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Re: Sched1 stability issue
- From: Alexander Monakov <amonakov at ispras dot ru>
- To: Kugan Vivekanandarajah <kugan dot vivekanandarajah at linaro dot org>
- Cc: gcc <gcc at gcc dot gnu dot org>
- Date: Wed, 4 Jul 2018 10:02:22 +0300 (MSK)
- Subject: Re: Sched1 stability issue
- References: <CAELXzTPj5isrWDDL21yTFxHdpjHPnmO2FPB92+RF-Eg=8KN3HA@mail.gmail.com>
On Wed, 4 Jul 2018, Kugan Vivekanandarajah wrote:
> We noticed a difference in the code generated for aarch64 gcc 7.2
> hosted in Linux vs mingw. AFIK, we are supposed to produce the same
> output.
>
> For the testacse we have (quite large and I am trying to reduce), the
> difference comes from sched1 pass. If I disable sched1 the difference
> is going away.
>
> Is this a known issue? Attached is the sched1 dump snippet where there
> is the difference.
The rank_for_schedule comparator used for qsort in the scheduler is known
to be invalid; some issues have been fixed in gcc-8, but some remain (you
can search the bugzilla for qsort_chk issues). Since the comparator is
invalid, different qsort implementations reorder the ready list differently.
In gcc-9 qsort calls use gcc_qsort instead and thus wouldn't diverge.
Alexander