When built with LTO enabled, GCC will warn about violations of C++'s one-definition rule. JSC currently has two violations. The first is JSC::SpeciesConstructResult, which has two different declarations: runtime/ArrayPrototype.cpp:enum class SpeciesConstructResult { runtime/JSArrayBufferPrototype.cpp:enum class SpeciesConstructResult : uint8_t { There are usually several different ways to fix any particular ODR violation, starting with renaming one or the other use. I decided to add : uint8_t to the version in ArrayPrototype.cpp to make the declarations match. I could alternatively have placed them in anonymous namespaces to restrict them to file scope. The next problem is JSC::SignalContext. We have two different versions of this struct, one in VMTraps.cpp and the other in SigillCrashAnalyzer.cpp. In this case, I decided the simplest approach would be to change the one in VMTraps.cpp from JSC::SignalContext to JSC::VMTraps::SignalContext.