Don't attempt to add 'nsw' when intermediate instructions had no such guarantee.

llvm-svn: 137572

Author: nlewycky

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -146,7 +146,6 @@ static bool MaintainNoSignedWrap(BinaryOperator &I, Value *B, Value *C) {
   return !Overflow;
 }
 
-
 /// SimplifyAssociativeOrCommutative - This performs a few simplifications for
 /// operators which are associative or commutative:
 //
@@ -197,7 +196,10 @@ bool InstCombiner::SimplifyAssociativeOrCommutative(BinaryOperator &I) {
           I.setOperand(1, V);
           // Conservatively clear the optional flags, since they may not be
           // preserved by the reassociation.
-          if (MaintainNoSignedWrap(I, B, C)) {
+          if (MaintainNoSignedWrap(I, B, C) &&
+	      (!Op0 || (isa<BinaryOperator>(Op0) && Op0->hasNoSignedWrap()))) {
+            // Note: this is only valid because SimplifyBinOp doesn't look at
+            // the operands to Op0.
             I.clearSubclassOptionalData();
             I.setHasNoSignedWrap(true);
           } else {
@@ -292,10 +294,11 @@ bool InstCombiner::SimplifyAssociativeOrCommutative(BinaryOperator &I) {
         I.setOperand(1, Folded);
         // Conservatively clear the optional flags, since they may not be
         // preserved by the reassociation.
-        if (MaintainNoSignedWrap(I, C1, C2)) {
+        if (MaintainNoSignedWrap(I, C1, C2) && Op0->hasNoSignedWrap() &&
+	    Op1->hasNoSignedWrap()) {
+          New->setHasNoSignedWrap(true);
           I.clearSubclassOptionalData();
           I.setHasNoSignedWrap(true);
-          New->setHasNoSignedWrap(true);
         } else {
           I.clearSubclassOptionalData();
         }

llvm/test/Transforms/InstCombine/nsw.ll

@@ -46,10 +46,38 @@ define i32 @preserve1(i32 %x) nounwind {
   ret i32 %add3
 }
 
+; CHECK: @preserve2
+; CHECK: add nsw i8 %A, %B
+; CHECK: add nsw i8
+define i8 @preserve2(i8 %A, i8 %B) nounwind {
+  %x = add nsw i8 %A, 10
+  %y = add nsw i8 %B, 10
+  %add = add nsw i8 %x, %y
+  ret i8 %add
+}
+
 ; CHECK: @nopreserve1
 ; CHECK: add i8 %x, -126
 define i8 @nopreserve1(i8 %x) nounwind {
   %add = add nsw i8 %x, 127
   %add3 = add nsw i8 %add, 3
   ret i8 %add3
-}
+}
+
+; CHECK: @nopreserve2
+; CHECK: add i8 %x, 3
+define i8 @nopreserve2(i8 %x) nounwind {
+  %add = add i8 %x, 1
+  %add3 = add nsw i8 %add, 2
+  ret i8 %add3
+}
+
+; CHECK: @nopreserve3
+; CHECK: add i8 %A, %B
+; CHECK: add i8
+define i8 @nopreserve3(i8 %A, i8 %B) nounwind {
+  %x = add i8 %A, 10
+  %y = add i8 %B, 10
+  %add = add nsw i8 %x, %y
+  ret i8 %add
+}