[ValueTracking] Use ConstantRange based overflow check for signed sub

[nikic]

Summary

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.

[lebedev.ri]

LG

llvm/lib/Analysis/ValueTracking.cpp

OverflowResult llvm::computeOverflowForSignedSub(const Value *LHS,
                                                  const DominatorTree *DT) {
   // If LHS and RHS each have at least two sign bits, the subtraction
   // cannot overflow.
   if (ComputeNumSignBits(LHS, DL, 0, AC, CxtI, DT) > 1 &&
       ComputeNumSignBits(RHS, DL, 0, AC, CxtI, DT) > 1)
     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,
                                      const DominatorTree &DT) {
 #ifndef NDEBUG
   auto IID = II->getIntrinsicID();
   assert((IID == Intrinsic::sadd_with_overflow ||
           IID == Intrinsic::uadd_with_overflow ||

llvm/test/Transforms/InstCombine/sub.ll

;
   %res = sub <2 x i32> zeroinitializer, %3
   ret <2 x i32> %res
 }
 
 define i32 @nsw_inference1(i32 %x, i32 %y) {
 ; 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
   %y2 = and i32 %y, 1
   %z = sub i32 %x2, %y2
   ret i32 %z
 }
 
 define i32 @nsw_inference2(i32 %x, i32 %y) {
 ; 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
   %y2 = or i32 %y, -2
   %z = sub i32 %x2, %y2
   ret i32 %z
 }