[PATCH] D60483: [InstCombine] Handle usubo always overflow

[Nikita Popov via Phabricator via llvm-commits]

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

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).


Repository:
  rL LLVM

https://reviews.llvm.org/D60483

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
@@ -544,8 +544,9 @@
 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)
Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3990,9 +3990,13 @@
       return SetResult(LHS, Builder.getFalse(), false);
 
     if (OCF == OCF_UNSIGNED_SUB) {
-      if (willNotOverflowUnsignedSub(LHS, RHS, OrigI))
+      OverflowResult OR = computeOverflowForUnsignedSub(LHS, RHS, &OrigI);
+      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 {
       if (willNotOverflowSignedSub(LHS, RHS, OrigI))
         return SetResult(Builder.CreateNSWSub(LHS, RHS), Builder.getFalse(),


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