[llvm-dev] [llvm dev] do we have allocator hook to use maximum different registers?

[Konstantin Vladimirov via llvm-dev]

Hi,

Adam,

Can you please be more verbose on how pre-RA scheduling may help in
this case? How should I schedule instructions with virtual registers
in order to instruct allocator to use maximum physical ones?

---
With best regards, Konstantin

On Mon, May 16, 2016 at 12:56 PM, Adam Husar <husar at codasip.com> wrote:
> Hello Konstantin,
>   we approach this issue by using pre-RA scheduling and it works fine,
> although for very large basic blocks, the effect is not so good, because it
> leads to more spilling
> comparted to the case when a pre-RA schedule is not used.
>
> I also remember that there is some interface for schedulers
> to do the register renaming, but as far as I know, we haven't used it yet.
>
> Best regards
>   Adam
>
>
> On Tue, 10 May 2016 22:41:01 +0200, Quentin Colombet via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>
>> Hi Konstantin,
>>
>>
>>> On May 10, 2016, at 1:29 PM, Konstantin Vladimirov via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>>
>>> Hi,
>>>
>>> Default register allocator tries to reuse the same registers over and
>>> over again even if register file have a plenty of registers to use.
>>> This creates parasite false dependencies and makes scheduling less
>>> effective.
>>>
>>> How to instruct allocator (may be override some virtual function in
>>> mine backend?) that it is profitable to use maximum number of
>>> available registers with minimal dependencies?
>>
>>
>> That is not possible and in the current scheme that would be hard to add.
>> Indeed, the greedy reg alloc assigns live-ranges in an order based on some
>> priority cost model. Thus, we do not know how close, in the scheduling
>> order, two live-ranges are when we assign them in a row.
>>
>> Cheers,
>> -Quentin
>>
>>> In GCC this option is
>>> known as -frename-registers.
>>
>>
>>>
>>> ---
>>> With best regards, Konstantin
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
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>
>
> --
> Adam Husar
> Compiler Group Leader
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