[llvm-dev] [GlobalISel][AArch64] Toward flipping the switch for O0: Please give it a try!

[Kristof Beyls via llvm-dev]

On 24 May 2017, at 19:31, Quentin Colombet <qcolombet at apple.com<mailto:qcolombet at apple.com>> wrote:

Hi Kristof,

Thanks for the measurements.

On May 24, 2017, at 6:00 AM, Kristof Beyls <kristof.beyls at arm.com<mailto:kristof.beyls at arm.com>> wrote:


- Comparing against -O0 without globalisel but with the above regalloc options: 5.6% performance drop, 1% code size drop.

In summary, the measurements indicate some good improvements.
I also haven't measure the impact on compile time.

Do you have a mean to make this measurement?
Ahmed did a bunch of compile time measurements on our side and I wanted to see if I need to put him on the hook again :).

I did a quick setup with CTMark (part of the test-suite). I ran each of
* '-O0 -g',
* '-O0 -g -mllvm -global-isel=true -mllvm -global-isel-abort=0', and
* '-O0 -g -mllvm -global-isel=true -mllvm -global-isel-abort=0 -mllvm -optimize-regalloc -mllvm -regalloc=greedy'
5 times, cross-compiling from X86 to AArch64, and took the median measured compile times.
In summary, I see GlobalISel having a compile time that's 3.5% higher than the current -O0 default.
With enabling the greedy register allocator, this increases to 28%.
28% is probably too high?

I think it is yes.
I have attached a quick hack to the greedy allocator to feature a fast mode.
Could you give it a try?

To enable the fast mode, please use (-mllvm) -regalloc-greedy-fast=true (default is false).

I'm afraid it doesn't seem to save much compile time. On geomean, I see about 26% compile time increase against the current -O0 default (compared to 28% increase for regalloc greedy without your patch).


At the moment I can't think of an alternative to having a "constant materialization localizer" pass at -O0 to hit all the metrics we thought of as necessary before enabling GISel by default.

It would be good if someone else could also do a compilation time experiment - just to make sure I didn't make any silly mistakes in my experiment.

Here are the details I see:

        gisel   gisel+greedy
CTMark/7zip/7zip-benchmark      102.8%  106.5%
CTMark/Bullet/bullet    100.5%  105.1%
CTMark/ClamAV/clamscan  101.6%  130.8%
CTMark/SPASS/SPASS      101.2%  120.0%
CTMark/consumer-typeset/consumer-typeset        105.7%  138.2%
CTMark/kimwitu++/kc     103.1%  122.6%
CTMark/lencod/lencod    106.2%  143.4%
CTMark/mafft/pairlocalalign     96.2%   135.4%
CTMark/sqlite3/sqlite3  109.1%  155.1%
CTMark/tramp3d-v4/tramp3d-v4    109.1%  132.0%
GEOMEAN 103.5%  128.0%


Thanks,

Kristof

Thanks,
-Quentin
<regalloc-fastmode.diff>

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