llvm dev - Mar 2017 - Why ISel Shifts operations can only be expanded for Value type vector ?

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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On 3/3/2017 9:21 AM, vivek pandya via llvm-dev 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.
There isn't anything fundamental; it just hasn't come up for any in-tree
target.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project

Why you can't still expand it through MUL with a Custom lowering? Or am I
missing something?

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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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
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> <javascript:_e(%7B%7D,'cvml','llvm-dev at
lists.llvm.org');>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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