[llvm-dev] Disable spilling sub-registers in LLVM

[ahmede via llvm-dev]

Hi Quentin,

Let me clarify if I understood this correctly.

If the accesses (writes and reads) to sub-registers are expressed always 
as sub-registers of the super-register register class (e.g., 
SuperReg.sub1;), then the spilling decision is for the super register.

But, if the accesses are in terms of the register class of the 
sub-registers directly (SubReg;), then the spilling decision will be for 
the sub-register.


So if we forced before register allocation all sub-register accesses to 
be accessed to "lanes" of the super register class then we guarantee 
that spilling is for the Super Registers only?

Thanks,
Ahmed


On 2018-01-30 00:22, Quentin Colombet wrote:
> Hi Ahmed,
> 
> If you access your values with sub-registers indices, IIRC the inline
> spiller will spill the super register.
> If you access your values directly (via sub-regclass), then the
> spiller uses this class.
> 
> Basically what I am saying is the spiller spills the value that
> contains the accesses.
> 
> E.g.,
> = v; will spill v
> = v.sub1; will spill v too, but v is a super register in that case.
> 
> Cheers,
> -Quentin
> 
>> On Jan 29, 2018, at 6:38 PM, ahmede via llvm-dev 
>> <llvm-dev at lists.llvm.org> wrote:
>> 
>> Hi Matthias,
>> 
>> No. I want the register allocator to spill the super-register (the 
>> large one e.g., 64-bit) and not just the sub-register (e.g., the 
>> 32-bit that is a piece of of the 64-bit register) because the stack 
>> loads/store width is 64-bit in this example.
>> 
>> RegClass1   (sub-registers):  sub_registers (32-bit)               --> 
>> can be natively used in arithmetic operations but no stack 
>> loads/stores for that width.
>> 
>> RegClass2 (super-registers):  [sub_register, subregister] (64-bit)  
>> --> can be natively used in arithmetic operations and can be used in 
>> loads/stores.
>> 
>> 
>> Thanks,
>> Ahmed
>> 
>> 
>> On 2018-01-29 20:20, Matthias Braun wrote:
>>>> On Jan 29, 2018, at 1:20 PM, ahmede via llvm-dev 
>>>> <llvm-dev at lists.llvm.org> wrote:
>>>> Hi,
>>>> I wonder if there is a way in LLVM to disable spilling a 
>>>> register-class while still enabling the super-registers of this 
>>>> register-class to be spilled.
>>> What would you have the register allocator do when it runs out of
>>> register and you have spilling disabled? Abort the compilation?
>>> If you just want a special instruction sequence (like using a bigger
>>> loads/stores for the spills) then you should be able to implement 
>>> that
>>> in storeRegToStackSlot()/loadRegFromStackSlot().
>>> - Matthias
>>>> If not, how can we implement spilling for sub-registers when stack 
>>>> load/stores can only operate on the super registers? Is there a way 
>>>> even if it is suboptimal?
>>>> Thanks,
>>>> Ahmed
>>>> _______________________________________________
>>>> 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