[InstCombine] convert fsub nsz with fneg operand to -(X + Y)

rotateright · rotateright · commit 242fed9d7fb9 · 2020-01-27T14:49:15.000-05:00
This was noted in D72521 - we need to match fneg specifically to
consistently handle that pattern along with (-0.0 - X).
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -2159,6 +2159,13 @@ Instruction *InstCombiner::visitFSub(BinaryOperator &I) {
     }
   }
 
+  // (-X) - Op1 --> -(X + Op1)
+  if (I.hasNoSignedZeros() && !isa<ConstantExpr>(Op0) &&
+      match(Op0, m_OneUse(m_FNeg(m_Value(X))))) {
+    Value *FAdd = Builder.CreateFAddFMF(X, Op1, &I);
+    return UnaryOperator::CreateFNegFMF(FAdd, &I);
+  }
+
   if (isa<Constant>(Op0))
     if (SelectInst *SI = dyn_cast<SelectInst>(Op1))
       if (Instruction *NV = FoldOpIntoSelect(I, SI))
diff --git a/llvm/test/Transforms/InstCombine/fsub.ll b/llvm/test/Transforms/InstCombine/fsub.ll
@@ -731,8 +731,8 @@ define float @fneg_fsub(float %x, float %y) {
 
 define float @fneg_fsub_nsz(float %x, float %y) {
 ; CHECK-LABEL: @fneg_fsub_nsz(
-; CHECK-NEXT:    [[NEGX:%.*]] = fneg float [[X:%.*]]
-; CHECK-NEXT:    [[SUB:%.*]] = fsub nsz float [[NEGX]], [[Y:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = fadd nsz float [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[SUB:%.*]] = fneg nsz float [[TMP1]]
 ; CHECK-NEXT:    ret float [[SUB]]
 ;
   %negx = fneg float %x
@@ -743,7 +743,7 @@ define float @fneg_fsub_nsz(float %x, float %y) {
 define float @fake_fneg_fsub_fast(float %x, float %y) {
 ; CHECK-LABEL: @fake_fneg_fsub_fast(
 ; CHECK-NEXT:    [[TMP1:%.*]] = fadd fast float [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[SUB:%.*]] = fsub fast float -0.000000e+00, [[TMP1]]
+; CHECK-NEXT:    [[SUB:%.*]] = fneg fast float [[TMP1]]
 ; CHECK-NEXT:    ret float [[SUB]]
 ;
   %negx = fsub float -0.0, %x
@@ -766,8 +766,8 @@ define float @fake_fneg_fsub_fast_extra_use(float %x, float %y) {
 
 define <2 x float> @fake_fneg_fsub_vec(<2 x float> %x, <2 x float> %y) {
 ; CHECK-LABEL: @fake_fneg_fsub_vec(
-; CHECK-NEXT:    [[NEGX:%.*]] = fsub <2 x float> <float -0.000000e+00, float -0.000000e+00>, [[X:%.*]]
-; CHECK-NEXT:    [[SUB:%.*]] = fsub nsz <2 x float> [[NEGX]], [[Y:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = fadd nsz <2 x float> [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[SUB:%.*]] = fneg nsz <2 x float> [[TMP1]]
 ; CHECK-NEXT:    ret <2 x float> [[SUB]]
 ;
   %negx = fsub <2 x float> <float -0.0, float -0.0>, %x
diff --git a/llvm/test/Transforms/Reassociate/fast-basictest.ll b/llvm/test/Transforms/Reassociate/fast-basictest.ll
@@ -671,7 +671,7 @@ define float @test19_reassoc(float %A, float %B) {
 define float @test20(float %a, float %b, float %c) nounwind  {
 ; CHECK-LABEL: @test20(
 ; CHECK-NEXT:    [[TMP1:%.*]] = fadd fast float [[B:%.*]], [[C:%.*]]
-; CHECK-NEXT:    [[T7:%.*]] = fsub fast float -0.000000e+00, [[TMP1]]
+; CHECK-NEXT:    [[T7:%.*]] = fneg fast float [[TMP1]]
 ; CHECK-NEXT:    ret float [[T7]]
 ;
   %t3 = fsub fast float %a, %b

Original file line number	Diff line number	Diff line change
`@@ -2159,6 +2159,13 @@ Instruction *InstCombiner::visitFSub(BinaryOperator &I) {`
`2159`	`2159`	`}`
`2160`	`2160`	`}`
`2161`	`2161`
	`2162`	`+ // (-X) - Op1 --> -(X + Op1)`
	`2163`	`+ if (I.hasNoSignedZeros() && !isa<ConstantExpr>(Op0) &&`
	`2164`	`+ match(Op0, m_OneUse(m_FNeg(m_Value(X))))) {`
	`2165`	`+ Value *FAdd = Builder.CreateFAddFMF(X, Op1, &I);`
	`2166`	`+ return UnaryOperator::CreateFNegFMF(FAdd, &I);`
	`2167`	`+ }`
	`2168`	`+`
`2162`	`2169`	`if (isa<Constant>(Op0))`
`2163`	`2170`	`if (SelectInst *SI = dyn_cast<SelectInst>(Op1))`
`2164`	`2171`	`if (Instruction *NV = FoldOpIntoSelect(I, SI))`
Original file line number	Diff line number	Diff line change
`@@ -671,7 +671,7 @@ define float @test19_reassoc(float %A, float %B) {`
`671`	`671`	`define float @test20(float %a, float %b, float %c) nounwind {`
`672`	`672`	`; CHECK-LABEL: @test20(`
`673`	`673`	`; CHECK-NEXT: [[TMP1:%.]] = fadd fast float [[B:%.]], [[C:%.*]]`
`674`		`-; CHECK-NEXT: [[T7:%.*]] = fsub fast float -0.000000e+00, [[TMP1]]`
	`674`	`+; CHECK-NEXT: [[T7:%.*]] = fneg fast float [[TMP1]]`
`675`	`675`	`; CHECK-NEXT: ret float [[T7]]`
`676`	`676`	`;`
`677`	`677`	`%t3 = fsub fast float %a, %b`