GCC and clang developers already do regularly compile and test a limited number of open source packages, and investigate any newly occurring failures. For now

GCC and clang developers already do regularly compile and test a limited number of open source packages, and investigate any newly occurring failures. For now they ignore failures if the code does not conform to standards (except for notifying upstream developers). In principle, in such cases they could expend effort to find similar (possibly simpler) optimizations that do not change the existing behavior of the code. Alternatively, they could disable the relevant optimizations at -O2 (and maybe -O3) level. I believe MSVC developers do put more effort than GCC/Clang developers to maintain source-level backwards compatibility. Alternative development strategies are certainly possible, but I understand that the GCC/Clang developers do face pressure from a significant subset of users to maximize performance at any cost.