llvm dev - Sep 2020 - spill to register not stack?

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

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>
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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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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
<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> 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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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