Re: [PATCH] In the ready lists of pipeline, put unrecog insns (such as CLOBBER, USE) at the latest to issue.

[Jeff Law <jeffreyalaw@gmail.com>]

On 3/27/23 11:01, Richard Sandiford wrote:
> Jin Ma via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>>    Unrecog insns (such as CLOBBER, USE) does not represent real instructions, but in the
>> process of pipeline optimization, they will wait for transmission in ready list like
>> other insns, without considering resource conflicts and cycles. This results in a
>> multi-issue CPU architecture that can be issued at any time if other regular insns
>> have resource conflicts or cannot be launched for other reasons. As a result, its
>> position is advanced in the generated insns sequence, which will affect register
>> allocation and often lead to more redundant mov instructions.
> 
> Is it the clobber rather than the use case that is causing problems?
> I would expect that scheduling a use ASAP would be better for register
> pressure, since it might close off the associated live range and so
> reduce the number of conflicts.
Agreed.  Issuing USES as soon as possible seems advisable from a 
register pressure standpoint.

A clobber can close off a range as well, but I suspect that is the 
exception rather than the norm.

> 
> I.e. is the problem that, when a live range starts with a clobber,
> the current code will tend to move the clobber up and so extend
> the associated live range?  If so, that sounds like something we
> should address more directly, for two reasons:
Agreed as well.  I would expect the normal case for clobbers is that 
deferring them as late as possible is best as I would expect they 
typically open a live range.

> 
> (1) We should try to prevent clobbers that start a live range from being
>      moved up even if first_cycle_insn_p.
Yes.

> 
> (2) Clobbers can also be used to close off a live range, which is useful
>      if a pseudo is only written to in parts.  The current behaviour is
>      probably better for those clobbers.
I thought these sequences started with a clobber, then the component 
sets.  In which case the clobber isn't closing a live range, but opening 
one and deferring it is advisable.

jeff