[llvm-dev] Why ISel Shifts operations can only be expanded for Value type vector ?

[vivek pandya via llvm-dev]

On Saturday, March 4, 2017, Ryan Taylor <ryta1203 at gmail.com> wrote:

> Why you can't still expand it through MUL with a Custom lowering? Or am I
> missing something?
>
> Yes we can but problem occurs when we know that it is shift with constant
value than if we return ISD::MUL with constant imm operand than LLVM will
convert it to SHL again because the constant will be power of 2. Thus it
creates loop.
So we may add target specific ISD node and lower it to mul instruction.

--Vivek

> Thanks.
>
> On Fri, Mar 3, 2017 at 12:21 PM, vivek pandya via llvm-dev <
> llvm-dev at lists.llvm.org
> <javascript:_e(%7B%7D,'cvml','llvm-dev at lists.llvm.org');>> wrote:
>
>> Hello LLVM Devs,
>>
>> I am working on a target on which no SHL instruction is available. So
>> wanted to expand it through MUL. But currently it is only possible to
>> expand SHL for vector types.
>>
>> One possible reason I can think is because LLVM tries to optimize MUL to
>> SHL in certain cases and that can make compiler co in loop or may end up
>> generating wrong code.
>>
>> But I think SHL should be able to expanded to MUL and to prevent looping
>> between MUL and SHL we can put a condition that only optimize MUL to SHL
>> when SHL is not expanded operation. The similar logic can be applied to DIV
>> and SRA.
>>
>> If there is any other reasons for not doing this, kindly explain.
>>
>> Sincerely,
>> Vivek
>>
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>>
>>
>
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