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

[Bruce Hoult via llvm-dev]

So then just generate assembly code for SHL as a MUL...

On Sat, Mar 4, 2017 at 10:59 AM, vivek pandya <vivekvpandya at gmail.com>
wrote:

>
>
> 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
>>>>>
>>>>>
>>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>
> --
> This message has been scanned for viruses and
> dangerous content by *MailScanner* <http://www.mailscanner.info/>, and is
> believed to be clean.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170304/1463ff9b/attachment.html>