InstCombine: Correctly propagate NSW/NUW for x-(-A) -> x+A

We can only propagate the nsw bits if both subtraction instructions are
marked with the appropriate bit.

N.B.  We only propagate the nsw bit in InstCombine because the nuw case
is already handled in InstSimplify.

This fixes PR20189.

llvm-svn: 214385

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index e80d6a9..ed0e1c9 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1462,11 +1462,17 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
   if (Value *V = SimplifyUsingDistributiveLaws(I))
     return ReplaceInstUsesWith(I, V);
 
-  // If this is a 'B = x-(-A)', change to B = x+A.  This preserves NSW/NUW.
+  // If this is a 'B = x-(-A)', change to B = x+A.
   if (Value *V = dyn_castNegVal(Op1)) {
     BinaryOperator *Res = BinaryOperator::CreateAdd(Op0, V);
-    Res->setHasNoSignedWrap(I.hasNoSignedWrap());
-    Res->setHasNoUnsignedWrap(I.hasNoUnsignedWrap());
+
+    if (const auto *BO = dyn_cast<BinaryOperator>(Op1)) {
+      assert(BO->getOpcode() == Instruction::Sub &&
+             "Expected a subtraction operator!");
+      if (BO->hasNoSignedWrap() && I.hasNoSignedWrap())
+        Res->setHasNoSignedWrap(true);
+    }
+
     return Res;
   }
 

diff --git a/llvm/test/Transforms/InstCombine/sub.ll b/llvm/test/Transforms/InstCombine/sub.ll
index 67b7c49..114aff7 100644 (file)
--- a/llvm/test/Transforms/InstCombine/sub.ll
+++ b/llvm/test/Transforms/InstCombine/sub.ll
@@ -464,3 +464,12 @@ define i32 @test38(i32 %A) {
 ; CHECK-NEXT: [[SEXT:%.*]] = sext i1 [[ICMP]] to i32
 ; CHECK-NEXT: ret i32 [[SEXT]]
 }
+
+define i32 @test39(i32 %A, i32 %x) {
+  %B = sub i32 0, %A
+  %C = sub nsw i32 %x, %B
+  ret i32 %C
+; CHECK-LABEL: @test39(
+; CHECK: %C = add i32 %x, %A
+; CHECK: ret i32 %C
+}