Merging r214385:

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r214385 | majnemer | 2014-07-30 21:49:29 -0700 (Wed, 30 Jul 2014) | 9 lines

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.
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_35@223644 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 18 insertions, 3 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index e80d6a9ee39b..ed0e1c908888 100644
--- a/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/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/test/Transforms/InstCombine/sub.ll b/test/Transforms/InstCombine/sub.ll
index 67b7c4996b07..114aff7efb85 100644
--- a/test/Transforms/InstCombine/sub.ll
+++ b/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
+}