https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97553
Bug ID: 97553 Summary: [missed optimization] constexprness not noticed when UBsan enabled Product: gcc Version: 10.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: eyalroz at technion dot ac.il Target Milestone: --- (GodBolt example: https://godbolt.org/z/Kvan5c) Consider the following code: #include <string_view> constexpr std::string_view f() { return "hello"; } static constexpr std::string_view g() { auto x { f() }; return x.substr(1, 3); } int foo() { return g().length(); } if you compile it with flags `--std=c++17 -O3`, it results in a pleasant: foo(): mov eax, 3 ret but if you also enabled undefined-behavior sanitization, i.e. `--std=c++17 -fsanitize=undefined -O3`, then you get a much longer program with UB-related instrumentation - which is never used. I'm not sure if it's because some optimizations are disabled with UBsan, in which case this might be a "misfeature", or whether they're enabled but the optimization is just missed.