[llvm-dev] spill to register not stack?

[Quentin Colombet via llvm-dev]

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
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