Reload with jump_insn that have a 'set' side effect.

Hendrik Greving hendrik.greving.intel@gmail.com
Tue Sep 17 20:44:00 GMT 2013


One follow-up question here. Is it of any disadvantage to always
generate a clobber (scratch) during expansion? Meaning that in the
case we don't need one (we won't split, which I am checking with
memory_operand(myoutput_operand), that register won't get allocated
and adds to register pressure, does it?
Thanks

On Tue, Sep 17, 2013 at 10:33 AM, Richard Sandiford
<rdsandiford@googlemail.com> wrote:
> Hendrik Greving <hendrik.greving.intel@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



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