Re: [RFC] Selective scheduling pass

[Vladimir Makarov <vmakarov@redhat.com>]

Andrey Belevantsev wrote:
> Hello,
>
> The patches in this thread introduce selective scheduler in GCC, 
> implemented by myself, Dmitry Melnik, Dmitry Zhurikhin, Alexander 
> Monakov, and Maxim Kuvyrkov while he was at ISP RAS.  Selective 
> scheduler is aimed at scheduling eager targets such as ia64, power6, 
> and cell.  The implementation contains both the scheduler and the 
> software pipeliner, which can be used on loops with control flow not 
> handled by SMS.  The scheduler can work either before or after 
> register allocation, but it is currently tuned to work after.
>
> The scheduler was bootstrapped and tested on ia64, with all default 
> languages, both as a first and as a second scheduler.  It was also 
> bootstrapped with c, c++, and fortran enabled on ppc64 and x86-64.
>
> On ia64, test results on SPEC2k FP comparing -O3 -ffast-math on trunk 
> and sel-sched branch show 3.8% speedup on average, SPEC INT shows both 
> small speedups and regressions, staying around neutral in average:
>
Congratulation!  I followed the project for a long time.  Finally some 
useful milestone is achieved and you have got a pretty big improvement.  
The scheduling algorithm is superior than what we had because it permits 
to improve insn schedules on all execution paths by insn cloning and 
other transformations.
> On power6, Revital Eres saw speedups on several tests; additional 
> tuning is required to get good results there, which is complicated 
> because we don't have power6.  On cell, there was some third-party 
> testing in 2007, showing 4-6% speedups, but I don't have more detailed 
> information.
>
> Compile time slowdown measured with --enable-checking=assert is quite 
> significant -- about 12% on spec int and about 18% on spec fp and 
> cc1-i-files collection.  For this reason, we have enabled selective 
> scheduler by default at -O3 on ia64 and disabled by default on other 
> targets.
>
Itanium is pretty specific target.  It would be interesting to know how 
big a slowdown for ppc.
> Our current plan is to work on further compile time improvements and 
> performance tuning for ppc and cell, hopefully with the help of IBM 
> Haifa folks.  If we will complete this work before the end of stage2, 
> then we can enable selective scheduling at -O3 also for ppc in 4.4.  
> In the mid-term, we will work on removing the ebb scheduler, as it is 
> now used on ia64 only and will be superseded by selective scheduler 
> when we'll further improve compile time.
>

I think we should rid of EBB scheduler finally.  You could try to 
improve compile-time problem preventing some transformations in the new 
scheduler in -O2 mode.

If you solve compile-time problem, I think we should work on removing 
haifa-scheduler too to have just one insn scheduler.  But as I 
understand it will not happen soon.