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<div>Hi, </div>
<div> </div>
<div>I observed below behavior with Instruction combiner (visitMul Canonicalization)</div>
<div>It tries to convert “(X+C1)*C2” to “X*C2+C1*C2”</div>
<div>While transforming if operation is guaranteed to not overflow it does not </div>
<div>reflect same property to transformed instructions.</div>
<div> </div>
<div>Consider following scenarios:</div>
<div>1) If input is ((X+C1)*C2)<nsw></div>
<div>Then post canonicalization output should be (X*C2+C1*C2) <nsw></div>
<div> </div>
<div>2) If input is (((X+C1)<nsw>)*C2)<nsw></div>
<div>Then post canonicalization output should be ((X*C2) <nsw>+(C1*C2) <nsw>) <nsw></div>
<div> </div>
<div>** C1 & C2 are constants.</div>
<div> </div>
<div>Current canonicalization transforms to (X*C2+C1*C2) in both scenarios (1 & 2).</div>
<div> </div>
<div>To me this looks like a limitation with canonicalization where its missing guarantee to not overflow property.</div>
<div>Please confirm my understanding.</div>
<div> </div>
<div><File: InstCombineMulDivRem.cpp ></div>
<div> 168 Instruction *InstCombiner::visitMul(BinaryOperator &I) { </div>
<div> 268 // Canonicalize (X+C1)*CI -> X*CI+C1*CI.</div>
<div> 269 {</div>
<div> 270 Value *X;</div>
<div> 271 Constant *C1;</div>
<div> 272 if (match(Op0, m_OneUse(m_Add(m_Value(X), m_Constant(C1))))) {</div>
<div> 273 Value *Mul = Builder->CreateMul(C1, Op1);</div>
<div> 274 // Only go forward with the transform if C1*CI simplifies to a tidier</div>
<div> 275 // constant.</div>
<div> 276 if (!match(Mul, m_Mul(m_Value(), m_Value())))</div>
<div> 277 return BinaryOperator::CreateAdd(Builder->CreateMul(X, Op1), Mul);</div>
<div> 278 }</div>
<div> 279 }</div>
<div> </div>
<div>If my understanding is correct then I like propose below change here:</div>
<div>If input operation to canonicalization is guaranteed to not overflow then transformed operation should have the same property.</div>
<div>Specifically I like to handle above explained 2 scenarios for ‘nsw’ & ‘nuw’.</div>
<div> </div>
<div>Regards,</div>
<div>Ashutosh</div>
<div> </div>
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