[PATCH] D60518: [InstCombine] Handle ssubo always overflow

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added a reviewer: lebedev.ri.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

Followup to D60483 <https://reviews.llvm.org/D60483> and D60497 <https://reviews.llvm.org/D60497>: Handle always overflow for ssubo. This it the last case we can handle right now.


Repository:
  rL LLVM

https://reviews.llvm.org/D60518

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
  llvm/test/Transforms/InstCombine/with_overflow.ll


Index: llvm/test/Transforms/InstCombine/with_overflow.ll
===================================================================
--- llvm/test/Transforms/InstCombine/with_overflow.ll
+++ llvm/test/Transforms/InstCombine/with_overflow.ll
@@ -582,8 +582,9 @@
 ; CHECK-LABEL: @ssub_always_overflow(
 ; CHECK-NEXT:    [[C:%.*]] = icmp sgt i8 [[X:%.*]], 29
 ; CHECK-NEXT:    [[Y:%.*]] = select i1 [[C]], i8 [[X]], i8 29
-; CHECK-NEXT:    [[A:%.*]] = call { i8, i1 } @llvm.ssub.with.overflow.i8(i8 -100, i8 [[Y]])
-; CHECK-NEXT:    ret { i8, i1 } [[A]]
+; CHECK-NEXT:    [[A:%.*]] = sub nuw i8 -100, [[Y]]
+; CHECK-NEXT:    [[TMP1:%.*]] = insertvalue { i8, i1 } { i8 undef, i1 true }, i8 [[A]], 0
+; CHECK-NEXT:    ret { i8, i1 } [[TMP1]]
 ;
   %c = icmp sgt i8 %x, 29
   %y = select i1 %c, i8 %x, i8 29
Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3997,15 +3997,15 @@
       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)
         return SetResult(Builder.CreateNSWSub(LHS, RHS), Builder.getFalse(),
                          true);
     }
+
+    if (OR == OverflowResult::AlwaysOverflows)
+      return SetResult(Builder.CreateSub(LHS, RHS), Builder.getTrue(), true);
     break;
   }
 


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