[llvm-dev] How to add optimizations to InstCombine correctly?

[Craig Topper via llvm-dev]

Your code seems fine. InstCombine can infinite loop if some other transform
is reversing your transform. Can you send the whole patch and a test case?

~Craig

On Wed, Sep 13, 2017 at 10:01 AM, Haidl, Michael via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> I am working on PR34474 and try to add a new optimization to
> InstCombine. Like in other parts of the visitMul function I add a Shl
> through the IR builder and create a new BinaryOp which I return from
> visitMul. If I understand correctly the new BinaryOp returned from
> visitMul should replace the original Instruction in the Worklist.
> However, I end up in an infinite loop and the Instruction I try to
> replace gets scheduled again and again. What is wrong in my code?
>
> // Replace X * (2^C+/-1) with (X << C) -/+ X
> APInt Plus1 = *IVal + 1;
> APInt Minus1 = *IVal - 1;
> int isPow2 = Plus1.isPowerOf2() ? 1 : Minus1.isPowerOf2() ? -1 : 0;
>
> if (isPow2) {
>    APInt &Pow2 = isPow2 > 0 ? Plus1 : Minus1;
>    Value *Shl = Builder.CreateShl(Op0, Pow2.logBase2());
>    return BinaryOperator::Create(isPow2 > 0 ? BinaryOperator::Sub :
> BinaryOperator::Add, Shl, Op0);
> }
>
> Thanks,
> Michael
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