[llvm-dev] Commit module to Git after each Pass

[Alexandre Isoard via llvm-dev]

Today it is entirely in llvm. It is even more costly than -print-after-all
as it:
  - print to file
  - print the entire module (after each basic block, for basic block
passes, it will still print the entire module where only one basic block
changed)
  - call git 2 times (add then commit) and wait for them to finish (I even
save all the empty commits)

The reason I print the entire module is so that git is able to
show/compress the change properly. Then a simple git log --patch show the
change of each pass.

Ideally, the -debug output of each pass would be piped as the git commit
message, and the passes name+options would be the commit title. But I
didn't have time to do that.

My goal was not particularly to be efficient but to be thorough so as to
preserve the maximum of behavioral information of each pass while not
affecting their order and behavior.

You are right, there could be major speed improvement gained by doing so as
external processing. For instance, the output would be in the format that
git patch takes as input. Could this be a GSoC?

Today I use git filter-branch + opt as a post processing tool to generate
the reg.dot files of the region info. In any way, the feature is a breath
of fresh air in our development here. That's why I wanted to share. It will
quickly become my reflex to debug with it... probably because I am at ease
with git though.

On Wed, Mar 7, 2018, 06:19 Alex Bradbury <asb at asbradbury.org> wrote:

> On 6 March 2018 at 18:43, Alexandre Isoard via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Hello,
> >
> > I had a stupid idea recently that turned out not so stupid after all. I
> > wanted to be able to "see" an entire pass pipeline in action to find
> > unnecessary transformations and/or missed opportunities and generally
> > improve the debug-ability of LLVM.
> >
> > So as the title suggest, I implemented an equivalent of
> "-print-after-all"
> > but instead of printing into stdout I dump into a file that get commit
> into
> > a temporary git. There are some quirks with it but it's working and is
> > actually awesome. For example, at first sight, I see multiple time lcssa
> and
> > instcombine cancelling each other's work.
> >
> > Of course, that has a big impact on compile time when enabled, but that's
> > still practical (git being quite good at its job) when debugging.
> >
> > There are improvement I can make, but would you guys be interested in
> such
> > feature?
>
> Hi Alexandre. I can definitely see how it could be useful to track
> changes through git commits, and take advantage of your favourite repo
> history viewer to see changes. How much of your current implementation
> is handled via modifications to LLVM vs an external helper script? For
> instance, I might imagine trying to achieve something similar through
> a script that parses the output of -print-after-all in order to create
> the desired files+commits.
>
> Best,
>
> Alex
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180307/0405b928/attachment.html>