vector_algorithms.cpp: Add [[nodiscard]] to _Use_avx2() and _Use_sse42()

[jovibor]

Adds [[nodiscard]] to _Use_avx2() and _Use_sse42() functions.

[YexuanXiao]

The query results can be cached as std::atomic<bool>s with namespace scope and internal linkage. This pattern helps generate cleaner and more stable code than magic static.

[YexuanXiao]

MSVC can optimize the function-scoped static atomic to be the same as one with internal linkage at namespace scope, without using the magic static mechanism, and magic static is worse, see: https://godbolt.org/z/r4zr9oE7s

[StephanTLavavej]

No - <atomic> is a non-core header, which we can't drag into the import lib. See #3011.

Aside from that, the complexity of introducing atomics isn't worth it here. We're reading an integer, not doing anything outrageously expensive, and branch prediction is going to be really happy with a result that doesn't change.

[jovibor]

Dragging-in atomics for such a tiny function would definitely be an overkill, imo.
I reverted changes back, leaving only [[nodiscard]].

@StephanTLavavej
This introduces magic statics, which we avoid (they have unusual interactions with certain code).

Can you give any hints, what exactly these unusual interactions is, or where to read about it?
As far as I know the "magic statics" are guaranteed to be thread-safe and one-time initialized since C++11.

[StephanTLavavej]

As I recall, they create a TLS section in the resulting binary, which had weird interactions with an optimization that Windows wanted to do.

[YexuanXiao]

I have re-showd my comments, which compare magic static and atomic. The conclusion is that if you want the best performance, then use lock-free atomics, or do not cache the result. Although I have only shown the results for MSVC, this actually applies to all compilers and operating systems.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo. Please notify me if any further changes are pushed, otherwise no action is required.

[StephanTLavavej]

Thanks for adding our favorite attribute! 😻