commit 193bd01e7e738e40dc86f910772b8c21b16cf821
author Nikita Popov <nikita.ppv@gmail.com> Thu Mar 21 17:23:51 2019 +0000
committer Nikita Popov <nikita.ppv@gmail.com> Thu Mar 21 17:23:51 2019 +0000
tree e5853f134f4e9e6e2efe0989d21e519925004851
parent 507cee0654899e3573c428297ee06c2d19c7b99f

[ValueTracking] Use ConstantRange based overflow check for signed sub

This is D59450, but for signed sub. This case is not NFC, because
the overflow logic in ConstantRange is more powerful than the existing
check. This resolves the TODO in the function.

I've added two tests to show that this indeed catches more cases than
the previous logic, but the main correctness test coverage here is in
the existing ConstantRange unit tests.

Differential Revision: https://reviews.llvm.org/D59617

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356685 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ValueTracking.cpp

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index 52328b1..e31ddf0 100644
--- a/lib/Analysis/ValueTracking.cpp
+++ b/lib/Analysis/ValueTracking.cpp
@@ -4190,17 +4190,12 @@
     return OverflowResult::NeverOverflows;
 
   KnownBits LHSKnown = computeKnownBits(LHS, DL, 0, AC, CxtI, DT);
-
   KnownBits RHSKnown = computeKnownBits(RHS, DL, 0, AC, CxtI, DT);
-
-  // Subtraction of two 2's complement numbers having identical signs will
-  // never overflow.
-  if ((LHSKnown.isNegative() && RHSKnown.isNegative()) ||
-      (LHSKnown.isNonNegative() && RHSKnown.isNonNegative()))
-    return OverflowResult::NeverOverflows;
-
-  // TODO: implement logic similar to checkRippleForAdd
-  return OverflowResult::MayOverflow;
+  ConstantRange LHSRange =
+      ConstantRange::fromKnownBits(LHSKnown, /*signed*/ true);
+  ConstantRange RHSRange =
+      ConstantRange::fromKnownBits(RHSKnown, /*signed*/ true);
+  return mapOverflowResult(LHSRange.signedSubMayOverflow(RHSRange));
 }
 
 bool llvm::isOverflowIntrinsicNoWrap(const IntrinsicInst *II,

test/Transforms/InstCombine/sub.ll

diff --git a/test/Transforms/InstCombine/sub.ll b/test/Transforms/InstCombine/sub.ll
index 9b10017..2a9aa09 100644
--- a/test/Transforms/InstCombine/sub.ll
+++ b/test/Transforms/InstCombine/sub.ll
@@ -1271,7 +1271,7 @@
 ; CHECK-LABEL: @nsw_inference1(
 ; CHECK-NEXT:    [[X2:%.*]] = or i32 [[X:%.*]], 1024
 ; CHECK-NEXT:    [[Y2:%.*]] = and i32 [[Y:%.*]], 1
-; CHECK-NEXT:    [[Z:%.*]] = sub nuw i32 [[X2]], [[Y2]]
+; CHECK-NEXT:    [[Z:%.*]] = sub nuw nsw i32 [[X2]], [[Y2]]
 ; CHECK-NEXT:    ret i32 [[Z]]
 ;
   %x2 = or i32 %x, 1024
@@ -1284,7 +1284,7 @@
 ; CHECK-LABEL: @nsw_inference2(
 ; CHECK-NEXT:    [[X2:%.*]] = and i32 [[X:%.*]], -1025
 ; CHECK-NEXT:    [[Y2:%.*]] = or i32 [[Y:%.*]], -2
-; CHECK-NEXT:    [[Z:%.*]] = sub i32 [[X2]], [[Y2]]
+; CHECK-NEXT:    [[Z:%.*]] = sub nsw i32 [[X2]], [[Y2]]
 ; CHECK-NEXT:    ret i32 [[Z]]
 ;
   %x2 = and i32 %x, -1025