AMDGPU: Disable some fneg combines unless nsz

For -(x + y) -> (-x) + (-y), if x == -y, this would
change the result from -0.0 to 0.0. Since the fma/fmad
combine is an extension of this problem it also
applies there.

fmul should be fine, and I don't think any of the unary
operators or conversions should be a problem either.

llvm-svn: 292473

2 files changed, 16 insertions, 0 deletions

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
index e48c1943cb0..07f627aab8b 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
@@ -2855,6 +2855,9 @@ SDValue AMDGPUTargetLowering::performFNegCombine(SDNode *N,
   SDLoc SL(N);
   switch (Opc) {
   case ISD::FADD: {
+    if (!mayIgnoreSignedZero(N0))
+      return SDValue();
+
     // (fneg (fadd x, y)) -> (fadd (fneg x), (fneg y))
     SDValue LHS = N0.getOperand(0);
     SDValue RHS = N0.getOperand(1);
@@ -2895,6 +2898,9 @@ SDValue AMDGPUTargetLowering::performFNegCombine(SDNode *N,
   }
   case ISD::FMA:
   case ISD::FMAD: {
+    if (!mayIgnoreSignedZero(N0))
+      return SDValue();
+
     // (fneg (fma x, y, z)) -> (fma x, (fneg y), (fneg z))
     SDValue LHS = N0.getOperand(0);
     SDValue MHS = N0.getOperand(1);
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h
index 69567aa5f71..171b0435850 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.h
@@ -119,6 +119,16 @@ protected:
 public:
   AMDGPUTargetLowering(const TargetMachine &TM, const AMDGPUSubtarget &STI);
 
+  bool mayIgnoreSignedZero(SDValue Op) const {
+    if (getTargetMachine().Options.UnsafeFPMath) // FIXME: nsz only
+      return true;
+
+    if (const auto *BO = dyn_cast<BinaryWithFlagsSDNode>(Op))
+      return BO->Flags.hasNoSignedZeros();
+
+    return false;
+  }
+
   bool isFAbsFree(EVT VT) const override;
   bool isFNegFree(EVT VT) const override;
   bool isTruncateFree(EVT Src, EVT Dest) const override;