Function-scope static initialization C++11 thread-safe guard variance across compilers

Direct answer

You cannot know the synchronization primitive portably. C++11 and later require thread-safe one-time initialization of function-scope statics with a single-initialization guarantee and happens-before publication, but the Standard does not specify how the guard is implemented. The choice of primitive and its contention profile is implementation-defined and can change with compiler, standard library, platform and release.

Confirmed facts

• C++11 mandates that a function-local static is initialized at most once and that initialization is thread-safe with defined happens-before semantics.
• The Standard does not specify the synchronization mechanism, latency, or contention cost of the guard.
• Observable performance is implementation-defined. It may vary between GCC/libstdc++, Clang/libc++, MSVC and their platform backends, and across versions.

Likely implementation behavior

Implementations typically use a per-static guard variable. The common pattern is a fast atomic fast-path that checks an "already initialized" state for steady-state calls, and a slower synchronization operation on first entry. The first-entry path is likely to involve an atomic compare-exchange plus a heavier primitive such as a mutex-like lock or a once primitive provided by the platform. Exact choices differ by implementation.

When multiple threads enter the same function for the first time they contend on that guard. The first thread performs the initialization while others wait on the implementation's wait primitive. This creates a serializing spike whose cost depends on the chosen primitive, its fairness and the work done inside the initializer. Do not assume lock-free or zero-cost first entry.

Portable reasoning without inspection

Treat first-time function-scope static initialization as potentially expensive and serializing. Portable design guidance is:

1. Avoid heavy work, I/O, allocation or locking inside a function-scope static initializer in latency-sensitive paths.
2. Keep initializers trivial and idempotent. If bounded latency is required, prefer an explicit once primitive you control, such as std::call_once, rather than relying on the implicit guard.
3. Measure first-call latency under concurrent threads versus steady-state calls on the target build to quantify the contention cost.

To narrow the implementation class you need one missing diagnostic detail: compiler name, version, standard library and target OS/arch for the build you care about. With that, you can inspect the binary symbols and generated code around the function for guard acquisition routines and measure the first-call contention on that exact configuration.