[llvm-dev] [RFC] Changing the default pass manager for the optimization pipeline

[Björn Pettersson A via llvm-dev]

Thanks for the answers Arthur!

I also noticed that you already put up some patches for review, to address some of my questions 😊
(I ran out of time today, but will take a look at those patches later)

/Björn
From: Arthur Eubanks <aeubanks at google.com>
Sent: den 4 februari 2021 20:36
To: Björn Pettersson A <bjorn.a.pettersson at ericsson.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [RFC] Changing the default pass manager for the optimization pipeline



On Thu, Feb 4, 2021 at 9:34 AM Björn Pettersson A <bjorn.a.pettersson at ericsson.com<mailto:bjorn.a.pettersson at ericsson.com>> wrote:
Hi!

Are there any existing plans for making the help text for opt more easy to understand?

Here are some things that have been bugging me so far:

Currently it for example doesn’t mention that –O0, –O1, etc are designed for legacy-pm (with new-pm it is a bit of a mess since you get the wrong aa-pipeline when for example using –O3).
So are those options supposed to be removed, or should they remain as a short-form for “-passes=default<O3> -aa-pipeline=default” etc?
I'd like to remove these at some point and just go with -passes='default<O#>', but they'll likely remain until we decide to start removing the legacy PM and we start cleaning up tests to only use -passes=. I'll update the description for these flags.

opt -help-hidden lists all the legacy passes (under “Optimizations available:”), but those now require -enable-new-pm=0. Maybe that is obvious.
But I’m missing a similar list mentioning all things that are legal to put in the -passes string.
The legacy PM registered passes to a global variable which made this easier to do in the legacy PM. The new PM doesn't have a global registry, but rather each PassBuilder is initialized with the list of passes on its own. I'll draft up something to try to get this working.

And options like -debug-pass-manager and -debug-pass=Arguments only work depending on which PM that is used, but that is not mentioned in the help text so one need to figure it out by trial and error. Although most of the time you don’t get an error, you just don’t get the requested functionality. So it would be helpful if the help text mention in what situation those options can be used or not (or there should be errors if using them in the wrong context).
I'll update the help text. Each pass manager's debug logging is separate, rather than being tied to a global variable like the legacy PM's -debug-pass, so it's a bit weird to check in each new PM PassManager if -debug-pass has been specified.

Downstream we implement fuzzy testing by using “opt -O3 -debug-pass=Arguments” in order to get a list of passes that are used randomly on the opt command line.
I haven’t figured out yet how to implement something similar with the new pm. Is there a way to make opt output the pass names available (that can be used in -passes string)?
There was some discussion about similar things in the bugpoint + new PM thread. The way the new PM is designed doesn't really lend itself to this sort of functionality. It could be possible for each type of PassManager to print out the names of all the passes it contains, but we'd need some change with the way we handle pass names (e.g. remove PassInfoMixin and have each pass declare its own name). Rather, I think something like the PassInstrumentation approach that -opt-bisect-limit does is better, where you can turn off a specific set of passes. But I could be convinced of a different approach.

Regards,
Björn

Thanks for the questions, these are good for making things clearer to users.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20210205/2f638f3a/attachment.html>