[Safety | C++26] `std::observable_checkpoint()`

[Taw3e8]

C++Weekly

Topics

• P1494s std::observable().
• P3641 renamed to std::observable_checkpoint
• Usage in Contracts (section 3.5.5). Anywhere else?

Length

Probably a quick one. I imagined a short example (implemented on gcc) but somehow couldn't make it to work.

[LB--]

The links aren't loading for me right now, but if this is what I think it is, I've been wanting something like this for a long time, some sort of "allow setting a breakpoint here but just be a NOP". Super useful for debugging since it's more efficient than conditional breakpoints.

[Taw3e8]

I've been wanting something like this for a long time, some sort of "allow setting a breakpoint here but just be a NOP".

No, it's a 'barrier' of sorts for the UB. Basically if your programm made some progress up to the point of the invication of std::observable_checkpoint(), then even if UB happens after it, compiler is not allowed to optimize before it.

[Taw3e8]

Came up with an example:
https://godbolt.org/z/EqhPjKceP

void protected_ub() {
	glob = 42;
	std::observable_checkpoint();
	
	std::unreachable(); // progress up to checkpoint won't be optimized out
}

and generated assembly:

"protected_ub()":
        mov     DWORD PTR "glob"[rip], 42

notice the lack of ret

[lefticus]

Thank you @Taw3e8 , I'll prioritize this for "soon" (at some point in the next 20 or so episodes, I have a long list :D )