[PATCH][ARM] PR target/71436: Restrict *load_multiple pattern till after LRA

[Kyrill Tkachov]

On 19/12/16 14:53, Jakub Jelinek wrote:
> On Thu, Dec 15, 2016 at 10:00:14AM +0000, Richard Earnshaw (lists) wrote:
>> sorry, pasted the wrong bit of code.
>>
>> That should read when we generate:
>>
>> (insn 55 19 67 3 (parallel [
>>              (set (reg:SI 0 r0)
>>                  (mem/u/c:SI (reg/f:SI 147) [2 c+0 S4 A32]))
>>              (set (reg:SI 158)
>>                  (mem/u/c:SI (plus:SI (reg/f:SI 147)
>>                          (const_int 4 [0x4])) [2 c+4 S4 A32]))
>>          ]) "ldm.c":25 404 {*load_multiple}
>>       (expr_list:REG_UNUSED (reg:SI 0 r0)
>>          (nil)))
>>
>> ie when we put a pseudo into the register load list.
> We put a pseudo there because the predicate on the insn allows it:
> (define_special_predicate "load_multiple_operation"
>    (match_code "parallel")
> {
>   return ldm_stm_operation_p (op, /*load=*/true, SImode,
>                                   /*consecutive=*/false,
>                                   /*return_pc=*/false);
> })
> and the consecutive = false argument says that (almost) no verification
> is performed on the SET_DEST, just that it is a REG and doesn't have
> REGNO smaller than the first reg.
> That said, RA is still not able to cope with such instructions, because
> only the first set is represented with constraints, so if such an insn
> needs any kind of reloading, it just will not happen.
> So I think the posted patch makes lots of sense, otherwise if you use
> such a pattern before reload, you just have to hope no reloading will be
> needed on it.

In that case I believe restricting the pattern till after LRA is the way to go.
However, we should still add the check from [1] to ldm_stm_operation_p for when
'consecutive' is true as consecutive order has no useful meaning on pseudos here.

Thanks,
Kyrill

[1] https://gcc.gnu.org/ml/gcc-patches/2016-12/msg01400.html

>
> 	Jakub