I am working to get the bugs out of Phoenix and have a situation where some tests are optimisation unstable - they run on Clang 3.5 with -O1 but not -O2 and on gcc 4.9.0 with -O0 but not -O1. The higher optimisation gives an erroneous return value. I am looking for tools to work with this. I have found one called Stack (http://css.csail.mit.edu/stack/). I have built it but it only has instructions for whole projects rather than individual tests. It is hard to search for because the word 'stack' has lots of other meanings. Has anyone else used this or any other tools to solve this sort of problem? It is worth noting that some of the tests run fine on develop testing. That leads me to wonder if the level of optimisation used there is documented anywhere. Thanks John