On Wed, Nov 22, 2023 at 10:31 AM Kewen.Lin <li...@linux.ibm.com> wrote:
>
> on 2023/11/17 20:55, Alexander Monakov wrote:
> >
> > On Fri, 17 Nov 2023, Kewen.Lin wrote:
> >>> I don't think you can run cleanup_cfg after sched_init. I would suggest
> >>> to put it early in schedule_insns.
> >>
> >> Thanks for the suggestion, I placed it at the beginning of haifa_sched_init
> >> instead, since schedule_insns invokes haifa_sched_init, although the
> >> calls rgn_setup_common_sched_info and rgn_setup_sched_infos are executed
> >> ahead but they are all "setup" functions, shouldn't affect or be affected
> >> by this placement.
> >
> > I was worried because sched_init invokes df_analyze, and I'm not sure if
> > cfg_cleanup can invalidate it.
>
> Thanks for further explaining! By scanning cleanup_cfg, it seems that it
> considers df, like compact_blocks checks df, try_optimize_cfg invokes
> df_analyze etc., but I agree that moving cleanup_cfg before sched_init
> makes more sense.
>
> >
> >>> I suspect this may be caused by invoking cleanup_cfg too late.
> >>
> >> By looking into some failures, I found that although cleanup_cfg is
> >> executed
> >> there would be still some empty blocks left, by analyzing a few failures
> >> there
> >> are at least such cases:
> >> 1. empty function body
> >> 2. block holding a label for return.
> >> 3. block without any successor.
> >> 4. block which becomes empty after scheduling some other block.
> >> 5. block which looks mergeable with its always successor but left.
> >> ...
> >>
> >> For 1,2, there is one single successor EXIT block, I think they don't
> >> affect
> >> state transition, for 3, it's the same. For 4, it depends on if we can
> >> have
> >> the assumption this kind of empty block doesn't have the chance to have
> >> debug
> >> insn (like associated debug insn should be moved along), I'm not sure.
> >> For 5,
> >> a reduced test case is:
> >
> > Oh, I should have thought of cases like these, really sorry about the slip
> > of attention, and thanks for showing a testcase for item 5. As Richard as
> > saying in his response, cfg_cleanup cannot be a fix here. The thing to check
> > would be changing no_real_insns_p to always return false, and see if the
> > situation looks recoverable (if it breaks bootstrap, regtest statistics of
> > a non-bootstrapped compiler are still informative).
>
> As you suggested, I forced no_real_insns_p to return false all the time, some
> issues got exposed, almost all of them are asserting NOTE_P insn shouldn't be
> encountered in those places, so the adjustments for most of them are just to
> consider NOTE_P or this kind of special block and so on. One draft patch is
> attached, it can be bootstrapped and regress-tested on ppc64{,le} and x86.
> btw, it's without the previous cfg_cleanup adjustment (hope it can get more
> empty blocks and expose more issues). The draft isn't qualified for code
> review but I hope it can provide some information on what kinds of changes
> are needed for the proposal. If this is the direction which we all agree on,
> I'll further refine it and post a formal patch. One thing I want to note is
> that this patch disable one assertion below:
>
> diff --git a/gcc/sched-rgn.cc b/gcc/sched-rgn.cc
> index e5964f54ead..abd334864fb 100644
> --- a/gcc/sched-rgn.cc
> +++ b/gcc/sched-rgn.cc
> @@ -3219,7 +3219,7 @@ schedule_region (int rgn)
> }
>
> /* Sanity check: verify that all region insns were scheduled. */
> - gcc_assert (sched_rgn_n_insns == rgn_n_insns);
> + // gcc_assert (sched_rgn_n_insns == rgn_n_insns);
>
> sched_finish_ready_list ();
>
> Some cases can cause this assertion to fail, it's due to the mismatch on
> to-be-scheduled and scheduled insn counts. The reason why it happens is that
> one block previously has only one INSN_P but while scheduling some other
> blocks
> it gets moved as well then we ends up with an empty block so that the only
> NOTE_P insn was counted then, but since this block isn't empty initially and
> NOTE_P gets skipped in a normal block, the count to-be-scheduled can't count
> it in. It can be fixed with special-casing this kind of block for counting
> like initially recording which block is empty and if a block isn't recorded
> before then fix up the count for it accordingly. I'm not sure if someone may
> have an argument that all the complication make this proposal beaten by
> previous special-casing debug insn approach, looking forward to more comments.
Just a comment that the NOTE_P thing is odd - do we only ever have those for
otherwise empty BBs? How are they skipped otherwise (and why does that not
work for otherwise empty BBs)?
Richard.
> BR,
> Kewen
>
