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 2023/11/23 16:20, Richard Biener wrote:
> On Thu, Nov 23, 2023 at 4:02 AM Kewen.Lin <linkw@linux.ibm.com> wrote:
>>
>> 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.
> 
> I see.  I expected most of them to be naturally part of another EBB.  So it's
> rather a limitation of the head/tail representation.
> 
> I wonder if there's a more minimal fix though.  But iff head or tail

Not sure I got the question correctly, if it is for this inconsistent states
issue, I think what Maxim suggested seems to be the minimal fixes.

> of an EBB then I
> guess either head or tail has to point to a stmt in said block which necessarily
> then means either a debug or note.

Yes, will only have NOTE_P for empty BB, if there is a debug insn then it wins
(all NOTEs get dropped).  remove_notes runs from head to tail, it's applied for
EBB's special head and tail.

BR,
Kewen