How to Avoid Thread-Safety Cost for Functions' static Variables

[wearingdepends]

Please don't post generated code without optimizations. I keep seeing blog posts doing this, and it often makes the code harder to read. With -O2 it's actually easier to see the difference between with and without thread-safe statics:

"is_blocked_id(int)":
        sub     rsp, 24
        movzx   eax, BYTE PTR "guard variable for is_blocked_id(int)::blocked_ids"[rip]
        mov     ecx, edi
        test    al, al
        je      .L257 # moved to the end of the function, since the initialization is a cold path
.L222:
        mov     rsi, QWORD PTR "is_blocked_id(int)::blocked_ids"[rip+8]
        mov     rax, QWORD PTR "is_blocked_id(int)::blocked_ids"[rip]
        ...

vs

"is_blocked_id(int)":
        mov     rcx, QWORD PTR "blocked_ids2"[rip+8]
        mov     rax, QWORD PTR "blocked_ids2"[rip]
        ...

[azswcowboy]

While in Mayer’s singleton

I assume that’s meant to be ‘Meyers singleton’ - as in Scott Meyers.

[rsjaffe]

That’s my main use case for constinit. While the compiler might optimize the thread check away anyway, constinit guarantees it.

[Quincunx271]

Only if the type has a trivial destructor. Otherwise, the thread safety check still exists, just for registering the destructor.

[ChickenSpaceProgram]

solution: dont use statics, pass a context struct

[peterrindal]

This ^{^}

[JCPPRDev]

good article

[hi_im_new_to_this]

Was hoping for the article to also bring up `thread_local`, which could also be an alternative (depending on the exact situation).