[LLVMdev] Register coalescer and reg_sequence (virtual super-regs)

[Jakob Stoklund Olesen]

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