From: Nikita Popov <nikita.ppv@gmail.com>
Date: Wed, 10 Apr 2019 07:10:53 +0000 (+0000)
Subject: [InstCombine] Handle usubo always overflow
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=b8b80606ab47cffbcac15f6e6664c09bfc0d56e4;p=llvm

[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 e274d7efddb..7ec75358f7b 100644
--- 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 d5854f4a2f1..d5a727ea194 100644
--- 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)