[LLVMdev] Greedy register allocation

[David A. Greene]

Jakob Stoklund Olesen <stoklund at 2pi.dk> writes:

>> That was my initial reaction.  Splitting should have at least
>> rematerialized the value just before header2.  That should significantly
>> improve things.  This is a classic motivational case for live range
>> splitting.
>
> Well, not really. Note there there are plenty of registers available
> and no spilling is neccessary.

Oh, I misunderstood then.  Thanks for clarifying.

> It's just that an REX prefix is required on some instructions when
> %xmm8 is used. Is it worth it to undo LICM just for that? In this
> case, probably. In general, no.

Ah, so you're saying the regression is due to the inner loop icache
footprint increasing.  Ok, that makes total sense to me.  I agree this
is a difficult thing to get right in a general sort of way.  Perhaps the
CostPerUse (or whatwever heuristics use it) can factor in the loop body
size so that tight loops are favored for smaller encodings.

                               -Dave