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

[Bruce Hoult via llvm-dev]

If your target does not have SHL then why don't you simply disable
converting MUL to SHL?


On Sat, Mar 4, 2017 at 8:22 AM, vivek pandya via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>
> 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> 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
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
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