[SelectionDAG] Check any use of negation result before removal

2508ef0 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 and will be fixed by this
patch.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D87614

(cherry picked from commit a2fb544)

Author: ecnelises

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -5751,8 +5751,10 @@ SDValue TargetLowering::getNegatedExpression(SDValue Op, SelectionDAG &DAG,
 
     // 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;
   }
@@ -5810,15 +5812,17 @@ SDValue TargetLowering::getNegatedExpression(SDValue Op, SelectionDAG &DAG,
     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;
@@ -5857,7 +5861,8 @@ SDValue TargetLowering::getNegatedExpression(SDValue Op, SelectionDAG &DAG,
     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;
     }
 
@@ -5870,7 +5875,8 @@ SDValue TargetLowering::getNegatedExpression(SDValue Op, SelectionDAG &DAG,
     if (NegY) {
       Cost = CostY;
       SDValue N = DAG.getNode(Opcode, DL, VT, X, NegY, Flags);
-      RemoveDeadNode(NegX);
+      if (NegX != N)
+        RemoveDeadNode(NegX);
       return N;
     }
     break;
@@ -5901,15 +5907,17 @@ SDValue TargetLowering::getNegatedExpression(SDValue Op, SelectionDAG &DAG,
     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;

llvm/test/CodeGen/X86/pr47517.ll

@@ -0,0 +1,28 @@
+; 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
+}