warn against using intrinsics that leave the scope of our memory model

[RalfJung]

No description provided.

[rustbot]

r? @joshtriplett

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[Urgau]

Can't we just remove them? At least for atomic_{load,store}_unordered the PR introducing them #19835 didn't have a use-case for them and they don't seems to be used anywhere in this repo (including stdarch).

nontemporal_store is used within stdarch, so that a no go for now

[the8472]

IIRC they were added for experimentation. Maybe the warning could be a bit more informative?

For whom is that warning anyway? std/compiler devs or for nightly users?
I.e. are you trying to convey that they shouldn't be exposed through stable std APIs or that nightly users are moving on thin ice when they experiment with this? Because the latter could be reasonable if they're doing that to experiment and demonstrate performance differences.

[RalfJung]

I have clarified: in terms of the AM, I think the best that we can do is that these intrinsics act like regular non-atomic reads and writes.

[the8472]

Hrrm, I still think the language is a bit too unspecific. Then again, people who are just doing experiments are free to ignore any such warning anyway, so whatever.

[RalfJung]

Can't we just remove them? At least for atomic_{load,store}_unordered the PR introducing them #19835 didn't have a use-case for them and they don't seems to be used anywhere in this repo (including stdarch).

They are used by compiler_builtins. We'll need to take a close look at that eventually, but for now we can at least discourage further usage of them.

[RalfJung]

@the8472 can you take the review of this one, since you already took a look?

[the8472]

@bors r+ rollup

[bors]

📌 Commit 79ad512 has been approved by the8472

It is now in the queue for this repository.

[bors]

⌛ Testing commit 79ad512 with merge 2f1ba4a...

[bors]

☀️ Test successful - checks-actions
Approved by: the8472
Pushing 2f1ba4a to master...

[rust-timer]

Finished benchmarking commit (2f1ba4a): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.1%, -0.1%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 672.67s -> 671.206s (-0.22%)
Artifact size: 314.11 MiB -> 314.12 MiB (0.01%)