[llvm-dev] Marking a register as reserved midway through register allocation

[Bruce Hoult via llvm-dev]

I assumed in that situation all those "spills" with non-overlapping ranges
will have been consolidated into a single stack slot? i.e. "the maximum
deficit of registers vs live values at any point in execution of the
function is one"

But I didn't look at the spilling logic.


On Mon, May 1, 2017 at 4:31 PM, Daniel Neilson <dneilson at azul.com> wrote:

> I think that the “one spill” case might be a little less restrictive than
> that. If you have N spills, but the live ranges for the spills do not
> overlap then you could just spill to Y instead of the stack.
>
> -Daniel
>
>
> On May 1, 2017, at 2:19 AM, Bruce Hoult via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Why not do it the other way around?
>
> Mark Y as reserved, and then at the end of the function if there was
> exactly one spill then put that variable in Y instead.
>
> If there were no spills then you lost nothing by reserving Y. If there
> were more than one then you need Y for this stack adjustment. So exactly
> one spill is the only case where you lost anything by reserving Y.
>
> Which is probably pretty rare, to be honest.
>
> On Mon, May 1, 2017 at 6:27 AM, Dylan McKay via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hey all,
>>
>> I'm attempting to rewrite something that the AVR backend used to support
>> (through an extremely dirty hack that was reverted before the upstreaming
>> of the target).
>>
>> The removal of the hack can be seen on GitHub
>>
>> https://github.com/avr-llvm/llvm/pull/226/files
>>
>> On the AVR architecture, the stack pointer is not in a standard register,
>> but rather it is a special register which can only be accessed via I/O
>> instructions. In order to implement stack spilling/restoring we need to
>> adjust the stack pointer, which is a problem because we need the value of
>> SP to be in a general purpose register for us to perform arithmetic on it.
>>
>> In AVR-GCC, a frame pointer (the 'Y' register) is reserved if there are
>> any spills in a method. This works fine because we can easily perform
>> arithmetic on it and then save it into the real stack pointer via IO
>> instructions.
>>
>> I would like to do the same for LLVM. This is tricky however, because we
>> don't know if we need a frame pointer until we've finished register
>> allocation and we've seen a spill.
>>
>> In the event that we do see a spill, it is likely that the 'Y' register
>> has already been allocated to some other variable, and so we somehow need
>> to deallocate it and then rewind the regalloc. That is what the old hack
>> above does.
>>
>> Is there any way today I can deallocate a register and rewind allocation
>> upon spilling?
>>
>>
>>
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>>
>>
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