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

[Adam Husar via llvm-dev]

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
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>
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-- 
Adam Husar
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