Tips and Tricks on using and contributing to Polly

Commiting to polly trunk

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

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.