[menaerus]

Hm, not sure. I can see that shared_ptr::_M_release [0] is implemented in terms of __exchange_and_add_dispatch [1] and which is implemented in terms of __is_single_threaded [2]. __is_single_threaded will use __gthread_active_p iff __GTHREADS is not defined and <sys/single_threaded.h> header not included.

Implementation of __gthread_active_p is indeed a runtime check [3] which AFAICS applies only to single-threaded programs. Perhaps the shared-library use-case also fits here?

Strange optimization IMHO so I wonder what was the motivation behind it. The cost function being optimized in this case is depending on WORD being atomic [4] without actually using the atomics [5].

[0] https://codebrowser.dev/llvm/include/c++/11/bits/shared_ptr_...

[1] https://codebrowser.dev/llvm/include/c++/11/ext/atomicity.h....

[2] https://codebrowser.dev/llvm/include/c++/11/ext/atomicity.h....

[3] https://codebrowser.dev/kde/include/x86_64-linux-gnu/c++/11/...

[4] https://codebrowser.dev/llvm/include/c++/11/ext/atomicity.h....

[5] https://codebrowser.dev/llvm/include/c++/11/ext/atomicity.h....

[aw1621107]

> Implementation of __gthread_active_p is indeed a runtime check [3] which AFAICS applies only to single-threaded programs. Perhaps the shared-library use-case also fits here?

The line you linked is for some FreeBSD/Solaris versions which appear to have some quirks with the way pthreads functions are exposed in their libc. I think the "normal" implementation of __gthread_active_p is on line 248 [0], and that is a pretty straightforwards check against a weak symbol.

> Strange optimization IMHO so I wonder what was the motivation behind it.

I believe the motivation is to avoid needing to pay the cost of atomics when there is no parallelism going on.

> The cost function being optimized in this case is depending on WORD being atomic [4] without actually using the atomics [5].

Not entirely sure what you're getting at here? The former is used for single-threaded programs so there's ostensibly no need for atomics, whereas the latter is used for non-single-threaded programs.

[0]: https://codebrowser.dev/kde/include/x86_64-linux-gnu/c++/11/...

[menaerus]

> Not entirely sure what you're getting at here?

> I believe the motivation is to avoid needing to pay the cost of atomics when there is no parallelism going on.

Obviously yes. What I am wondering is what benefit does it bring in practice. Single-threaded program with shared-ptr's using atomics vs shared-ptr's using WORDs seem like a non-problem to me - e.g. I doubt it has a measurable performance impact. Atomics are slowing down the program only when it comes to contention, and single-threaded programs can't have them.

[aw1621107]

> What I am wondering is what benefit does it bring in practice. Single-threaded program with shared-ptr's using atomics vs shared-ptr's using WORDs seem like a non-problem to me - e.g. I doubt it has a measurable performance impact.

I mean, the blog post basically starts with an example where the performance impact is noticeable:

> I found that my Rust port of an immutable RB tree insertion was significantly slower than the C++ one.

And:

> I just referenced pthread_create in the program and the reference count became atomic again.

> Although uninteresting to the topic of the blog post, after the modifications, both programs performed very similarly in the benchmarks.

So in principle an insert-heavy workload for that data structure could see a noticeable performance impact.

> Atomics are slowing down the program only when it comes to contention, and single-threaded programs can't have them.

Not entirely sure I'd agree? My impression is that while uncontended atomics are not too expensive they aren't exactly free compared to the corresponding non-atomic instruction. For example, Agner Fog's instruction tables [0] states:

> Instructions with a LOCK prefix have a long latency that depends on cache organization and possibly RAM speed. If there are multiple processors or cores or direct memory access (DMA) devices, then all locked instructions will lock a cache line for exclusive access, which may involve RAM access. A LOCK prefix typically costs more than a hundred clock cycles, even on single-processor systems. This also applies to the XCHG instruction with a memory operand.

And there's this blog post [1], which compares the performance of various concurrency mechanisms/implementations including uncontended atomics and "plain" code and shows that uncontended atomics are still slower than non-atomic operations (~3.5x if I'm reading the raw data table correctly).

So if the atomic instruction is in a hot loop then I think it's quite plausible that it'll be noticeable.

[0]: https://www.agner.org/optimize/instruction_tables.pdf

[1]: https://travisdowns.github.io/blog/2020/07/06/concurrency-co...

[menaerus]

Thanks, I'll revisit your comment. Some interesting things you shared.