[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 22:01, Quentin Colombet <qcolombet at apple.com<mailto:qcolombet at apple.com>> wrote:

Hi Kristof,

Thanks for going back so fast!

On May 24, 2017, at 12:57 PM, Kristof Beyls <kristof.beyls at arm.com<mailto:kristof.beyls at arm.com>> wrote:


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).

Interesting, I guess a lot of time is spent in the coalescer. Could you give a try with -join-liveintervals=false?

With adding -join-liveintervals=false, I see the compile time increase going up to 28% again.


Do you know where the time is spent (-time-passes)?

I'm afraid I won't have time to have a closer look in the next couple of days - I don't know where the time is spent at the moment.


Anyhow, fixing all of those, although this is I think the right approach, will take time, so we can go with the localizer.

Right, I don't understand the register allocator well enough to know if that compile time overhead can be fixed, while still getting the necessary codegen benefits the greedy allocator gives.
Is there any specific help you're looking for with getting the localizer work well enough for production use?

Thanks,

Kristof

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