Re: [PATCH] emit-rtl.c: Allow splitting of RTX_FRAME_RELATED_P insns?

[Segher Boessenkool <segher@kernel.crashing.org>]

On Mon, Aug 10, 2020 at 05:16:15PM +0100, Richard Sandiford wrote:
> Senthil Kumar via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> >   The wiki suggests using post-reload splitters, so that's the
> >   direction I took, but I ran into an issue where split_insn
> >   bails out early if RTX_FRAME_RELATED_P is true - this means
> >   that splits for REG_CC clobbering insns with
> >   RTX_FRAME_RELATED_P will never execute, resulting in a
> >   could-not-split insn ICE in the final stage.
> >
> >   I see that the recog.c:peep2_attempt allows splitting of a
> >   RTX_FRAME_RELATED_P insn, provided the result of the split is a
> >   single insn. Would it be ok to modify try_split also to
> >   allow those kinds of insns (tentative patch attached, code
> >   copied over from peep2_attempt, only setting old and new_insn)? Or is there
> >   a different approach to fix this?
> 
> I agree there's no obvious reason why splitting to a single insn
> should be rejected but a peephole2 to a single instruction should be OK.
> And reusing the existing, tried-and-tested code is the way to go.

The only obvious difference is that the splitters run many times, while
peep2 runs only once, very late.  If you make this only do stuff for
reload_completed splitters, that difference is gone as well.

> But could you split the code out of peep2_attempt into a subroutine
> (probably still in recog.c) and reuse it in try_split?

Yes please :-)


Segher