Re: [PATCH][MIPS] P5600 scheduling

[Richard Sandiford <rdsandiford at googlemail dot com>]

Hi Jaydeep,

Thanks for the write-up and updated patches.  I'll try to get to them
this weekend.  In the meantime...

Jaydeep Patil <Jaydeep.Patil@imgtec.com> writes:
> The -msched-weight option:
> We are using ~650 hot-spot functions from VP9/VP8/H264/MPEG4 codes 
> available to us as a test suite. The default Haifa-scheduler worked 
> well for most of the functions, but excess spilling was observed in 
> cases where register pressure was more than ~20. The 
> -fsched-pressure flag proved good in some cases, but the algorithm 
> focuses more on reducing register pressure.
> We observed increase in stalls (but less spilling) with the -fsched- 
> pressure option. When the register pressure goes beyond a certain 
> threshold, the -msched-weight option tries to keep it down by 
> promoting certain instructions up in the order. It has been 
> implemented as part of TARGET_SCHED_REORDER target hook (function 
> mips_sched_weight). The change is generic and there is nothing 
> specific to MIPS.
>
> When the register pressure goes beyond 15 then an instruction with 
> maximum forward dependency is promoted ahead of the instruction at
> READY[NREADY-1].
> Scheduling of an INSN with maximum forward dependency enables early 
> scheduling of instructions dependent on it.
>
> When the register pressure goes beyond 25 and if consumer of the 
> instruction in question (INSN) has higher priority than the 
> instruction at READY[NREADY-1] then INSN is promoted. This chooses 
> an INSN which has a high priority instruction dependent on it. This 
> triggers the scheduling of that consumer instruction as early as 
> possible to free up the registers used by that instruction.

Yeah, this sounds similar to what I was seeing for Cortex-A8
with the default -fsched-pressure (which is tuned for and known
to work well on x86).  Did you try with:

  -fsched-pressure --param sched-pressure-heuristic=2

?  That isn't advertised in the documentation because we don't
want to make it a user-level option that would then need to be
supported in future.  But if you find that the above works better
than plain -fsched-pressure, which is equivalent to:

  -fsched-pressure --param sched-pressure-heuristic=1

then we could consider making sched-pressure-heuristic=2 the
default for MIPS.  I'd certainly be interested to know how it
compares with -msched-weight.

FWIW the write-up of the alternative -fsched-pressure is here:

  https://gcc.gnu.org/ml/gcc-patches/2011-12/msg01684.html

It sounds like the Linaro guys have a performance fix pending, so if
running the tests takes a lot of your time or a lot of resources,
it might be worth waiting until that's submitted.

E.g. one of the things I noticed with the default -fsched-pressure
at the time -- not sure whether this has changed since -- is that
the pressure at the start of the EBB was based on the total number
of live values, including those that are live across a loop but
not used in it.  So in many cases the starting pressure was too high
and so like you say the heuristic was too conservative.  That was
one of the main things that the alternative heuristic was supposed
to help with.

It looks like you solve that by starting with a pressure of 0
for each EBB, is that right?  That's a bit more aggressive than
what I did, since AIUI starting with 0 will ignore loop invariants.
On the other hand, being more aggressive (i.e. closer to what you'd get
with the default scheduling heuristic) also means that it's more likely
to be usable by default.

Thanks,
Richard