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

[Craig Topper via llvm-dev]

And that is less instructions. So from InstCombine's perspective the
multiply is the correct answer. I think this transformation is better left
to codegen where we know whether multiply or shift is truly better.

~Craig

On Wed, Sep 13, 2017 at 10:18 AM, Craig Topper <craig.topper at gmail.com>
wrote:

> There is in fact a transform out there somewhere that reverses yours.
>
> define i64 @foo(i64 %a) {
>   %b = shl i64 %a, 5
>   %c = add i64 %b, %a
>   ret i64 %c
> }
>
> becomes
>
> define i64 @foo(i64 %a) {
>
>   %c = mul i64 %a, 33
>
>   ret i64 %c
>
> }
>
> ~Craig
>
> On Wed, Sep 13, 2017 at 10:11 AM, Craig Topper <craig.topper at gmail.com>
> wrote:
>
>> 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
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170913/70f3bc07/attachment.html>