[llvm-dev] Pass and Transformation-level debugging in LLVM (original) (raw)

Zhizhou Yang via llvm-dev llvm-dev at lists.llvm.org
Wed Sep 26 13:36:54 PDT 2018

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>

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