Hi,

on 2023/11/22 17:30, Kewen.Lin 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.
> 

The attached one is the improved draft patch v2 for skipping empty BB, against
the previous draft, it does:
  1) use NONDEBUG_INSN_P for !DEBUG_INSN_P && !NOTE_P when it's appropriate;
  2) merge NOTE_P special handling into the one on DEBUG_INSN_P;
  3) fix exposed issue on broad testing on EBB;
  4) introduce rgn_init_empty_bb for mismatch count issue;
  5) add/refine some comments;

It's bootstrapped and regress-tested on x86_64-redhat-linux and
powerpc64{,le}-linux-gnu.  I also tested with EBB turned on by default, one
issue in schedule_ebb got exposed and fixed, bootstrapped on x86_64-redhat-linux
and powerpc64{,le}-linux-gnu, regress-tested on powerpc64{,le}-linux-gnu, one
guality failure on x86_64-redhat-linux.  With seletive-scheduling turned on by
default, it's bootstrapped on x86_64-redhat-linux and a few guality failures
and some failures with extra warnings (like with -fvar-tracking-assignments),
those failures are trivial.  Note that I'm unable to test forced seletive-scheduling
on Power since it's even broken without this patch, there seems a few issues,
I'll file some PRs separately.

Any comments are highly appreciated.  I'll go forward to make a formal patch if
there is no objection this week.  I think we still can keep this no_real_insns_p
since in some places like free_block_dependencies it's still good for early
return, we can just remove its uses in some places. 

BR,
Kewen

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