[InstCombine] Drop nsw in negation of select

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp

@@ -334,6 +334,17 @@ std::array<Value *, 2> Negator::getSortedOperandsOfBinOp(Instruction *I) {
       NewSelect->swapValues();
       // Don't swap prof metadata, we didn't change the branch behavior.
       NewSelect->setName(I->getName() + ".neg");
+      // Poison-generating flags should be dropped
+      Value *TV = NewSelect->getTrueValue();
+      Value *FV = NewSelect->getFalseValue();
+      if (match(TV, m_Neg(m_Specific(FV))))
+        cast<Instruction>(TV)->dropPoisonGeneratingFlags();
+      else if (match(FV, m_Neg(m_Specific(TV))))
+        cast<Instruction>(FV)->dropPoisonGeneratingFlags();
+      else {
+        cast<Instruction>(TV)->dropPoisonGeneratingFlags();
+        cast<Instruction>(FV)->dropPoisonGeneratingFlags();
+      }
       Builder.Insert(NewSelect);
       return NewSelect;
     }

llvm/test/Transforms/InstCombine/sub-of-negatible.ll

@@ -1377,7 +1377,7 @@ define i8 @negate_select_of_op_vs_negated_op(i8 %x, i8 %y, i1 %c) {
 
 define i8 @negate_select_of_op_vs_negated_op_nsw(i8 %x, i8 %y, i1 %c) {
 ; CHECK-LABEL: @negate_select_of_op_vs_negated_op_nsw(
-; CHECK-NEXT:    [[T0:%.*]] = sub nsw i8 0, [[X:%.*]]
+; CHECK-NEXT:    [[T0:%.*]] = sub i8 0, [[X:%.*]]
 ; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[C:%.*]], i8 [[X]], i8 [[T0]]
 ; CHECK-NEXT:    [[T2:%.*]] = add i8 [[TMP1]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i8 [[T2]]
@@ -1390,7 +1390,7 @@ define i8 @negate_select_of_op_vs_negated_op_nsw(i8 %x, i8 %y, i1 %c) {
 
 define i8 @negate_select_of_op_vs_negated_op_nsw_commuted(i8 %x, i8 %y, i1 %c) {
 ; CHECK-LABEL: @negate_select_of_op_vs_negated_op_nsw_commuted(
-; CHECK-NEXT:    [[T0:%.*]] = sub nsw i8 0, [[X:%.*]]
+; CHECK-NEXT:    [[T0:%.*]] = sub i8 0, [[X:%.*]]
 ; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[C:%.*]], i8 [[T0]], i8 [[X]]
 ; CHECK-NEXT:    [[T2:%.*]] = add i8 [[TMP1]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i8 [[T2]]
@@ -1403,8 +1403,8 @@ define i8 @negate_select_of_op_vs_negated_op_nsw_commuted(i8 %x, i8 %y, i1 %c) {
 
 define i8 @negate_select_of_op_vs_negated_op_nsw_xyyx(i8 %x, i8 %y, i8 %z, i1 %c) {
 ; CHECK-LABEL: @negate_select_of_op_vs_negated_op_nsw_xyyx(
-; CHECK-NEXT:    [[SUB1:%.*]] = sub nsw i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[SUB2:%.*]] = sub nsw i8 [[Y]], [[X]]
+; CHECK-NEXT:    [[SUB1:%.*]] = sub i8 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[SUB2:%.*]] = sub i8 [[Y]], [[X]]
 ; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[C:%.*]], i8 [[SUB2]], i8 [[SUB1]]
 ; CHECK-NEXT:    [[T2:%.*]] = add i8 [[TMP1]], [[Z:%.*]]
 ; CHECK-NEXT:    ret i8 [[T2]]