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

[Mikael Holmén via llvm-dev]

On Thu, 2021-02-25 at 10:25 +0000, Florian Hahn wrote:
> > 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://protect2.fireeye.com/v1/url?k=b28a4060-ed117a79-b28a00fb-8682aaa22bc0-2fe31fd4ca0dd9ee&q=1&e=71cf8e51-b719-4c90-b79a-9aad4f1804c5&u=https%3A%2F%2Freviews.llvm.org%2FD86657
> > > 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.

+1

This is what I tried to describe in the 

"Another case is when a number of passes is needed to make a certain
pass expose something interesting."

case.

Thanks,
Mikael
> 
> Cheers,
> Florian