[SelectionDAG] Check any use of negation result before removal

[qiucf]

Summary

D87614 fixed a bug about constant removal in negation. But after sanitizing check I found there's still some issue about it so it's reverted.

Temporary nodes will be removed if useless in negation. Before the removal, they'd be checked if any other nodes used it. So the removal was moved after getNode. However in rare cases the node to be removed is the same as result of getNode. We missed that.

The part is fragile, so we need to be careful about it..

[steven.zhang]

LGTM.

[hans]

Cherry-picked to 11.x as b78e5de029c26c309f541ab883fa5d6d953b073d.

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

       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);
         }))

     NegatibleCost CostY = NegatibleCost::Expensive;
     SDValue NegY =
         getNegatedExpression(Y, DAG, LegalOps, OptForSize, CostY, Depth);
 
     // 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);
+      if (NegY != N)
+        RemoveDeadNode(NegY);
       return N;
     }
 
     // Negate the Y if it is not expensive.
     if (NegY) {
       Cost = CostY;
       SDValue N = DAG.getNode(ISD::FSUB, DL, VT, NegY, X, Flags);
-      RemoveDeadNode(NegX);
+      if (NegX != N)
+        RemoveDeadNode(NegX);
       return N;
     }
     break;
   }
   case ISD::FSUB: {
     // We can't turn -(A-B) into B-A when we honor signed zeros.
     if (!Options.NoSignedZerosFPMath && !Flags.hasNoSignedZeros())
       break;

     NegatibleCost CostY = NegatibleCost::Expensive;
     SDValue NegY =
         getNegatedExpression(Y, DAG, LegalOps, OptForSize, CostY, Depth);
 
     // Negate the X if its cost is less or equal than Y.
     if (NegX && (CostX <= CostY)) {
       Cost = CostX;
       SDValue N = DAG.getNode(Opcode, DL, VT, NegX, Y, Flags);
-      RemoveDeadNode(NegY);
+      if (NegY != N)
+        RemoveDeadNode(NegY);
       return N;
     }
 
     // Ignore X * 2.0 because that is expected to be canonicalized to X + X.
     if (auto *C = isConstOrConstSplatFP(Op.getOperand(1)))
       if (C->isExactlyValue(2.0) && Op.getOpcode() == ISD::FMUL)
         break;
 
     // Negate the Y if it is not expensive.
     if (NegY) {
       Cost = CostY;
       SDValue N = DAG.getNode(Opcode, DL, VT, X, NegY, Flags);
-      RemoveDeadNode(NegX);
+      if (NegX != N)
+        RemoveDeadNode(NegX);
       return N;
     }
     break;
   }
   case ISD::FMA:
   case ISD::FMAD: {
     if (!Options.NoSignedZerosFPMath && !Flags.hasNoSignedZeros())
       break;

     NegatibleCost CostY = NegatibleCost::Expensive;
     SDValue NegY =
         getNegatedExpression(Y, DAG, LegalOps, OptForSize, CostY, Depth);
 
     // Negate the X if its cost is less or equal than Y.
     if (NegX && (CostX <= CostY)) {
       Cost = std::min(CostX, CostZ);
       SDValue N = DAG.getNode(Opcode, DL, VT, NegX, Y, NegZ, Flags);
-      RemoveDeadNode(NegY);
+      if (NegY != N)
+        RemoveDeadNode(NegY);
       return N;
     }
 
     // Negate the Y if it is not expensive.
     if (NegY) {
       Cost = std::min(CostY, CostZ);
       SDValue N = DAG.getNode(Opcode, DL, VT, X, NegY, NegZ, Flags);
-      RemoveDeadNode(NegX);
+      if (NegX != N)
+        RemoveDeadNode(NegX);
       return N;
     }
     break;
   }
 
   case ISD::FP_EXTEND:
   case ISD::FSIN:
     if (SDValue NegV = getNegatedExpression(Op.getOperand(0), DAG, LegalOps,

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 correctly removed
+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
+}