Re: [PATCH] In the pipeline, UNRECOG INSN is not executed in advance if it starts a live range.

[Jin Ma]

> On 5/29/23 04:51, Jin Ma wrote:
> >    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.
> > 
> > A simple example:
> > https://github.com/majin2020/gcc-test/blob/master/test.c
> > This is a function in the dhrystone benchmark.
> > 
> > https://github.com/majin2020/gcc-test/blob/0b08c1a13de9663d7d9aba7539b960ec0607ca24/test.c.299r.sched1
> > This is a log of the pass 'sched1' When issue_rate == 2. Among them, insn 13 and 14 are
> > much ahead of schedule, which risks generating redundant mov instructions, which seems
> > unreasonable.
> > 
> > Therefore, I submit patch again on the basis of the last review opinions to try to solve
> > this problem.
> > 
> > This is the new log of shed1 after patch is added.
> > https://github.com/majin2020/gcc-test/commit/efcb43e3369e771bde702955048bfe3f501263dd
> > 
> > gcc/ChangeLog:
> > 
> >          * haifa-sched.cc (unrecog_insn_for_forw_only_p): New.
> >          (prune_ready_list): UNRECOG INSN is not executed in advance if it starts a
> > 	live range.
> > ---
> >   gcc/haifa-sched.cc | 44 +++++++++++++++++++++++++++++++++++++++-----
> >   1 file changed, 39 insertions(+), 5 deletions(-)
> > 
> > diff --git a/gcc/haifa-sched.cc b/gcc/haifa-sched.cc
> > index 2c881ede0ec..205680a4936 100644
> > --- a/gcc/haifa-sched.cc
> > +++ b/gcc/haifa-sched.cc
> > @@ -765,6 +765,23 @@ real_insn_for_shadow (rtx_insn *insn)
> >     return pair->i1;
> >   }
> >   
> > +/* Return true if INSN is unrecog that starts a live range.  */
> I would rewrite this as
> 
> /* Return TRUE if INSN (a USE or CLOBBER) starts a new live
>     range, FALSE otherwise.  */

Ok.

> > +
> > +static bool
> > +unrecog_insn_for_forw_only_p (rtx_insn *insn)
> I would call this "use_or_clobber_starts_range_p" or something like that.

Ok.

> > +{
> > +  if (insn && !INSN_P (insn) && recog_memoized (insn) >= 0)
> > +    return false;
> I would drop the test that INSN is not NULL in this test.  There's no 
> way it can ever be NULL here.
> 
> If you really want to check that, then I'd do something like
> 
> gcc_assert (INSN);
> 
> Instead of checking it in that condition.

Ok.

> > @@ -6320,11 +6337,28 @@ prune_ready_list (state_t temp_state, bool first_cycle_insn_p,
> >   	    }
> >   	  else if (recog_memoized (insn) < 0)
> >   	    {
> > -	      if (!first_cycle_insn_p
> > -		  && (GET_CODE (PATTERN (insn)) == ASM_INPUT
> > -		      || asm_noperands (PATTERN (insn)) >= 0))
> > -		cost = 1;
> > -	      reason = "asm";
> > +	      if (GET_CODE (PATTERN (insn)) == ASM_INPUT
> > +		  || asm_noperands (PATTERN (insn)) >= 0)
> > +		{
> > +		  reason = "asm";
> > +		  if (!first_cycle_insn_p)
> > +		    cost = 1;
> > +		}
> > +	      else if (unrecog_insn_for_forw_only_p (insn))
> > +		{
> > +		  reason = "unrecog insn";
> > +		  if (!first_cycle_insn_p)
> > +		    cost = 1;
> > +		  else
> > +		    {
> > +		      int j = i;
> > +		      while (n > ++j)
> > +			if (!unrecog_insn_for_forw_only_p (ready_element (&ready, j)))
> > +			  break;
> > +
> > +		      cost = (j == n) ? 0 : 1;
> > +		    }
> Why do you need a different cost based on what's in the ready list? 
> Isn't the only property we're looking for whether or not the USE/CLOBBER 
> opens a live range?
> 
> Jeff

For this, I found that if I only look for the USE/CLOBBER  that opens a live range,
when there is only the USE/CLOBBERs left in the ready list, there will be an infinite
loop, because we will always postpone it to the next cycle(cost = 1), causing it to
never be emitted and always be in the ready list.

So I think (may not be correct) when there is only the USE/CLOBBERs left in the ready
list, the cost should be set to 0, and the USE/CLOBBER can be emitted immediately.

Maybe there's a better way?