Re: [PATCH] Ignore debug insns with CONCAT and CONCATN for insn scheduling

[Jeff Law <jeffreyalaw@gmail.com>]

On 9/26/22 17:43, Jakub Jelinek wrote:
> On Mon, Sep 26, 2022 at 05:23:45PM -0600, Jeff Law via Gcc-patches wrote:
>> On 9/26/22 13:52, H.J. Lu wrote:
>>> On Sat, Sep 24, 2022 at 1:37 PM Jeff Law <jeffreyalaw@gmail.com> wrote:
>>>> On 9/21/22 16:11, H.J. Lu wrote:
>>>>> On Wed, Sep 7, 2022 at 10:03 AM Jeff Law via Gcc-patches
>>>>> <gcc-patches@gcc.gnu.org> wrote:
>>>>>> On 9/2/2022 8:36 AM, H.J. Lu via Gcc-patches wrote:
>>>>>>> CONCAT and CONCATN never appear in the insn chain.  They are only used
>>>>>>> in debug insn.  Ignore debug insns with CONCAT and CONCATN for insn
>>>>>>> scheduling to avoid different insn orders with and without debug insn.
>>>>>>>
>>>>>>> gcc/
>>>>>>>
>>>>>>>          PR rtl-optimization/106746
>>>>>>>          * sched-deps.cc (sched_analyze_2): Ignore debug insns with CONCAT
>>>>>>>          and CONCATN.
>>>>>> Shouldn't we be ignoring everything in a debug insn?   I don't see why
>>>>>> CONCAT/CONCATN are special here.
>>>>> Debug insns are processed by insn scheduling.   I think it is to improve debug
>>>>> experiences.  It is just that there are no matching usages of CONCAT/CONCATN
>>>>> in non-debug insns.
>>>> But from a dependency standpoint ISTM all debug insn can be ignored.  I
>>>> still don't see why concat/concatn should be special here.
>>>>
>>> I tried to ignore everything in a debug insn.  It caused many regressions in
>>> the GCC testsuite.
>> Not terribly useful -- what failed and why?
> I think the design for debug insns in the scheduler is that they do affect
> scheduling decisions, but what is in debug insns should only affect actual
> scheduling of the debug insns and not the rest.
> So it wouldn't surprise me if ignoring everything in a debug insn broke a
> lot.  But I admit I never fully understood how it works, hopefully Alex or
> Vlad do.

But if we  look at debug insns in scheduling decisions, wouldn't that 
likely trigger a variety of compare-debug failures. Something here 
doesn't make sense.


jeff