[llvm-dev] Pass and Transformation-level debugging in LLVM

[Zhizhou Yang via llvm-dev]

I read this tool and I believe this could do a perfect job as a general
bisecting script using DebugCounter.

Just one small nit that I noticed that you are using (1<<32) as a default
upper bound. I have a patch that
could print the total number of transformations in a single pass:
https://reviews.llvm.org/D50031. I think it
may be helpful to let the user know exactly how many transformations are
there in the pass, so that they
could have a high level idea of the position of bad transformation.


On Thu, Sep 20, 2018 at 9:56 AM David Greene <dag at cray.com> wrote:

> Zhizhou Yang via llvm-dev <llvm-dev at lists.llvm.org> writes:
>
> > To bring it to the next level, DebugCounter provides features for me
> > to have an in-pass (transformation) level limit to tell which
> > transform in the pass exactly caused the error. When we set StopAfter
> > value for a DebugCounter, it will eventually stop there as a limit.
> >
> > And in D50031 and rL337748, I added a method to print DebugCounter
> > info: the `-print-debug-counter` flag. With this, writing a
> > transformation level bisection script will be more straightforward.
> >
> > I have already built a bisection tool to help Android toolchain debug
> > on our side. So I will say that the bisecting idea with OptBisect and
> > DebugCounter helps us save time while debugging mis-compilations.
>
> There is already a DebugCounter bisect tool in utils/bisect-skip-count.
>
> It is not documented, unfortunately.  I had to figure it out by
> inspection, but you use it by including "%(skip)d" and "$(count)d" in
> the command you specify to bisect-skip-count.  Then those values get
> filled in and your command should respond to them appropriately.  For
> example:
>
> bisect-skip-count bisect-command.sh "%(skip)d" "%(count)d" 2>&1 | tee
> bisect.out
>
> bisect-command.sh presumably looks something like this:
>
>   #!/bin/bash
>
>   skip=$1
>   count=$2
>
>   opt --debug-counter=my-counter-skip=${skip},my-counter-count=${count}
>   ...
>
> I recently used bisect-skip-count in this way very successfully to track
> down an aliasing bug deep in the machine scheduler.  I'm working on
> documenting bisect-skip-count so people know about it.  I can add
> comments to the script but I haven't looked at updating web page sources
> yet.  I was thinking of adding something to the existing opt-bisect
> page.  Guidance here would be helpful.
>
> I agree that anyone who adds DebugCounters should propose those changes
> on Phabricator.  We can incrementally improve the debuggability of LLVM
> with such a process.
>
>                                 -David
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180926/850029a3/attachment.html>