[llvm-dev] Disabling DAGCombine's specific optimization

[vivek pandya via llvm-dev]

On Mon, May 15, 2017 at 1:43 PM, Matt Arsenault <arsenm2 at gmail.com> wrote:

>
> On May 15, 2017, at 07:54, vivek pandya via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hello LLVM Developers,
>
> I am working on an architecture which have one bit shift operation if
> barrel shiftier hardware is not present in such cases some DAGCombine
> optimizations reduces performance of certain benchmarks upto 5% for example
> consider follwing optimization:
> fold (select_cc seteq (and x, y), 0, 0, A) -> (and (shr (shl x)) A)
> Here it introduce 2 shift operations and when barrel shiftier is not
> present these shifts will be converted to loops thus making it worst. I am
> sure there few architectures which have similar features. So how to disable
> these kind of optimizations in DAGCombine particularly when I want build
> and use LLVM for more than one target ?
>
> Sincerely,
> Vivek
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>
> Sounds to me like this combine is missing a TLI.isOperationLegal(ISD::LSHR)
> or similar check
>
I think for me, problem will still be there because shift operation is
legal on my target but it is able to shift only 1 bit at a time and that's
why I need to convert to loop.
-Vivek

>
> -Matt
>
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