Use pointers in `cell::{Ref,RefMut}` to avoid `noalias` #97027

cuviper · 2022-05-13T19:09:32Z

When Ref and RefMut were based on references, they would get LLVM noalias attributes that were incorrect, because that alias guarantee is only true until the guard drops. A &RefCell on the same value can get a new borrow that aliases the previous guard, possibly leading to miscompilation. Using NonNull pointers in Ref and RefCell avoids noalias.

Fixes the library side of #63787, but we still might want to explore language solutions there.

rust-highfive · 2022-05-13T19:09:36Z

r? @thomcc

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CAD97

Ref doesn't need a PhantomData<&T> for correctness, but it might make sense to include it for explicitness / symmetry with RefMut?

Also, I forget how NonNull acts w.r.t. UnwindSafe; this is a reminder to someone to check that it's not changed by this PR.

RalfJung · 2022-05-13T21:26:34Z

library/core/src/cell.rs

@@ -1548,23 +1557,20 @@ impl<'b, T: ?Sized> RefMut<'b, T> {
    /// ```
    #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
    #[inline]
-    pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
+    pub fn filter_map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
    where
        F: FnOnce(&mut T) -> Option<&mut U>,
    {
        // FIXME(nll-rfc#40): fix borrow-check


I assume this no longer applies now?

Maybe? I'm not really sure what needed fixing -- was it that orig had to be destructured, then rebuilt in the Err case?

cuviper · 2022-05-13T23:04:00Z

@CAD97 :

Ref doesn't need a PhantomData<&T> for correctness, but it might make sense to include it for explicitness / symmetry with RefMut?

I could just add a comment addressing covariance, but are there more effects you want from PhantomData?

Also, I forget how NonNull acts w.r.t. UnwindSafe; this is a reminder to someone to check that it's not changed by this PR.

All of the auto traits are unchanged, at least according to rustdoc. NonNull does implement UnwindSafe for T: RefUnwindSafe, but Ref and RefMut do not.

thomcc · 2022-05-14T04:01:24Z

I think this is fine as-is, no need for additional PhantomData.

@bors r+

bors · 2022-05-14T04:01:26Z

📌 Commit 15d8c00 has been approved by thomcc

Use pointers in `cell::{Ref,RefMut}` to avoid `noalias` When `Ref` and `RefMut` were based on references, they would get LLVM `noalias` attributes that were incorrect, because that alias guarantee is only true until the guard drops. A `&RefCell` on the same value can get a new borrow that aliases the previous guard, possibly leading to miscompilation. Using `NonNull` pointers in `Ref` and `RefCell` avoids `noalias`. Fixes the library side of rust-lang#63787, but we still might want to explore language solutions there.

bors · 2022-05-15T10:46:32Z

⌛ Testing commit 15d8c00 with merge 0b2b6a751bfaaef94c621925159a481da0356b9d...

bors · 2022-05-15T11:19:23Z

💔 Test failed - checks-actions

cuviper · 2022-05-17T00:40:08Z

I took a stab at fixing natvis, but I'm just guessing...

thomcc · 2022-05-19T15:34:54Z

(I'm assuming I need to r+ this again)

@bors r+

bors · 2022-05-19T15:34:56Z

📌 Commit 1c3921f has been approved by thomcc

bors · 2022-05-19T15:34:57Z

🌲 The tree is currently closed for pull requests below priority 1000. This pull request will be tested once the tree is reopened.

bors · 2022-05-20T01:05:56Z

⌛ Testing commit 1c3921f with merge 4d6992b...

bors · 2022-05-20T03:26:56Z

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 4d6992b to master...

rust-highfive · 2022-05-20T03:27:25Z

📣 Toolstate changed by #97027!

Tested on commit 4d6992b.
Direct link to PR: #97027

💔 miri on windows: test-pass → test-fail (cc @oli-obk @eddyb @RalfJung).
💔 miri on linux: test-pass → test-fail (cc @oli-obk @eddyb @RalfJung).

@oli-obk

Tested on commit rust-lang/rust@4d6992b. Direct link to PR: <rust-lang/rust#97027> 💔 miri on windows: test-pass → test-fail (cc @oli-obk @eddyb @RalfJung). 💔 miri on linux: test-pass → test-fail (cc @oli-obk @eddyb @RalfJung).

rust-timer · 2022-05-20T05:15:04Z

Finished benchmarking commit (4d6992b): comparison url.

Instruction count

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

Max RSS (memory usage)

Results

Primary benchmarks: 😿 relevant regression found
Secondary benchmarks: mixed results

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count¹	1	1	0	2	1
mean²	0.1%	3.3%	N/A	-3.1%	0.1%
max	0.1%	3.3%	N/A	-3.2%	0.1%

Cycles

Results

Primary benchmarks: no relevant changes found
Secondary benchmarks: mixed results

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count¹	0	1	0	2	0
mean²	N/A	1.7%	N/A	-2.4%	N/A
max	N/A	1.7%	N/A	-3.0%	N/A

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

number of relevant changes ↩ ↩²
the arithmetic mean of the percent change ↩ ↩²

RalfJung · 2022-05-20T06:07:48Z

The Miri tests fail because this affects the Display output of Ref and RefMut: they used to print the value, now they print the address of the RefCell. That is probably a bug?

rustup `Display` of `Ref`/`RefMut` was broken by rust-lang/rust#97027, let's deref them to use the underlying reference `Display`. Cc rust-lang/rust#97204

cuviper · 2022-05-20T18:05:50Z

Ah, yes, I'll fix that. The Debug impls use Deref, but Display is directly reaching for self.value (now a pointer).

These guards changed to pointers in rust-lang#97027, but their `Display` was formatting that field directly, which made it show the raw pointer value. Now we go through `Deref` to display the real value again.

Fix `Display` for `cell::{Ref,RefMut}` These guards changed to pointers in rust-lang#97027, but their `Display` was formatting that field directly, which made it show the raw pointer value. Now we go through `Deref` to display the real value again. Miri noticed this change, rust-lang#97204, so hopefully that will be fixed.

cuviper added 3 commits May 13, 2022 11:42

Use a pointer in cell::Ref so it's not noalias

d369045

Use a pointer in cell::RefMut so it's not noalias

2b8041f

Test RefCell aliasing

15d8c00

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cuviper mentioned this pull request May 13, 2022

Ref parameter incorrectly decorated with noalias attribute #63787

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 14, 2022

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GuillaumeGomez mentioned this pull request May 14, 2022

Rollup of 4 pull requests #97050

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Dylan-DPC mentioned this pull request May 15, 2022

Rollup of 4 pull requests #97054

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bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels May 15, 2022

cuviper added 3 commits May 16, 2022 17:22

Remove outdated references to nll-rfc#40

1e53fab

Add a comment for covariant Ref

1f33c92

Read the Ref/RefMut pointer in natvis

1c3921f

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 19, 2022

bors added the merged-by-bors This PR was explicitly merged by bors. label May 20, 2022

bors merged commit 4d6992b into rust-lang:master May 20, 2022

rustbot added this to the 1.63.0 milestone May 20, 2022

rust-highfive mentioned this pull request May 20, 2022

miri no longer builds after rust-lang/rust#97027 #97204

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RalfJung mentioned this pull request May 20, 2022

rustup rust-lang/miri#2130

Merged

cuviper mentioned this pull request May 20, 2022

Fix Display for cell::{Ref,RefMut} #97225

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m-ou-se mentioned this pull request Jun 11, 2022

Make RwLockReadGuard covariant #96820

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cuviper deleted the yesalias-refcell branch October 15, 2022 00:16

Ten0 mentioned this pull request Nov 23, 2022

Possible approach to fix #71 Kimundi/owning-ref-rs#72

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yvt mentioned this pull request Nov 23, 2022

Stop storing a reference in a lock guard r3-os/r3#15

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Imberflur mentioned this pull request Feb 4, 2023

Use NonNull pointer in the place of references in AtomicRef and AtomicRefMut to avoid noalias related soundness bug. bholley/atomic_refcell#18

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Use pointers in `cell::{Ref,RefMut}` to avoid `noalias` #97027

Use pointers in `cell::{Ref,RefMut}` to avoid `noalias` #97027

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Use pointers in cell::{Ref,RefMut} to avoid noalias #97027

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RalfJung commented May 20, 2022

cuviper commented May 20, 2022

Use pointers in `cell::{Ref,RefMut}` to avoid `noalias` #97027

Use pointers in `cell::{Ref,RefMut}` to avoid `noalias` #97027