[polly] r305633 - Add information about bisecting

[Davide Italiano via llvm-commits]

On Sat, Jun 17, 2017 at 12:45 AM, Tobias Grosser via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: grosser
> Date: Sat Jun 17 02:45:44 2017
> New Revision: 305633
>
> URL: http://llvm.org/viewvc/llvm-project?rev=305633&view=rev
> Log:
> Add information about bisecting
>
> Modified:
>     polly/trunk/docs/TipsAndTricks.rst
>
> Modified: polly/trunk/docs/TipsAndTricks.rst
> URL: http://llvm.org/viewvc/llvm-project/polly/trunk/docs/TipsAndTricks.rst?rev=305633&r1=305632&r2=305633&view=diff
> ==============================================================================
> --- polly/trunk/docs/TipsAndTricks.rst (original)
> +++ polly/trunk/docs/TipsAndTricks.rst Sat Jun 17 02:45:44 2017
> @@ -37,3 +37,16 @@ Understanding which pass makes a particu
>      - ``$ 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-project/llvm-project-20170507>`_. How to bisect on a
> +git repository is explained here
> +`<https://www.metaltoad.com/blog/beginners-guide-git-bisect-process-elimination`.
>

Hi Tobias,
the article you link describes a semi-manual procedure for bisecting.
I think something better can be done in case you have a test of what
fails using `git bisect run`. I created a gist because I kept
forgetting how to do it :) (in particular the return code being `125`
in case of "dubious" revisions).
https://gist.github.com/dcci/891cd98d80b1b95352a407d80914f7cf

I also found this useful when reducing regressions:
https://gcc.gnu.org/wiki/A_guide_to_testcase_reduction

-- 
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare