Re: PING^1 [PATCH v3] sched: Change no_real_insns_p to no_real_nondebug_insns_p [PR108273]

["Kewen.Lin" <linkw@linux.ibm.com>]

on 2023/11/22 18:25, Richard Biener wrote:
> On Wed, Nov 22, 2023 at 10:31 AM Kewen.Lin <linkw@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)?

Yes, previously (bypassing empty BBs) there is no chance to encounter NOTE_P
when scheduling insns, as for notes in normal BBs, when setting up the head
and tail, some are skipped (like get_ebb_head_tail), and there are also some
special handlings remove_notes and unlink_bb_notes to guarantee they are
gone.  By disabling empty BB bypassing, all empty BBs will be actually
uniformed as (head == tail && NOTE_P (head)), we have to deal with NOTE_P.

BR,
Kewen