Hendrik Greving <hendrik.greving.in...@gmail.com> writes:
> For a special mechanism we are generating jump_insn with a 'set' side
> effect in our backend. RTL looks e.g. like this:
>
> (jump_insn 25 24 26 (nil) (parallel [
> (set (pc)
> (if_then_else (ne (unspec_volatile [
> (const_string ("<myinsn> %0,[%1] =%2"))
> (const_int 0 [0x0])
> (reg:SI 348)
> ] 21)
> (const_int 0 [0x0]))
> (label_ref:SI 43)
> (pc)))
> (clobber (reg:SI 321 link))
> (set (reg/v:QI 346)
> (unspec:QI [
> (const_int 0 [0x0])
> ] 0))
> ]) -1 (nil)
> (nil))
>
> After working out some issues initially, this all works fine and the
> 'set' seems to be properly recognized by RTL optimization phases (e.g.
> CSE). I am now running into an issue however with reload. The problem
> seems to be that if the parallel 'set' from e.g. RTL above feeds into
> a (mem (reg)). This happens when e.g. compiling in debug mode, most
> variables are at memory locations of the stack. In this case, compiler
> needs to reload the instruction above (I am actually not sure why, but
> I guess this could happen all the time). The further problem seems to
> be that there is a hard constraint in reload that jump_insn can't have
> output operands / reloads.
>
> if (GET_CODE (insn) == JUMP_INSN || GET_CODE (insn) == CALL_INSN)
> no_output_reloads = 1;
>
> so what I am doing is, when generating RTL above, I always 'reload'
> myself in the backend, generating mov's to/from memory, and always
> making sure only hard registers are put into RTL above. The problem is
> that I need hard registers for that. I would strongly prefer using
> pseudo's, but pseudo's also seem to require reload.
This might be what you mean by doing reload yourself, but the usual
way of handling this is to add a memory alternative to the pattern
and split that alternative after reload. If the split requires
a temporary register, you can allocate one by adding a
(clobber (match_scratch ...)) to the insn pattern. The scratch can
be "X" for the normal register case that doesn't need a temporary.
See the *ctr<mode>_internal1 pattern in config/rs6000/rs6000.md and
the doloop_si64 pattern in config/s390/s390.md for examples.
Thanks,
Richard
