[PATCH] D74141: [InstCombine] Simplify a umul overflow check to a != 0 && b != 0.

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp:2726-2743
+  CmpInst::Predicate Pred;
+  Value *UMul1, *UMul2;
+  // Check if the OR weakens the overflow condition for umul.with.overflow by
+  // treating any non-zero result as overflow. In that case, we overflow if both
+  // umul.with.overflow operands are != 0, as in that case the result can only
+  // be 0, iff the multiplication overflows.
+  auto P = m_ICmp(Pred, m_ExtractValue<0>(m_Value(UMul2)), m_SpecificInt(0));
----------------
fhahn wrote:
> lebedev.ri wrote:
> > lebedev.ri wrote:
> > > Consider doing the following instead:
> > > ```
> > >   // Check if the OR weakens the overflow condition for umul.with.overflow by
> > >   // treating any non-zero result as overflow. In that case, we overflow if both
> > >   // umul.with.overflow operands are != 0, as in that case the result can only
> > >   // be 0, iff the multiplication overflows.
> > >   CmpInst::Predicate Pred;
> > >   Value *MulWithOv;
> > >   if (match(&I,
> > >             m_c_Or(m_ICmp(Pred, m_OneUse(m_ExtractValue<0>(m_Value(MulWithOv))),
> > >                           m_ZeroInt()),
> > >                    m_OneUse(m_ExtractValue<1>(m_Deferred(MulWithOv))))) &&
> > >       Pred == CmpInst::ICMP_NE) {
> > >     Value *A, *B;
> > >     if (match(MulWithOv, m_Intrinsic<Intrinsic::umul_with_overflow>(
> > >                              m_Value(A), m_Value(B))))
> > >       return BinaryOperator::CreateAnd(
> > >           Builder.CreateICmpNE(A, ConstantInt::get(A->getType(), 0)),
> > >           Builder.CreateICmpNE(A, ConstantInt::get(B->getType(), 0)));
> > >   }
> > > ```
> > Also, use `Builder.CreateIsNotNull()`
> Nice, m_c_Or and m_Deferred are very helpful!
Please adjust one-use checks as they were in the snippet,
both operands of `or` must be single-use, and we don't care if the intrinsic goes away or not.


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  https://reviews.llvm.org/D74141/new/

https://reviews.llvm.org/D74141