A common scenario in when working in the build system is to want to check if a particular change results in a change in the build output or not, or a more detailed analysis of such changes.

The compare.sh tool will provide such an analysis, but only if you have a pre-built baseline to compare with.

To facilitate testing changes, especially on distributed job servers, we should add a COMPARE_BUILD make control variable. If this variable is present, the build will run twice, the second time with the code that should be tested (particular make targets, configure arguments, or with a patch file applied), and then automatically run the compare script afterwards against the baseline.