Optimally a compiler's version of a language is better defined simply by it's definitions. It's just problematic if the behavior is changed between versions, eg

Optimally a compiler's version of a language is better defined simply by it's definitions. It's just problematic if the behavior is changed between versions, eg. for the sake of optimizations. It's also bad if undefined behavior is expected to be reliable where the original implementation wasn't intended to be meaningful in the first place. If those two cases are hard to tell apart, the process of announcing this is at fault, I guess, e.g. the error messages and warnings, the documentation, the info hidden in discussions on the mailinglists or in bug reports, but the gcc docs seem fairly extensive and I can't claim to have read it completely or the c standard.