LRA reloads of subregs

[Vladimir Makarov]

On 09/04/2015 09:02 PM, David Miller wrote:
> From: David Miller <davem@davemloft.net>
> Date: Fri, 04 Sep 2015 11:27:31 -0700 (PDT)
>
>> From: Vladimir Makarov <vmakarov@redhat.com>
>> Date: Fri, 4 Sep 2015 10:00:54 -0400
>>
>>> I don't think we should add a new LRA code calling process_address
>>> before adding insns for further processing.  LRA just needs to get
>>> operands from insns to make them valid.  So again I'd try to make insn
>>> recognizable for LRA first and only if it does not work then think
>>> about other solutions in case when such change creates other problems
>>> (it is hard for me to predict LRA behaviour definitely just reading
>>> source files and not knowing sparc port well).
> I've taken some time to see exactly what is going on here, perhaps
> you can give me some guidance, I'm quite happy to implement anything
> :-)

Ok, if modifying constraint/insn definitions does not work, then you 
could try to split the 1st loop in 
lra-constraints.c::curr_insn_transform onto 2 loops: one with processing 
subregs and another one with the rest of the original loop code.  Then 
you need to put the new loop processing subregs after the loop 
processing addresses.  I think it will work.  Of course the change will 
need a lot of testing on other platforms (at least on x86/x86-64).
> We start with:
>
> (insn 1100 1099 1101 14 (set (reg:SI 3376)
>          (ior:SI (subreg:SI (reg:QI 287) 0)
>              (subreg:SI (reg:QI 289) 0))) x.c:104 234 {iorsi3}
>       (expr_list:REG_DEAD (reg:QI 289)
>          (expr_list:REG_DEAD (reg:QI 287)
>              (nil))))
>
> LRA emits, in curr_insn_transform():
>
> 	(set (reg:SI 13423) (subreg:SI (reg:QI 287) 0))
>
> LRA then spills the subreg onto the stack, which gives us:
>
> (insn 18631 1099 1100 14 (set (reg:SI 13423)
>          (subreg:SI (mem/c:QI (plus:SI (reg/f:SI 101 %sfp)
>                      (const_int -14269 [0xffffffffffffc843])) [0 %sfp+-14269 S1 A8]) 0)) x.c:104 63 {*movsi_insn}
>       (expr_list:REG_DEAD (reg:QI 287)
>          (nil)))
>
> And this is where we run into trouble in simplify_operand_subreg(),
> which seems to force reloads for all SUBREGs of MEM.
>
> Normally, if there were no SUBREG here, LRA would run
> process_address() over the MEMs in this instruction and all would be
> well.
>
> It is also the case that I cannot do anything special in the SPARC
> move emitter to handle this, as address validization is disabled when
> lra_in_progress is true.