It started to happen today in docker-based builds. There is high chance that an executable built with GCC6 or clang5 will be invalid it will crash on startup with exception “17254 Illegal instruction”.
See this branch: https://circleci.com/gh/ethereum/cpp-ethereum/tree/circleci-mac
Rebuilding sometimes helps, so the issue might be with the host machine. E.g.:
The failing build: https://circleci.com/gh/ethereum/cpp-ethereum/521
The rebuild of the previous one: https://circleci.com/gh/ethereum/cpp-ethereum/523
It looks the crash happens on host with arch: arch1-linux-amd64-6_62 but not on arch1-linux-amd64-6_63.