Re: LRA Stuck in a loop until aborting

[pshortis at dataworx dot com dot au]

On 17.04.2014 13:00, Jeff Law wrote:
> On 04/16/14 16:19, Richard Henderson wrote:
> > The register allocators only select an alternative for a move.  They 
> > do not
> > choose between N different patterns, separately describing loads, 
> > stores, and
> > register-to-register movement.
> >
> > I'm fairly sure the documentation is quite clear on this, and GCC had 
> > required
> > this since the beginning of time.
> Correct on all counts; many an hour was spent reimplementing the PA
> movXX patterns to satisfy that requirement.
>
> jeff
I'm convinced :-) but...

gcc internals info about movm is fairly comprehensive and I had taken 
care to ensure that I satisfied ...

"The constraints on a âmovmâ must permit moving any hard register to 
any other hard register provided..."

by providing a define_expand that assigns from a general_operand to a 
nonimmediate_operand and ...

*ldsi instruction that can load from a general_operand to a 
nonimmediate_operand
and a
*storesi instruction that can store a register_operand to a 
memory_operand

In any case, out of curiosity and to convice myself I hadn't imagined 
the old reload pass handling this I reverted my recent fixes so that 
ldsi and storesi were once again as described above then repeated the 
exercise with full rtl dumping on and compared the rtl generated both 
with and without LRA enabled.

In both cases the *.ira dmp produced the triggering ...

(insn 57 61 58 5 (set (reg/v:SI 46 [orig:31 s ] [31])
        (reg/v:SI 31 [ s ])) 48 {*ldsi}
     (expr_list:REG_DEAD (reg/v:SI 31 [ s ])
        (nil)))

The non-LRA reload rtl produced ..

(insn 57 61 67 3 (set (reg:SI 1 r1)
        (mem/c:SI (plus:HI (reg/f:HI 3 r3)
                (const_int 4 [0x4])) [4 %sfp+4 S4 A16])) 48 {*ldsi}
     (nil))
(insn 67 57 58 3 (set (mem/c:SI (plus:HI (reg/f:HI 3 r3)
                (const_int 4 [0x4])) [4 %sfp+4 S4 A16])
        (reg:SI 1 r1)) 47 {*storesi}
     (nil))

While the LRA just got stuck in a loop unable to perform the reload of 
insn 57 that the old reload pass handled (or more correctly didn't choke 
over - it seems to be a redundant load/store).

I'm really just highlighting this because I know the LRA is quite young 
and this might be a hint towards a deeper/other issues.

Paul.