[InstCombine] Handle usubo always overflow

Check AlwaysOverflow condition for usubo. The implementation is the
same as the existing handling for uaddo and umulo. Handling for saddo
and ssubo will follow (smulo doesn't have the necessary ValueTracking
support).

Differential Revision: https://reviews.llvm.org/D60483

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358052 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index e274d7efddbe2d59f163dbbb205b84fdb8c4dda0..7ec75358f7b0a460f3109a8f6cee840addbe805c 100644 (file)
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3997,6 +3997,9 @@ bool InstCombiner::OptimizeOverflowCheck(OverflowCheckFlavor OCF, Value *LHS,
       if (OR == OverflowResult::NeverOverflows)
         return SetResult(Builder.CreateNUWSub(LHS, RHS), Builder.getFalse(),
                          true);
+
+      if (OR == OverflowResult::AlwaysOverflows)
+        return SetResult(Builder.CreateSub(LHS, RHS), Builder.getTrue(), true);
     } else {
       OR = computeOverflowForSignedSub(LHS, RHS, &OrigI);
       if (OR == OverflowResult::NeverOverflows)

diff --git a/test/Transforms/InstCombine/with_overflow.ll b/test/Transforms/InstCombine/with_overflow.ll
index d5854f4a2f1bad2603c0ab643d54068a38750f84..d5a727ea194564c4912a9586577511fdb02073d4 100644 (file)
--- a/test/Transforms/InstCombine/with_overflow.ll
+++ b/test/Transforms/InstCombine/with_overflow.ll
@@ -544,8 +544,9 @@ define { i8, i1 } @uadd_always_overflow(i8 %x) nounwind {
 define { i8, i1 } @usub_always_overflow(i8 %x) nounwind {
 ; CHECK-LABEL: @usub_always_overflow(
 ; CHECK-NEXT:    [[Y:%.*]] = or i8 [[X:%.*]], 64
-; CHECK-NEXT:    [[A:%.*]] = call { i8, i1 } @llvm.usub.with.overflow.i8(i8 63, i8 [[Y]])
-; CHECK-NEXT:    ret { i8, i1 } [[A]]
+; CHECK-NEXT:    [[A:%.*]] = sub nsw i8 63, [[Y]]
+; CHECK-NEXT:    [[TMP1:%.*]] = insertvalue { i8, i1 } { i8 undef, i1 true }, i8 [[A]], 0
+; CHECK-NEXT:    ret { i8, i1 } [[TMP1]]
 ;
   %y = or i8 %x, 64
   %a = call { i8, i1 } @llvm.usub.with.overflow.i8(i8 63, i8 %y)