[llvm-dev] spill to register not stack?

[Bagel via llvm-dev]

Thank you Nemanja and Quentin!  I did the following:

1. Defined a register class AC that is the union of A and C.

2. In *RegisterInfo, implement getLargestLegalSuperClass, which when queried 
for C returns AC.

3. In *InstrInfo, teach copyPhysReg how to copy between registers in A and C.

And voila, spills to register work!

Maybe there should be a FAQ for implementers of backends and this should be in it.

Thanks again,
brian


On 9/9/20 1:48 PM, Quentin Colombet wrote:
> Hi Brian,
> 
> +1 on what Nemanja said: specifying large register classes is the key.
> 
> More details here:
> http://lists.llvm.org/pipermail/llvm-dev/2019-December/137700.html
> 
> Cheers,
> -Quentin
> 
>> On Sep 9, 2020, at 11:13 AM, Nemanja Ivanovic via llvm-dev 
>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> On PowerPC, we implemented support for spilling GPR registers to vector 
>> registers. Essentially, it requires specifying the correct register class in 
>> `getLargestLegalSuperClass()` as well as 
>> `storeRegToStackSlot()/loadRegFromStackSlot()`. You can see what is 
>> controlled with the `EnableGPRToVecSpills` option.
>>
>> If you also want to use this method for spilling callee-saved registers to 
>> other registers, you can do what we do on PPC in 
>> `assignCalleeSavedSpillSlots()` to specify the destination register. This 
>> should also ensure that the CFI directives are emitted correctly. Keep in 
>> mind that this can likely only be done for leaf functions since you'll likely 
>> be spilling these to caller-saved registers of the other class.
>>
>> Hope this helps,
>> Nemanja
>>
>>
>> On Wed, Sep 9, 2020 at 1:30 PM Bagel via llvm-dev <llvm-dev at lists.llvm.org 
>> <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>>     Given an architecture with two classes of registers: A is general purpose
>>     and
>>     has an "adequate" number of registers, and C which is special purpose and
>>     has
>>     very few (e.g. one) register.  There are cheap instructions that directly
>>     copy
>>     from C to A and vice versa.
>>
>>     If we need another C register and they are all live, we need to spill one.
>>     Currently as far as I can tell, the only way to do that is spill to the
>>     stack.
>>     Is there a knob I can turn to get the spill from C to go to an A register?
>>
>>     Thanks,
>>     brian
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