[llvm-dev] How to get from "opt -O3" to "opt <a-single-pass>" with the new PM?

[Florian Hahn via llvm-dev]

> On Feb 25, 2021, at 07:03, Mikael Holmén <mikael.holmen at ericsson.com> wrote:
> 
> On Wed, 2021-02-24 at 11:25 -0800, Arthur Eubanks wrote:
>> There's no way to get a list of passes. Currently the name used to
>> specify a pass, e.g. "instcombine" in -passes=instcombine is mostly a
>> one way street to creating the pass, it's hard to go backwards from a
>> pass to the original name due to the implementation. This could be
>> fixed, but it'd be a fairly large change.
>> 
>> Personally I'd prefer an addition to https://reviews.llvm.org/D86657
>> where we dump the IR on a crash to a file. Then with -debug-pass-manager, we can see which pass caused the crash. Then use llvm-reduce to reduce the IR, running only that one pass.
>> 
> 
> Ok. So more or less the tool we have is -print-before-all -debug-pass-
> manager and copy/paste the last input and rerun with the last
> interesting pass. -print-on-crash makes it a little bit more convenient
> when a pass crashes. I suppose this will work ok in many cases.

Unfortunately this won’t work when the problem is caused by state not captured in LLVM IR. E.g. a pass could optimize the MemorySSA for a function and then the optimized MemorySSA is used by a subsequent pass, which crashes because the optimized MemorySSA enables an optimization. If you just use the IR before the crash, the pass will use unoptimized MemorySSA and may not crash. There are other analysis results that may expose similar effects.

I think we need a way to reduce the passes involved in such scenarios.

Cheers,
Florian