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

[Quentin Colombet via llvm-dev]

Hi,

GlobalISel, the SelectionDAG replacement, has come a long way since initially discussed on the mailing list and its last discussion at the EuroLLVM BoF (https://etherpad.net/p/GlobalISel <https://etherpad.net/p/GlobalISel>).
We believe we are close to the point of enabling it by default on AArch64 at O0. We now would like to enlist your help to get there.


*** Quick Status ***

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.


*** We Need Your Help ***

Please try GlobalISel for AArch64 at O0 (preferably O0 g) and file PR for:
- Performance problems (compile time, runtime, code size)
- Miscompile
- Crashers
- Poor debug information
- etc.

There is a component GlobalISel in llvm.org/bugs <http://llvm.org/bugs> for that.

GlobalISel cannot handle some inputs (e.g., some vector construction), but it falls back to SDISel when it hits such cases. We are also interested to know whether or not GlobalISel fallback on your favorite workloads and why (see next section for the actual options to run). So please file PRs for that too.

As always, patches are welcome!


*** Concretely What Do I Run ***

Please test your favorite workload/scenario on AArch64 at O0 using one of the following additional category of options:
Recommended: (-llvm) -global-isel (-mllvm) -global-isel-abort=2
OkToFallBack:: (-llvm) -global-isel (-mllvm) -global-isel-abort=0
AbortOnFallBack: (-llvm) -global-isel

The Recommended way will issue a warning if it falls back to SelectionDAG, the OkToFallBack will silently fallback to SDISel if it needs to, and the AbortOnFallBack will kill the compiler if GlobalISel cannot handle the input completely.


*** What Is Next? ***

We would like to turn on GobalISel by default in the next couple of weeks. Please help identify any critical issue that needs to be resolved before that can happen. We expect that minor hiccups and outliers in any metrics can be fixed quickly, but are worried about harmful run-time (miscompile) or compile-time crashes.


Many thanks,
-Quentin
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