Effect of SMS register move scheduling

[Richard Sandiford]

Richard Sandiford <richard.sandiford at linaro.org> writes:
> Revital Eres <revital.eres at linaro.org> writes:
>> btw, do you also have numbers of how much SMS (hopefully) improves
>> performance on top of the vectorized code?
>
> OK, here's a comparison of:
>
>     -mcpu=cortex-a8 -mfpu=neon -mfloat-abi=softfp -mvectorize-with-neon-quad
>     -fno-auto-inc-dec
>
> vs:
>
>     -mcpu=cortex-a8 -mfpu=neon -mfloat-abi=softfp -mvectorize-with-neon-quad
>     -fmodulo-sched -fmodulo-sched-allow-regmoves -fno-auto-inc-dec

Revital pointed out that I'd forgotten to list:

    -O2 -ffast-math -funsafe-loop-optimizations -ftree-vectorize

for both cases, which does make quite a big difference :-)

I looked at the mjpegenc regression, and the register pressure looks OK.
I think it maxes out at around 20 vector double registers if you just
consider the loop body.  So I think this is actually a regalloc failure
rather than an SMS one per se.

-fira-algorithm=priority removes all but one spill from the loop.
I ran another test comparing:

   -O2 -ffast-math -funsafe-loop-optimizations -ftree-vectorize
   -mcpu=cortex-a8 -mfpu=neon -mfloat-abi=softfp -mvectorize-with-neon-quad
   -fmodulo-sched -fmodulo-sched-allow-regmoves -fno-auto-inc-dec

with:

   -O2 -ffast-math -funsafe-loop-optimizations -ftree-vectorize
   -mcpu=cortex-a8 -mfpu=neon -mfloat-abi=softfp -mvectorize-with-neon-quad
   -fmodulo-sched -fmodulo-sched-allow-regmoves -fno-auto-inc-dec
   -fira-algorithm=priority

(soon this lot won't fit in my emacs window).  I've attached the
results below.  In both cases, the compiler was current trunk with my
move-scheduling patch applied.

I haven't rerun an SMS-vs-non-SMS test, but based on previous results,
mjpegenc and aacsbr-2 become faster with SMS than without.

This doesn't hide the fact that SMS doesn't take register pressure
into account.  But if I haven't completely miscalculated (and I might
have) it seems that even if SMS did have some pressure-tracking
capability, it probably wouldn't have triggered for mjpegenc,
at least not unless it was very conservative.

Richard


a3dec
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  speedup: x1.01
aacsbr-1
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  speedup: x1
aacsbr-2
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aacsbr-3
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aes
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avs
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cdgraphics
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dwt
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dxa
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mjpegenc
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qtrle
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wmavoice
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