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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214385 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index e80d6a9ee39b6ba346abf17c4fd6a8abae3ee8a0..ed0e1c908888ee0286a1a53e4bb9a305fad4f854 100644 (file)
--- 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 67b7c4996b075fce5b29b98d012120a84bd93027..114aff7efb8545f3fb6799db520c070edfcea122 100644 (file)
--- 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
+}