[PATCH] D85446: [InstCombine] Add vector support to mul(add(x,c),negpow2) -> mul(sub(-c,x),pow2) folds

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

Aha, good catch.
But this is not enough of a generalization:

  diff
  diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
  index 172b304b776..06a0fcbf0fa 100644
  --- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
  +++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
  @@ -233,29 +233,11 @@ Instruction *InstCombinerImpl::visitMul(BinaryOperator &I) {
       }
     }
   
  -  if (ConstantInt *CI = dyn_cast<ConstantInt>(Op1)) {
  -    // (Y - X) * (-(2**n)) -> (X - Y) * (2**n), for positive nonzero n
  -    // (Y + const) * (-(2**n)) -> (-constY) * (2**n), for positive nonzero n
  -    // The "* (2**n)" thus becomes a potential shifting opportunity.
  -    {
  -      const APInt &   Val = CI->getValue();
  -      const APInt &PosVal = Val.abs();
  -      if (Val.isNegative() && PosVal.isPowerOf2()) {
  -        Value *X = nullptr, *Y = nullptr;
  -        if (Op0->hasOneUse()) {
  -          ConstantInt *C1;
  -          Value *Sub = nullptr;
  -          if (match(Op0, m_Sub(m_Value(Y), m_Value(X))))
  -            Sub = Builder.CreateSub(X, Y, "suba");
  -          else if (match(Op0, m_Add(m_Value(Y), m_ConstantInt(C1))))
  -            Sub = Builder.CreateSub(Builder.CreateNeg(C1), Y, "subc");
  -          if (Sub)
  -            return
  -              BinaryOperator::CreateMul(Sub,
  -                                        ConstantInt::get(Y->getType(), PosVal));
  -        }
  -      }
  -    }
  +  if (match(Op1, m_NegatedPower2())) {
  +    // Interpret  X * (-1<<C)  as  (-X) * (1<<C)  and try to sink the negation.
  +    if (Value *NegOp0 = Negator::Negate(/*IsNegation*/ false, Op0, *this))
  +      return BinaryOperator::CreateMul(
  +          NegOp0, ConstantExpr::getNeg(cast<Constant>(Op1)), I.getName());
     }
   
     if (Instruction *FoldedMul = foldBinOpIntoSelectOrPhi(I))

We miss a similar thing for `sdiv exact %x, -1<<C  -->  -(ashr exact %x, C)`
I can take over this patch if you prefer.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D85446/new/

https://reviews.llvm.org/D85446