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

[vivek pandya via llvm-dev]

On Sat, Mar 4, 2017 at 1:19 PM, Bruce Hoult <bruce at hoult.org> wrote:

> If your target does not have SHL then why don't you simply disable
> converting MUL to SHL?
>
> MUL is converted to SHL by target independent passes when second operand
is power of 2.

-Vivek

>
> 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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>>
>
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