[llvm-dev] spill to register not stack?

[Nemanja Ivanovic via llvm-dev]

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
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