Simple attempt at adding InstCombine::visitFNeg(...)

[cameron.mcinally]

Summary

This is essentially the same direction as D61693, but might be easier to review...

Create InstCombine::visitFNeg(...) with a call to InstSimplify to do the heavy lifting. More transforms will need to be copied over from InstCombine::visitFSub(...) in future patches.

[craig.topper]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

Instruction *InstCombiner::visitSub(BinaryOperator &I) {
   if (!I.hasNoUnsignedWrap() && willNotOverflowUnsignedSub(Op0, Op1, I)) {
     Changed = true;
     I.setHasNoUnsignedWrap(true);
   }
 
   return Changed ? &I : nullptr;
 }
 
+Instruction *InstCombiner::visitFNeg(UnaryOperator &I) {
+  if (Value *V = SimplifyFNegInst(I.getOperand(0), I.getFastMathFlags(),
+                                  SQ.getWithInstruction(&I)))
+    return replaceInstUsesWith(I, V);
+
+  return nullptr;
+}
+
+
 Instruction *InstCombiner::visitFSub(BinaryOperator &I) {
   if (Value *V = SimplifyFSubInst(I.getOperand(0), I.getOperand(1),
                                   I.getFastMathFlags(),
                                   SQ.getWithInstruction(&I)))
     return replaceInstUsesWith(I, V);
 
   if (Instruction *X = foldVectorBinop(I))
     return X;

llvm/lib/Transforms/InstCombine/InstCombineInternal.h

public:
 
   // Visitation implementation - Implement instruction combining for different
   // instruction types.  The semantics are as follows:
   // Return Value:
   //    null        - No change was made
   //     I          - Change was made, I is still valid, I may be dead though
   //   otherwise    - Change was made, replace I with returned instruction
   //
+  Instruction *visitFNeg(UnaryOperator &I);
   Instruction *visitAdd(BinaryOperator &I);
   Instruction *visitFAdd(BinaryOperator &I);
   Value *OptimizePointerDifference(Value *LHS, Value *RHS, Type *Ty);
   Instruction *visitSub(BinaryOperator &I);
   Instruction *visitFSub(BinaryOperator &I);
   Instruction *visitMul(BinaryOperator &I);
   Instruction *visitFMul(BinaryOperator &I);
   Instruction *visitURem(BinaryOperator &I);

llvm/test/Transforms/InstCombine/fneg.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -instcombine -S | FileCheck %s
 
 declare void @use(float)
 
 define float @fneg_fneg(float %a) {
 ;
 ; CHECK-LABEL: @fneg_fneg(
-; CHECK-NEXT:    [[F:%.*]] = fneg float [[A:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = fneg float [[F]]
-; CHECK-NEXT:    ret float [[R]]
+; CHECK-NEXT:    ret float [[A:%.*]]
 ;
   %f = fneg float %a
   %r = fneg float %f
   ret float %r
 }
 
 ; -(X * C) --> X * (-C)