[InstCombine] Fold '((%x * %y) u/ %x) != %y' to '@llvm.umul.with.overflow' + overflow bit extraction
Summary:
`((%x * %y) u/ %x) != %y` is one of (3?) common ways to check that
some unsigned multiplication (will not) overflow.
Currently, we don't catch it. We could:
```
$ /repositories/alive2/build-Clang-unknown/alive -root-only ~/llvm-patch1.ll
Processing /home/lebedevri/llvm-patch1.ll..
----------------------------------------
Name: no overflow
%o0 = mul i4 %y, %x
%o1 = udiv i4 %o0, %x
%r = icmp ne i4 %o1, %y
ret i1 %r
=>
%n0 = umul_overflow i4 %x, %y
%o0 = extractvalue {i4, i1} %n0, 0
%o1 = udiv %o0, %x
%r = extractvalue {i4, i1} %n0, 1
ret %r
Done: 1
Optimization is correct!
----------------------------------------
Name: no overflow
%o0 = mul i4 %y, %x
%o1 = udiv i4 %o0, %x
%r = icmp eq i4 %o1, %y
ret i1 %r
=>
%n0 = umul_overflow i4 %x, %y
%o0 = extractvalue {i4, i1} %n0, 0
%o1 = udiv %o0, %x
%n1 = extractvalue {i4, i1} %n0, 1
%r = xor %n1, -1
ret i1 %r
Done: 1
Optimization is correct!
```
Reviewers: nikic, spatel, efriedma, xbolva00, RKSimon
Reviewed By: nikic
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D65144
llvm-svn: 370348
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 955886c..8cb0a26 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3527,20 +3527,21 @@
/// Fold
/// (-1 u/ x) u< y
+/// ((x * y) u/ x) != y
/// to
/// @llvm.umul.with.overflow(x, y) plus extraction of overflow bit
-/// Note that the comparison is commutative, while inverted (u>=) predicate
+/// Note that the comparison is commutative, while inverted (u>=, ==) predicate
/// will mean that we are looking for the opposite answer.
-static Value *
-foldUnsignedMultiplicationOverflowCheck(ICmpInst &I,
- InstCombiner::BuilderTy &Builder) {
+Value *InstCombiner::foldUnsignedMultiplicationOverflowCheck(ICmpInst &I) {
ICmpInst::Predicate Pred;
Value *X, *Y;
+ Instruction *Mul;
bool NeedNegation;
// Look for: (-1 u/ x) u</u>= y
if (!I.isEquality() &&
match(&I, m_c_ICmp(Pred, m_OneUse(m_UDiv(m_AllOnes(), m_Value(X))),
m_Value(Y)))) {
+ Mul = nullptr;
// Canonicalize as-if y was on RHS.
if (I.getOperand(1) != Y)
Pred = I.getSwappedPredicate();
@@ -3556,12 +3557,34 @@
default:
return nullptr; // Wrong predicate.
}
+ } else // Look for: ((x * y) u/ x) !=/== y
+ if (I.isEquality() &&
+ match(&I, m_c_ICmp(Pred, m_Value(Y),
+ m_OneUse(m_UDiv(m_CombineAnd(m_c_Mul(m_Deferred(Y),
+ m_Value(X)),
+ m_Instruction(Mul)),
+ m_Deferred(X)))))) {
+ NeedNegation = Pred == ICmpInst::Predicate::ICMP_EQ;
} else
return nullptr;
+ BuilderTy::InsertPointGuard Guard(Builder);
+ // If the pattern included (x * y), we'll want to insert new instructions
+ // right before that original multiplication so that we can replace it.
+ bool MulHadOtherUses = Mul && !Mul->hasOneUse();
+ if (MulHadOtherUses)
+ Builder.SetInsertPoint(Mul);
+
Function *F = Intrinsic::getDeclaration(
I.getModule(), Intrinsic::umul_with_overflow, X->getType());
CallInst *Call = Builder.CreateCall(F, {X, Y}, "umul");
+
+ // If the multiplication was used elsewhere, to ensure that we don't leave
+ // "duplicate" instructions, replace uses of that original multiplication
+ // with the multiplication result from the with.overflow intrinsic.
+ if (MulHadOtherUses)
+ replaceInstUsesWith(*Mul, Builder.CreateExtractValue(Call, 0, "umul.val"));
+
Value *Res = Builder.CreateExtractValue(Call, 1, "umul.ov");
if (NeedNegation) // This technically increases instruction count.
Res = Builder.CreateNot(Res, "umul.not.ov");
@@ -3918,7 +3941,7 @@
}
}
- if (Value *V = foldUnsignedMultiplicationOverflowCheck(I, Builder))
+ if (Value *V = foldUnsignedMultiplicationOverflowCheck(I))
return replaceInstUsesWith(I, V);
if (Value *V = foldICmpWithLowBitMaskedVal(I, Builder))