[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));
----------------
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()`


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https://reviews.llvm.org/D74141