> 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?
