Following on from yesterday's call about what it would take to enable
SMS by default: one of the problems I was seeing with the SMS+IV patch
was that we ended up with excessive moves. E.g. a loop such as:
void
foo (int *__restrict a, int n)
{
int i;
for (i = 0; i < n; i += 2)
a[i] = a[i] * a[i + 1];
}
would end up being scheduled with an ii of 3, which means that in the
ideal case, each loop iteration would take 3 cycles. However, we then
added ~8 register moves to the loop in order to satisfy dependencies.
Obviously those 8 moves add considerably to the iteration time.
I played around with a heuristic to see whether there were enough
free slots in the original schedule to accomodate the moves.
That avoided the problem, but it was a hack: the moves weren't
actually scheduled in those slots. (In current trunk, the moves
generated for an instruction are inserted immediately before that
instruction.)
I mentioned this to Revital, who told me that Mustafa Hagog had
tried a more complete approach that really did schedule the moves.
That patch was quite old, so I ended up reimplementing the same kind
of idea in a slightly different way. (The main functional changes
from Mustafa's version were to schedule from the end of the window
rather than the start, and to use a cyclic window. E.g. moves for
an instruction in row 0 column 0 should be scheduled starting at
row ii-1 downwards.)
The effect on my flawed libav microbenchmarks was much greater
than I imagined. I used the options:
-mcpu=cortex-a8 -mfpu=neon -mfloat-abi=softfp -mvectorize-with-neon-quad
-fmodulo-sched -fmodulo-sched-allow-regmoves -fno-auto-inc-dec
The "before" code was from trunk, the "after" code was trunk + the
register scheduling patch alone (not the IV patch). Only the tests
that have different "before" and "after" code are run. The results were:
a3dec
before: 500000 runs take 4.68384s
after: 500000 runs take 4.61395s
speedup: x1.02
aes
before: 500000 runs take 20.0523s
after: 500000 runs take 16.9722s
speedup: x1.18
avs
before: 1000000 runs take 15.4698s
after: 1000000 runs take 2.23676s
speedup: x6.92
dxa
before: 2000000 runs take 18.5848s
after: 2000000 runs take 4.40607s
speedup: x4.22
mjpegenc
before: 500000 runs take 28.6987s
after: 500000 runs take 7.31342s
speedup: x3.92
resample
before: 1000000 runs take 10.418s
after: 1000000 runs take 1.91016s
speedup: x5.45
rgb2rgb-rgb24tobgr16
before: 1000000 runs take 1.60513s
after: 1000000 runs take 1.15643s
speedup: x1.39
rgb2rgb-yv12touyvy
before: 1500000 runs take 3.50122s
after: 1500000 runs take 3.49887s
speedup: x1
twinvq
before: 500000 runs take 0.452423s
after: 500000 runs take 0.452454s
speedup: x1
Taking resample as an example: before the patch we had an ii of 27,
stage count of 6, and 12 vector moves. Vector moves can't be dual
issued, and there was only one free slot, so even in theory, this loop
takes 27 + 12 - 1 = 38 cycles. Unfortunately, there were so many new
registers that we spilled quite a few.
After the patch we have an ii of 28, a stage count of 3, and no moves,
so in theory, one iteration should take 28 cycles. We also don't spill.
So I think the difference really is genuine. (The large difference
in moves between ii=27 and ii=28 is because in the ii=27 schedule,
a lot of A--(T,N,0)-->B (intra-cycle true) dependencies were scheduled
with time(B) == time(A) + ii + 1.)
I also saw benefits in one test in a "real" benchmark, which I can't
post here.
Richard
Hello,
Following today performance call
(https://wiki.linaro.org/WorkingGroups/ToolChain/Meetings/2011-08-23)
here are some points raised regarding the steps towards enabling SMS by default:
* Benchmarks testing:
-- Running benchmarks as EEMBC and SPEC2006 with SMS enabled is
crucial to expose loops where SMS degrades the performance. those
loops need to be analysed to construct a cost model.
-- SMS increases code size by introducing prologue and epilogue to the
loop kernel. This should also be measured.
-- Measure increase in compile time: on native or cross build?
Currently SMS fails to bootstrap trunk on ARM machine. this should
also be taken into account when considering enabling it by default.
Should it be turned on with -O2 or -O3?
SMS flags to use for testing:
-O3 -fmodulo-sched-allow-regmoves -fmodulo-sched
-funsafe-loop-optimizations -fno-auto-inc-dec
Thanks,
Revital
Hi
Some time ago we agreed that not everyone here uses Ubuntu distribution
and decided to provide so called 'generic linux' cross toolchain.
Recently I managed to get it done and now need brave testers to tell is
it working or not.
Get it here: http://people.linaro.org/~hrw/generic-linux/ (64bit only)
Needed files are toolchain-11.07.tar.xz and init.sh script. Unpack
tarball from / so /opt/linaro/11.07/ will be populated and put init.sh
anywhere you want (it will be integrated into tarball later).
How to use:
$ source init.sh
this will add cross toolchain into PATH and also set LD_LIBRARY_PATH to
two directories:
- one with binutils libraries
- second with all extra libraries which may be needed
Feel free to experiment with second dir by removing files from there and
checking are system provided libs are fine too.
So far I checked this toolchain under few distributions:
- Ubuntu 10.04 'lucid' LTS
- Ubuntu 11.04 'natty'
- Fedora 14
- OpenSUSE 11.4
- CentOS 5.6
It failed only under CentOS (which was expected due to it's age).
How did I checked? So far compilation of 'gpm' and 'zlib' were tested.
==GCC==
===Progress===
* Continue to look at the test failure with mvectorize-with-neon-quad.
Should be able to commit the backend workaround in on Monday .
* Having some problems getting my panda board working reliably. I'm
not sure if its the temperature or what but when it gets hot in the
office as it was on Tuesday keeping it working reliably is hard. The
board locks up and then crashes quite often.
* Looked at VFP moves again for some more time.
* Committed tbh range change.
* Committed fixes for PR50022
=== Plans ===
* Finish off VFP moves patch.
* Look at BRANCH_COST results.
* Breakdown the T2 performance blueprints into smaller blueprints.
* Backport tbh range changes to Linaro 4.6
* Test the intrinsics patch once with some more intrinsics tests and
then merge it in to Linaro gcc 4.6
Meetings:
* 1-1s
* TCWG calls
Absences.
* 29th Aug - Sept. 2 - Holiday booked and approved.
* 31st Oct - 4th Nov - Linaro Summit Orlando - Travel booked - hotel
to be booked.
Hi all,
I'm having real trouble here :(
I just can't seem to get bzr to work! I've tried to branch
gcc-linaro/4.6 again and again, and it just won't. My other machine
refuses to do the merge from lp:gcc/4.6, presumable because the bzr on
there is too old.
I'm stuck. Can anybody else do the merge from upstream?
I'm going to keep trying.
Andrew
* GCC
Completed merging GCC 4.5 from FSF to Linaro.
Spun release tarballs for Linaro GCC 4.5 and 4.6. Uploaded them to
Michael's server, and kicked off the test builds remotely.
Submitted expenses for Linaro Connect.
Finally (!) committed my widening multiplies patches to FSF. :)
Continued trying to figure out what's wrong with my thumb2 constants
patches. I think I have identified a possible flaw, but I'm having
trouble reproducing the problem as I have been unable to pin down a
specific constant/expression combination that makes it through all the
other optimizations intact, and triggers the problem. I've not run out
of idea yet though ...
* Other
On leave all day Wednesday.
Prepared for the big CodeSourcery to Mentor switch-over by moving all my
work-in-progress data over to the new servers.
== String routines ==
* Working through updating my eglibc patch for memchr, I think I'm
nearly there - took way too long
to persuade an eglibc make check to work cross (can't get native
build happy).
== QEMU ==
* Sent a new version of my QEMU patch for the atomic helpers to Peter.
* Tested the Android beagle image on a real beagle - it fails in
pretty much the same way as the
QEMU run.
== Other ==
* Had a brief look at bug 825711 - scribus ftbfs on ARM - this is QT
being built to define qreal as
float on ARM when it's double on most other things, scribus
having a qreal variable and something
it's defined as a double and then passing it to a template that
requires two arguments of the same type;
not really sure which one is to blame here!
I'm on holiday next week.
Dave
