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

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri removed reviewers: RKSimon, dexonsmith, Bigcheese, aemerson, majnemer.
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));
----------------
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)));
  }
```


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Comment at: llvm/test/Transforms/InstCombine/umul-signed.ll:22
 ;
   %res = tail call { i64, i1 } @llvm.umul.with.overflow.i64(i64 %a, i64 -4775807)
   %overflow = extractvalue { i64, i1 } %res, 1
----------------
There shouldn't be any `-4775807` here, please replace it with `%b`


================
Comment at: llvm/test/Transforms/InstCombine/umul-signed.ll:67
 
 ; Do not simplify if %overflow has multiple uses.
 define i1 @test3_multiple_overflow_users(i64 %a, i64 %b, i64* %ptr) {
----------------
Please add the same test for `%mul`, and `%res`


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

https://reviews.llvm.org/D74141