llvm dev - May 2013 - [LLVMdev] Register coalescer and reg_sequence (virtual super-regs)

The register coalescer treats virtual super register classes -- a
sequential register range composed of multiple hardware registers -- as a
register with sub registers. When making coalescing decisions it thinks
that the virtual super reg interferes with sub reg instances, even though
in reality they shouldn't conflict. That is, they are individual registers
and would be better compared as such for register coalescing decisions
(CoalescerPair::Partial = 0).

For example, I have a super reg that has r20, r21, r22, and r23 physical
registers. This super reg is the dest of a reg_sequence which generates 4
COPY MIs. The first COPY coalesces (merging into r20), but the vregs for
r21-r23 (SUPER_RC:%vreg50:subreg1..subreg3) are never coalesced after that
because doing so generates inteference on %vreg50, the "parent" super
reg.

Is there a way to work around this? It causes unnecessary copies.

Thanks,
Joe
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On May 31, 2013, at 4:07 PM, Joe Matarazzo <joe.matarazzo at gmail.com>
wrote:
> The register coalescer treats virtual super register classes -- a
sequential register range composed of multiple hardware registers -- as a
register with sub registers. When making coalescing decisions it thinks that the
virtual super reg interferes with sub reg instances, even though in reality they
shouldn't conflict. That is, they are individual registers and would be
better compared as such for register coalescing decisions
(CoalescerPair::Partial = 0).
> 
> For example, I have a super reg that has r20, r21, r22, and r23 physical
registers. This super reg is the dest of a reg_sequence which generates 4 COPY
MIs. The first COPY coalesces (merging into r20), but the vregs for r21-r23
(SUPER_RC:%vreg50:subreg1..subreg3) are never coalesced after that because doing
so generates inteference on %vreg50, the "parent" super reg.
> 
> Is there a way to work around this? It causes unnecessary copies.
Is this happening on trunk, or are you using an old version of LLVM?

/jakob

I think the last time I pulled from trunk was probably end of last year.
Some time ago. Does your reply intimate it's fixed on trunk? That would be
great. (I don't sync too often to avoid churn with my TD.)

Joe


On Fri, May 31, 2013 at 4:21 PM, Jakob Stoklund Olesen <stoklund at
2pi.dk>wrote:
>
> On May 31, 2013, at 4:07 PM, Joe Matarazzo <joe.matarazzo at
gmail.com>
> wrote:
>
> > The register coalescer treats virtual super register classes -- a
> sequential register range composed of multiple hardware registers -- as a
> register with sub registers. When making coalescing decisions it thinks
> that the virtual super reg interferes with sub reg instances, even though
> in reality they shouldn't conflict. That is, they are individual
registers
> and would be better compared as such for register coalescing decisions
> (CoalescerPair::Partial = 0).
> >
> > For example, I have a super reg that has r20, r21, r22, and r23
physical
> registers. This super reg is the dest of a reg_sequence which generates 4
> COPY MIs. The first COPY coalesces (merging into r20), but the vregs for
> r21-r23 (SUPER_RC:%vreg50:subreg1..subreg3) are never coalesced after that
> because doing so generates inteference on %vreg50, the "parent"
super reg.
> >
> > Is there a way to work around this? It causes unnecessary copies.
>
> Is this happening on trunk, or are you using an old version of LLVM?
>
> /jakob
>
>
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