Re: [RFC][PATCH][MIPS] Patch to enable LRA for MIPS backend

[Vladimir Makarov <vmakarov@redhat.com>]

On 2014-04-21, 8:23 AM, Richard Sandiford wrote:
> Robert Suchanek <Robert.Suchanek@imgtec.com> writes:
>>>>> Did you see the failures even after your mips_regno_mode_ok_for_base_p
>>>>> change?  LRA should know how to reload a "W" address.
>>>>
>>>> Yes but I realize there is more. It fails because $sp is now included
>>>> in BASE_REG_CLASS and "W" is based on it. However, I suppose that
>>>> it would be too eager to say it is wrong and likely there is
>>>> something missing
>>>> in LRA if we want to keep all alternatives. Currently there is no check
>>>> if a reloaded operand has a valid address, use of $sp in lbu/lhu cases.
>>>> Even if we added extra checks we are less likely to benefit as we need
>>>> to reload the base into register.
>>>
>>> Not sure what you mean, sorry.  "W" exists specifically to exclude
>>> $sp-based and $pc-based addresses.  LRA AFAIK should already be able
>>> to reload addresses that are valid in the TARGET_LEGITIMATE_ADDRESS_P
>>> sense but which do not match the constraints for a particular insn.
>>>
>>> Can you remember one of the tests that fails?
>>
>> I couldn't trigger the problem with the original testcase but found
>> another one that reveals it. The following needs to compiled with
>> -mips32r2 -mips16 -Os:
>>
>> struct { int addr; } c;
>> struct command { int args[1]; };
>> unsigned short a;
>>
>> fn1 (struct command *p1)
>> {
>>      unsigned short d;
>>      d = fn2 ();
>>      a = p1->args[0];
>>      fn3 (a);
>>      if (c.addr)
>>      {
>>          fn4 (p1->args[0]);
>>          return;
>>      }
>>      (&c)->addr = fn5 ();
>>      fn6 (d);
>> }
>
> Thanks.
>
>> Not sure how the constraint would/should exclude $sp-based address in
>> LRA.  In this particular case, a spilled pseudo is changed to memory
>> giving the following RTL form:
>>
>> (insn 30 29 31 4 (set (reg:SI 4 $4)
>>          (and:SI (mem/c:SI (plus:SI (reg/f:SI 78 $frame)
>>                      (const_int 16 [0x10])) [7 %sfp+16 S4 A32])
>>              (const_int 65535 [0xffff]))) shell.i:17 161 {*andsi3_mips16}
>>       (expr_list:REG_DEAD (reg:SI 194 [ D.1469 ])
>>          (nil)))
>>
>> The operand 1 during alternative selection is not marked as a bad
>> operand as it is a memory operand. $frame appears to be fine as it
>> could be eliminated later to hard register. No reloads are inserted
>> for the instructions concerned. Unless, $frame should be temporarily
>> eliminated and then a reload would be inserted?
>
> Yeah, I think the lack of elimination is the problem.  process_address
> eliminates $frame temporarily before checking whether the address
> is valid, but the places that check EXTRA_CONSTRAINT_STR pass the
> original uneliminated address.  So the legitimate_address_p hook sees
> the $sp-based address but the "W" constraint only sees the $frame-based
> address (which might or might not be valid, depending on whether $frame
> is eliminated to the stack or hard frame pointer).  I think the constraints
> should see the eliminated address too.
>
> This patch seems to fix it for me.  Tested on x86_64-linux-gnu.
> Vlad, is this OK for trunk?
>
> BTW, we might want to define something like:
>
> #define MODE_BASE_REG_CLASS(MODE) \
>    (TARGET_MIPS16 \
>     ? ((MODE) == SImode || (MODE) == DImode ? M16_SP_REGS : M16_REGS) \
>     : GR_REGS)
>
> instead of BASE_REG_CLASS.  It might lead to slightly better code
> (or not -- if it doesn't then don't bother :-)).
>
> If this patch is OK then I think the only thing blocking the switch
> to LRA is the asm-subreg-1.c failure.  I think it'd be fine to XFAIL
> that test on MIPS for now, until there's a consensus about what "X" means
> for asms.
>
>
> gcc/
> 	* lra-constraints.c (valid_address_p): Move earlier in file.
> 	Add a constraint argument to the address_info version.
> 	(satisfies_memory_constraint_p): New function.
> 	(satisfies_address_constraint_p): Likewise.
> 	(process_alt_operands, curr_insn_transform): Use them.
> 	(process_address): Pass the constraint to valid_address_p when
> 	checking address operands.
>
>

Yes, it looks ok for me, Richard.  Thanks on working on this.

I am on vacation till May 4th. If the patch results in problems on other 
targets, I hope you revert it.  But to be honest, I believe it is very 
safe and don't expect any problems at all.