Re: [PATCH] dse: Handle SUBREGs of word REGs differently for WORD_REGISTER_OPERATIONS targets [PR109040]

[Jeff Law <jeffreyalaw@gmail.com>]

On 4/6/23 04:15, Eric Botcazou wrote:
>> Originally I didn't really see this as an operation.  But the more and
>> more I ponder it feels like it's an operation and thus should be subject
>> to WORD_REGISTER_OPERATIONS.
>>
>> While it's not really binding on RTL semantics, if we look at how some
>> architectures implement reg->reg copies, they're actually implemented
>> with an ADD or IOR -- so a real operation under the hood.
>>
>> If we accept a subreg copy as an operation and thus subject to
>> WORD_REGISTER_OPERATIONS then that would seem to imply the combine is
>> the real problem here.  Otherwise dse is the culprit.
> 
> Yes, I agree that there is an ambiguity for subreg copy operations.  At some
> point I tried to define what register operations are and added a predicate to
> that effect (word_register_operation_p ); while it returns true for SUBREG,
> it's an opt-out predicate so this does not mean much.
Yea, I saw word_register_operation_p.  I was hesitant to treat it as a 
canonical definition of what ops are and are not subject to 
WORD_REGISTER_OPERATIONS.

> 
> I don't think that DSE does anything wrong: as I wrote in the PR, defining
> WORD_REGISTER_OPERATIONS should not prevent any particular form of RTL.
That was the conclusion I'd come to, predicated on treating SUBREGs as 
affected by WORD_REGISTER_OPERATIONS.

> 
> I therefore think that the problem is in the combiner and probably in the
> intermediate step shown by Jakub:
> 
> "Then after that try_combine we do:
> 13325           record_value_for_reg (dest, record_dead_insn,
> 13326                                 WORD_REGISTER_OPERATIONS
> 13327                                 && word_register_operation_p (SET_SRC
> (setter))
> 13328                                 && paradoxical_subreg_p (SET_DEST
> (setter))
> 13329                                 ? SET_SRC (setter)
> 13330                                 : gen_lowpart (GET_MODE (dest),
> 13331                                                SET_SRC (setter)));
> and the 3 conditions are true here and so record value of the whole setter.
> That then records among other things nonzero_bits as 0x8084c."
> 
> That's a recent addition of mine (ae20d760b1ed69f631c3bf9351bf7e5005d52297)
> and I think that it probably abuses WORD_REGISTER_OPERATIONS and should either
> be reverted or restricted to the load case documented in its comment.  I can
> provide testing on SPARC if need be.
I think that's the job for today.  Pan2, Jakub and myself have all 
zero'd in on this code in combine.

jeff