[LLVMdev] Instruction combiner multiplication canonicalization

[Nema, Ashutosh]

Hi,

I observed below behavior with Instruction combiner (visitMul  Canonicalization)
It tries to convert "(X+C1)*C2" to "X*C2+C1*C2"
While transforming if operation is guaranteed to not overflow it does not
reflect same property to transformed instructions.

Consider following scenarios:
1) If input is ((X+C1)*C2)<nsw>
Then post canonicalization output should be (X*C2+C1*C2) <nsw>

2) If input is (((X+C1)<nsw>)*C2)<nsw>
Then post canonicalization output should be ((X*C2) <nsw>+(C1*C2) <nsw>) <nsw>

** C1 & C2 are constants.

Current canonicalization transforms to (X*C2+C1*C2) in both scenarios (1 & 2).

To me this looks like a limitation with canonicalization where its missing guarantee to not overflow property.
Please confirm my understanding.

<File: InstCombineMulDivRem.cpp >
 168 Instruction *InstCombiner::visitMul(BinaryOperator &I) {
 268     // Canonicalize (X+C1)*CI -> X*CI+C1*CI.
 269     {
 270       Value *X;
 271       Constant *C1;
 272       if (match(Op0, m_OneUse(m_Add(m_Value(X), m_Constant(C1))))) {
 273         Value *Mul = Builder->CreateMul(C1, Op1);
 274         // Only go forward with the transform if C1*CI simplifies to a tidier
 275         // constant.
 276         if (!match(Mul, m_Mul(m_Value(), m_Value())))
 277           return BinaryOperator::CreateAdd(Builder->CreateMul(X, Op1), Mul);
 278       }
 279     }

If my understanding is correct then I like propose below change here:
If input operation to canonicalization is guaranteed to not overflow then transformed operation should have the same property.
Specifically I like to handle above explained 2 scenarios for 'nsw' & 'nuw'.

Regards,
Ashutosh

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