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std: move check for destroyed TLS variable #148037

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@joboet joboet commented Oct 23, 2025

When accessing a TLS variable, the comparison that checks whether the variable is initialised can be inlined to the call site. The comparison that checks whether the variable has been destroyed is however performed in the #[cold] accessor function. Since all call-sites need to check whether the returned pointer is null, this split prevents these two checks from being merged into one. Thus moving the liveliness check into the inlineable accessor may yield more optimal codegen. This is especially true in the case where the variable does not need destruction and thus no null pointer will ever be returned, as the null-pointer check can now be optimised out.

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 23, 2025
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joboet commented Oct 23, 2025

@bors try @rust-timer queue

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rust-bors bot added a commit that referenced this pull request Oct 23, 2025
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Auto merge of #148037 - joboet:destroyed-tls-check, r=<try>
db38fd3 
std: move check for destroyed TLS variable
@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 23, 2025
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The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot) 
finished building tool typos
error: `Uninitalized` should be `Uninitialized`
   ╭▸ library/std/src/sys/thread_local/native/lazy.rs:67:40
   │
67 │     /// * The state of `self` must be `Uninitalized`.
   ╰╴                                       ━━━━━━━━━━━━
rerun tidy with `--extra-checks=spellcheck --bless` to fix typos
tidy [extra_checks]: checks with external tool 'typos' failed
tidy [extra_checks]: FAIL
tidy: The following check failed: extra_checks
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1280:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:02:33
  local time: Thu Oct 23 15:55:15 UTC 2025
  network time: Thu, 23 Oct 2025 15:55:15 GMT
##[error]Process completed with exit code 1.

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rust-bors bot commented Oct 23, 2025

☀️ Try build successful (CI)
Build commit: db38fd3 (db38fd3601bd22cc534c47281f839697adae4273, parent: 11d2046fe9962720558cb15f72541f7fd170dec9)

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Finished benchmarking commit (db38fd3): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

mean range count
Regressions ❌
(primary)		 0.1% [0.0%, 0.2%] 30
Regressions ❌
(secondary)		 0.3% [0.0%, 0.9%] 38
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) 0.1% [0.0%, 0.2%] 30

Max RSS (memory usage)

Results (primary -0.6%, secondary -2.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

mean range count
Regressions ❌
(primary)		 3.4% [3.4%, 3.5%] 2
Regressions ❌
(secondary)		 - - 0
Improvements ✅
(primary)		 -2.6% [-4.6%, -1.1%] 4
Improvements ✅
(secondary)		 -2.5% [-2.5%, -2.5%] 1
All ❌✅ (primary) -0.6% [-4.6%, 3.5%] 6

Cycles

Results (primary 2.7%, secondary 2.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

mean range count
Regressions ❌
(primary)		 2.7% [2.7%, 2.7%] 1
Regressions ❌
(secondary)		 2.1% [2.1%, 2.1%] 1
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) 2.7% [2.7%, 2.7%] 1

Binary size

Results (primary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

mean range count
Regressions ❌
(primary)		 0.1% [0.0%, 0.1%] 6
Regressions ❌
(secondary)		 - - 0
Improvements ✅
(primary)		 -0.4% [-0.6%, -0.1%] 2
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) -0.0% [-0.6%, 0.1%] 8

Bootstrap: 474.995s -> 473.568s (-0.30%)
Artifact size: 390.51 MiB -> 390.48 MiB (-0.01%)

@rustbot rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Oct 23, 2025
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