[llvm-dev] "Earlyclobber" but for a subset of the inputs

[Roger Ferrer Ibáñez via llvm-dev]

Hi Quentin,

Thanks! I was worried I might be missing something very obvious. I'll look
into your suggestions.

Kind regards,

Missatge de Quentin Colombet <qcolombet at apple.com> del dia dt., 5 de maig
2020 a les 19:59:

> Hi Roger,
>
> On May 4, 2020, at 11:37 PM, Roger Ferrer Ibáñez <rofirrim at gmail.com>
> wrote:
>
> Hi Quentin,
>
>
>> It sounds like you only need the earlyclobber description for the N, N
>> variant.
>> In other words, as long as you use different opcodes for widen-op NN and
>> widen-op WN, you model exactly what you want.
>>
>> What am I missing?
>>
>
> we are using different opcodes for widen-op NN and widen-op WN.
>
> My understanding is that not setting earlyclobber to the W, N variant
> would allow the RegAlloc to do an allocation like this
>
> W1 = widen-op W2, N3
>
>
> Sorry I mixed up earlyclobber and tie-operand. For some reason I thought
> earlyclobber could be set individually on the src operands to say that they
> interfere with the definition...
> That’s obviously wrong.
>
> So yeah there isn’t anything in LLVM right now that conveys the semantic
> you want.
>
> If you’re really concerned that you would use too many registers, you can
> either:
> 1. Add this concept to llvm
> 2. Repair after regalloc
>
> For #2, basically you don’t set any constraints in regalloc then after
> regalloc (i.e., expand pseudo), you check if there are some overlapping and
> if so, you change the allocation locally.
> E.g.,
> W1 = W2, N3
> =>
> N4 = copy N3
> W1 = copy W2, N4
> or
> W3 = copy W2, N3
> W1 = copy W3
>
> Depending on what is the cheapest with respect to what registers are
> available (you may have to spill).
>
> Cheers,
> -Quentin
>
>
> but this is not correct in that target because W1 and N3 are of different
> kind and W1 (being the group of registers N2, N3) overlaps N3.
>
> If I understand earlyclobber semantics correctly, earlyclobber would
> allocate the destination to something that doesn't overlap W2 and also
> doesn't overlap N3. For instance
>
> W3 = widen-op W2, N3
>
> But because the dest is a W register the target's constraint only applies
> to N3, not to W2, so the following should be OK (however RegAlloc would
> never make such an assignment under earlyclobber)
>
> W2 = widen-op W2, N3
>
> In principle earlyclobber is always going to do allocations that are
> correct for the target but there are a valid ones that will be missed.
>
> Kind regards,
>
> --
> Roger Ferrer Ibáñez
>
>
>

-- 
Roger Ferrer Ibáñez
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