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

[Kristof Beyls via llvm-dev]

I've been digging a little bit deeper into the biggest performance regressions I've observed.

What I've observed so far is:
* A lot of the biggest regressions are caused by unnecessarily moving floating point values through general purpose registers. I've raised http://bugs.llvm.org/show_bug.cgi?id=32550 for this. I think this one definitely needs fixing before enabling GlobalISel by default at -O0.
* FastISel seems to transform division-by-constant-power-of-2 into right shift (see https://github.com/llvm-mirror/llvm/blob/master/lib/CodeGen/SelectionDAG/FastISel.cpp#L456-L468). GlobalISel does not. It seems to me that at -O0 there may be reasons not perform this transformation, but maybe there is a good reason why FastISel does this?
* FastISel doesn't seem to handle functions with switch statements, so it falls back to DAGISel. DAGISel produces code that's a lot better than GlobalISel for switch statement at -O0. I'm not sure if we need to do something here before enabling GlobalISel by default. I'm thinking we may need to add a smarter way to lower switch statements rather than just a cascaded sequence of conditional branches.

I'll try to add the above content to the document Diana created at https://goo.gl/IS2Bdw too.

Thanks,

Kristof



On 3 Apr 2017, at 17:10, Kristof Beyls <Kristof.Beyls at arm.com<mailto:Kristof.Beyls at arm.com>> wrote:

I've kicked off a run to compare "-O0 -g" versus "-O0 -g -mllvm -global-isel -mllvm -global-isel-abort=2".
I've selected the test-suite (albeit a version which is a couple of months old now) and a few short-running proprietary benchmarks to get data back quickly for an initial feel of where things are.
This was running on Cortex-A57 AArch64 Linux.

I saw one assertion failure in GlobalISel, see http://bugs.llvm.org/show_bug.cgi?id=32471. This is in a program compiled at -O2 (my out-dated test-suite still overrides -O0 and instead uses -O for that program). The root cause of the failure seems to be due to LowLevelType not supporting vectors of pointers. I think this demonstrates that for correctness, we should be trying to test more than -O0, or even more than just LLVM-IR produced by clang, as other front-ends could run into this even at -O0.

Due to this assertion failure and the infrastructure I used, the numbers below do not include test-suite/MultiSource/Benchmarks results.

On the non-correctness aspects, LNT tells me that:
- The programs that report execution time, on geomean are about 17% slower.
- The programs that report scores, on geomean are about 21% slower.
- Code size is up on geomean about 11%.
I'm afraid I don't have compile time numbers, nor any feel for debug info quality.

I'll need quite a bit more time to dig into the details to come up with something actionable, although the fact that LowLevelType doesn't support vectors of pointers is already actionable.
Nevertheless, I thought to share what I see as is, to see if others see similar results so far.

I thought Diana was going to look into fallback rate on the test-suite on AArch64 linux?

Thanks,

Kristof

On 30 Mar 2017, at 10:54, Renato Golin <renato.golin at linaro.org<mailto:renato.golin at linaro.org>> wrote:

On 30 March 2017 at 00:27, Quentin Colombet <qcolombet at apple.com<mailto:qcolombet at apple.com>> wrote:
On iOS we are at 100% pass rate in 00 g for the LLVM test suite, standard
benchmarks and unit tests. In about 5% of all functions GlobalIsel falls
back to SDIsel.
(Kristof Beyls would have the linux numbers.)
The self host compiler correctly builds and runs the LLVM test suite in O0.

Having done no tests at all on my side, I think we need to have
similar numbers on Linux to be able to flip across the board.

I don't want to flip it only for Darwin and not Linux, as that will
fragment the effort too much.

I'll check with Diana and Kristof to know what's the best way forward,
but it should be reasonably quick.

cheers,
--renato


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