::SETEQ)
• return SDValue();
• EVT VT = N->getValueType(0);
• SDValue LHS = N->getOperand(0);
• SDValue RHS = N->getOperand(1);
• SDLoc DL(N);
• EVT OpVT = LHS.getValueType();
• // 0-x == y --> x+y == 0
• // 0-x != y --> x+y != 0
• if (LHS.getOpcode() == ISD::SUB && isNullConstant(LHS.getOperand(0)) &&
• LHS.hasOneUse()) {
• SDValue Add = DAG.getNode(ISD::ADD, DL, OpVT, RHS, LHS.getOperand(1));
• return DAG.getSetCC(DL, VT, Add, DAG.getConstant(0, DL, OpVT), CC);
• }
• // x == 0-y --> x+y == 0
• // x != 0-y --> x+y != 0
• if (RHS.getOpcode() == ISD::SUB && isNullConstant(RHS.getOperand(0)) &&
• RHS.hasOneUse()) {
• SDValue Add = DAG.getNode(ISD::ADD, DL, OpVT, LHS, RHS.getOperand(1));
• return DAG.getSetCC(DL, VT, Add, DAG.getConstant(0, DL, OpVT), CC);
• }
• return SDValue();
  +}

SDValue AArch64TargetLowering::PerformDAGCombine(SDNode *N,
DAGCombinerInfo &DCI) const {
SelectionDAG &DAG = DCI.DAG;
@@ -10249,6 +10278,8 @@ SDValue AArch64TargetLowering::PerformDAGCombine(SDNode *N,
break;
case ISD::STORE:
return performSTORECombine(N, DCI, DAG, Subtarget);

• case ISD::SETCC:
• return performSetCCCombine(N, DAG);
  case AArch64ISD::BRCOND:
  return performBRCONDCombine(N, DCI, DAG);
  case AArch64ISD::TBNZ:

The above patch will fix the problem, but I'm not sure it's the best solution and it doesn't impact any of the workloads I care about. Therefore, I'm not interested in pursuing this further, but figured I'd file a bug..

[Arnaud-de-Grandmaison-ARM]

Changing to confirmed as this can be reproduced with ToT as of today : r346643

[c-rhodes]

this has been fixed (since at least clang 9.0.0): https://godbolt.org/z/MTd7hff8W