[SelectionDAG] Remove unused FP constant in getNegatedExpression

[qiucf]

Summary

960cbc53 immediately removes nodes that won't be used to avoid compilation time explosion. This patch adds the removal to constants to fix PR47517.

[RKSimon]

Have you tried to reduce the test case any further?

[qiucf]

Reduce test case

[steven.zhang]

llvm/test/CodeGen/X86/pr47517.ll

2

Please update the test with FileCheck

[qiucf]

Update test using script

[steven.zhang]

LGTM and thank you for doing this. But please for a while to see if RKSimon has more comments.

[RKSimon]

LGTM

[qiucf]

After running sanitizer, I think the bug is not totally covered by this patch.

In D86689, we introduced such pattern:

// Negate the X if its cost is less or equal than Y.
if (NegX && (CostX <= CostY)) {
  Cost = CostX;
  SDValue N = DAG.getNode(ISD::FSUB, DL, VT, NegX, Y, Flags);
  RemoveDeadNode(NegY);
  return N;
}

Somewhere in this test case, N=3.0, NegY=3.0, NegY->use_size()=0, so NegY is just deleted but N also gets deleted.

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

if (LegalOps && !IsOpLegal)
       break;
 
     APFloat V = cast<ConstantFPSDNode>(Op)->getValueAPF();
     V.changeSign();
     SDValue CFP = DAG.getConstantFP(V, DL, VT);
 
     // If we already have the use of the negated floating constant, it is free
     // to negate it even it has multiple uses.
-    if (!Op.hasOneUse() && CFP.use_empty())
+    if (!Op.hasOneUse() && CFP.use_empty()) {
+      RemoveDeadNode(CFP);
       break;
+    }
     Cost = NegatibleCost::Neutral;
     return CFP;
   }
   case ISD::BUILD_VECTOR: {
     // Only permit BUILD_VECTOR of constants.
     if (llvm::any_of(Op->op_values(), [&](SDValue N) {
           return !N.isUndef() && !isa<ConstantFPSDNode>(N);
         }))

llvm/test/CodeGen/X86/pr47517.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple x86_64 < %s | FileCheck %s
+
+; To ensure unused floating point constant is removed in negation
+define float @test(float %src, float* %p) {
+; CHECK-LABEL: test:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    movq $0, (%rdi)
+; CHECK-NEXT:    xorps %xmm0, %xmm0
+; CHECK-NEXT:    retq
+entry:
+  %a0 = getelementptr inbounds float, float* %p, i32 0
+  %a1 = getelementptr inbounds float, float* %p, i32 1
+  store float 0.000000e+00, float* %a0
+  store float 0.000000e+00, float* %a1
+  %zero = load float, float* %a0
+  %fmul1 = fmul fast float %zero, %src
+  %fadd1 = fadd fast float %fmul1, %zero
+  %fmul2 = fmul fast float %fadd1, 2.000000e+00
+  %fmul3 = fmul fast float %fmul2, %fmul2
+  %fmul4 = fmul fast float %fmul2, 2.000000e+00
+  %fadd2 = fadd fast float %fmul4, -3.000000e+00
+  %fmul5 = fmul fast float %fadd2, %fmul2
+  %fadd3 = fadd fast float %fmul2, %src
+  %fadd4 = fadd fast float %fadd3, %fmul5
+  %fmul6 = fmul fast float %fmul3, %fadd4
+  ret float %fmul6
+}