================================================== Tips and Tricks on using and contributing to Polly

Committing to polly trunk

- [General reference to git-svn workflow](https://stackoverflow.com/questions/190431/is-git-svn-dcommit-after-merging-in-git-dangerous)

Using bugpoint to track down errors in large files

If you know the ``opt`` invocation and have a large ``.ll`` file that causes
an error, ``bugpoint`` allows one to reduce the size of test cases.

The general calling pattern is:

- ``$ bugpoint <file.ll> <pass that causes the crash> -opt-args <opt option flags>``

An example invocation is:

- ``$ bugpoint crash.ll -polly-codegen -opt-args  -polly-canonicalize -polly-process-unprofitable``

For more documentation on bugpoint, [Visit the LLVM manual](https://llvm.org/docs/Bugpoint.html)

Understanding which pass makes a particular change

If you know that something like `opt -O3 -polly` makes a change, but you wish to
isolate which pass makes a change, the steps are as follows:

- ``$ bugpoint -O3 file.ll -opt-args -polly``  will allow bugpoint to track down the pass which causes the crash.

To do this manually:

- ``$ opt -O3 -polly -debug-pass=Arguments`` to get all passes that are run by default. ``-debug-pass=Arguments`` will list all passes that have run.
- Bisect down to the pass that changes it.

Debugging regressions introduced at some unknown earlier point

In case of a regression in performance or correctness (e.g., an earlier version of Polly behaved as expected and a later version does not), bisecting over the version history is the standard approach to identify the commit that introduced the regression.

LLVM has a single repository that contains all projects. It can be cloned at: <https://github.com/llvm/llvm-project>. How to bisect on a git repository is explained here <https://www.metaltoad.com/blog/beginners-guide-git-bisect-process-elimination>. The bisect process can also be automated as explained here: <https://www.metaltoad.com/blog/mechanizing-git-bisect-bug-hunting-lazy>. An LLVM specific run script is available here: <https://gist.github.com/dcci/891cd98d80b1b95352a407d80914f7cf>.