[TargetLowering] fix SETCC SETLT folding with FP types

The bug was introduced with: https://reviews.llvm.org/rL294863 ...and manifests as a selection failure in x86, but that's actually another bug. This fix prevents wrong codegen with -0.0, but in the more common case when we have NSZ and NNAN (-ffast-math), we should still be able to fold this setcc/compare. llvm-svn: 294924
llvm · Feb 12, 2017 · 0557a44 · 0557a44
1 parent 31e1b8f
commit 0557a44
Showing 2 changed files with 37 additions and 9 deletions.
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -751,25 +751,29 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op,
     KnownOne &= KnownOne2;
     KnownZero &= KnownZero2;
     break;
-  case ISD::SETCC:
+  case ISD::SETCC: {
+    SDValue Op0 = Op.getOperand(0);
+    SDValue Op1 = Op.getOperand(1);
+    ISD::CondCode CC = cast<CondCodeSDNode>(Op.getOperand(2))->get();
     // If (1) we only need the sign-bit, (2) the setcc operands are the same
     // width as the setcc result, and (3) the result of a setcc conforms to 0 or
     // -1, we may be able to bypass the setcc.
-    if (NewMask.isSignBit() &&
-        Op.getOperand(0).getScalarValueSizeInBits() == BitWidth &&
+    if (NewMask.isSignBit() && Op0.getScalarValueSizeInBits() == BitWidth &&
         getBooleanContents(Op.getValueType()) ==
             BooleanContent::ZeroOrNegativeOneBooleanContent) {
-      ISD::CondCode CC = cast<CondCodeSDNode>(Op.getOperand(2))->get();
-      // If we're testing if X < 0, then this compare isn't needed - just use X!
-      if (CC == ISD::SETLT &&
-          (isNullConstant(Op.getOperand(1)) ||
-           ISD::isBuildVectorAllZeros(Op.getOperand(1).getNode())))
-        return TLO.CombineTo(Op, Op.getOperand(0));
+      // If we're testing X < 0, then this compare isn't needed - just use X!
+      // FIXME: We're limiting to integer types here, but this should also work
+      // if we don't care about FP signed-zero. The use of SETLT with FP means
+      // that we don't care about NaNs.
+      if (CC == ISD::SETLT && Op1.getValueType().isInteger() &&
+          (isNullConstant(Op1) || ISD::isBuildVectorAllZeros(Op1.getNode())))
+        return TLO.CombineTo(Op, Op0);
 
       // TODO: Should we check for other forms of sign-bit comparisons?
       // Examples: X <= -1, X >= 0
     }
     break;
+  }
   case ISD::SHL:
     if (ConstantSDNode *SA = dyn_cast<ConstantSDNode>(Op.getOperand(1))) {
       unsigned ShAmt = SA->getZExtValue();

diff --git a/llvm/test/CodeGen/X86/vselect-pcmp.ll b/llvm/test/CodeGen/X86/vselect-pcmp.ll
@@ -317,3 +317,27 @@ define <8 x double> @signbit_sel_v8f64(<8 x double> %x, <8 x double> %y, <8 x i6
   ret <8 x double> %z
 }
 
+; If we have a floating-point compare:
+; (1) Don't die.
+; (2) FIXME: If we don't care about signed-zero (and NaN?), the compare should still get folded.
+
+define <4 x float> @signbit_sel_v4f32_fcmp(<4 x float> %x, <4 x float> %y, <4 x float> %mask) #0 {
+; AVX12F-LABEL: signbit_sel_v4f32_fcmp:
+; AVX12F:       # BB#0:
+; AVX12F-NEXT:    vxorps %xmm2, %xmm2, %xmm2
+; AVX12F-NEXT:    vcmpltps %xmm2, %xmm0, %xmm2
+; AVX12F-NEXT:    vblendvps %xmm2, %xmm0, %xmm1, %xmm0
+; AVX12F-NEXT:    retq
+;
+; AVX512VL-LABEL: signbit_sel_v4f32_fcmp:
+; AVX512VL:       # BB#0:
+; AVX512VL-NEXT:    vpxor %xmm2, %xmm2, %xmm2
+; AVX512VL-NEXT:    vcmpltps %xmm2, %xmm0, %k1
+; AVX512VL-NEXT:    vblendmps %xmm0, %xmm1, %xmm0 {%k1}
+; AVX512VL-NEXT:    retq
+  %cmp = fcmp olt <4 x float> %x, zeroinitializer
+  %sel = select <4 x i1> %cmp, <4 x float> %x, <4 x float> %y
+  ret <4 x float> %sel
+}
+
+attributes #0 = { "no-nans-fp-math"="true" }