Update `__rust_[rd]ealloc` to take `NonNull<u8>` instead of `*mut u8` by scottmcm · Pull Request #153251 · rust-lang/rust

scottmcm · 2026-03-01T07:52:42Z

Passing null to it is already UB per Miri anyway, and this is ABI-compatible as NonNull is repr(transparent).

Inspired by #152702 (comment) ; Similar to #152605 which changed align: usize to align: ptr::Alignment.

Passing null to it is [already UB](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=8dcd25a549c11de72adc94a668277779) anyway.

rustbot · 2026-03-01T07:52:47Z

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scottmcm · 2026-03-01T07:56:46Z

...mir-opt/pre-codegen/drop_boxed_slice.generic_in_place.PreCodegen.after.32bit.panic-abort.mir

-        StorageLive(_9);
-        _9 = copy _8 as *mut u8 (Transmute);
-        _10 = alloc::alloc::__rust_dealloc(move _9, move _5, move _6) -> [return: bb3, unwind unreachable];
+        _9 = alloc::alloc::__rust_dealloc(move _8, move _5, move _6) -> [return: bb3, unwind unreachable];


Notably, this lets us avoid the transmute back from NonNull to *mut.

All the alloc code is doing the NonNull conversion in practice anyway, as it's using https://doc.rust-lang.org/std/alloc/struct.Global.html#method.deallocate not the alloc::dealloc function directly.

joboet · 2026-03-01T13:18:27Z

Makes sense to me. Let's see if this has any effect on performance...
@bors try @rust-timer queue

Update `__rust_[rd]ealloc` to take `NonNull<u8>` instead of `*mut u8`

rust-bors · 2026-03-01T15:28:46Z

☀️ Try build successful (CI)
Build commit: 0fd2a86 (0fd2a86d7a2a7268acb5d51b0f12ac84d755c77f, parent: 765fd2d8c77a570e7069d9f30bb6d3d8fe437f9e)

rust-timer · 2026-03-01T16:09:18Z

Finished benchmarking commit (0fd2a86): comparison URL.

Overall result: ❌ regressions - no action needed

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.

@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)	0.9%	[0.9%, 0.9%]	1
Regressions ❌ (secondary)	0.5%	[0.5%, 0.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.9%	[0.9%, 0.9%]	1

Max RSS (memory usage)

Results (primary -0.1%, secondary 2.3%)

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

	mean	range	count
Regressions ❌ (primary)	3.2%	[2.4%, 4.0%]	3
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-3.4%	[-4.9%, -2.2%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-4.9%, 4.0%]	6

Cycles

Results (secondary -6.3%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.3%	[-7.2%, -5.2%]	3
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 477.793s -> 479.944s (0.45%)
Artifact size: 397.19 MiB -> 395.22 MiB (-0.50%)

scottmcm · 2026-03-01T18:46:32Z

That perf run looks fine to me. The one icount regression looks like just inlining noise from different MIR

With no cycle regressions I think there's nothing problematic.

joboet · 2026-03-01T21:57:34Z

Yeah, that looks acceptable.
@bors r+

rust-bors · 2026-03-01T21:57:37Z

📌 Commit 2c6ec51 has been approved by joboet

It is now in the queue for this repository.

bjorn3 · 2026-03-02T08:37:52Z

library/alloc/src/alloc.rs

+#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
+unsafe fn dealloc_nonnull(ptr: NonNull<u8>, layout: Layout) {
    unsafe { __rust_dealloc(ptr, layout.size(), layout.alignment()) }
 }


Might be worth it to manually inline these functions. They are tiny and not even encapsulating any unsafeness or tricky logic. Would make optimizations slightly faster and backtraces slightly shorter.

I would expect that it's already disappeared in the shipped liballoc rlib since the mir inliner should have done that for us already. So it would remove a inlined entry in the mir scopes, but that's it, afaict.

MIR inlined calls do still show in backtraces.

rust-bors · 2026-03-02T12:01:29Z

☀️ Test successful - CI
Approved by: joboet
Duration: 3h 51m 12s
Pushing 8d50bcc to main...

github-actions · 2026-03-02T12:04:26Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing ddd36bd (parent) -> 8d50bcc (this PR)

Test differences

Show 11 test diffs

11 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 8d50bccc5bd70be9f5a2fc98c0857e24b3dcdf85 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

x86_64-rust-for-linux: 52m 46s -> 42m 43s (-19.0%)
x86_64-gnu-gcc: 1h 8m -> 55m 20s (-18.7%)
aarch64-gnu-llvm-20-1: 1h 1m -> 52m 58s (-13.9%)
i686-gnu-1: 2h 23m -> 2h 6m (-12.1%)
pr-check-1: 30m 53s -> 27m 42s (-10.3%)
i686-gnu-2: 1h 41m -> 1h 30m (-10.3%)
dist-apple-various: 1h 43m -> 1h 53m (+9.6%)
i686-gnu-nopt-1: 2h 19m -> 2h 6m (-9.3%)
pr-check-2: 42m 29s -> 38m 33s (-9.3%)
x86_64-gnu-aux: 2h 24m -> 2h 11m (-9.0%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-03-02T12:42:10Z

Finished benchmarking commit (8d50bcc): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -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)	0.8%	[0.8%, 0.8%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.8%	[0.8%, 0.8%]	1

Max RSS (memory usage)

Results (primary 1.6%, secondary 1.4%)

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

	mean	range	count
Regressions ❌ (primary)	4.2%	[2.2%, 7.8%]	5
Regressions ❌ (secondary)	4.7%	[1.1%, 8.3%]	2
Improvements ✅ (primary)	-2.7%	[-2.7%, -2.6%]	3
Improvements ✅ (secondary)	-5.3%	[-5.3%, -5.3%]	1
All ❌✅ (primary)	1.6%	[-2.7%, 7.8%]	8

Cycles

Results (primary 2.0%, secondary -1.0%)

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

	mean	range	count
Regressions ❌ (primary)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	4.6%	[4.6%, 4.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-4.5%, -3.1%]	2
All ❌✅ (primary)	2.0%	[2.0%, 2.0%]	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)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.6%	[-0.6%, -0.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-0.6%, 0.6%]	2

Bootstrap: 478.623s -> 479.657s (0.22%)
Artifact size: 397.00 MiB -> 396.97 MiB (-0.01%)

Update __rust_[rd]ealloc to take NonNull<u8> instead of *mut u8

2c6ec51

Passing null to it is [already UB](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=8dcd25a549c11de72adc94a668277779) anyway.

rustbot assigned joboet Mar 1, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Mar 1, 2026

scottmcm commented Mar 1, 2026

View reviewed changes

This was referenced Mar 1, 2026

GVN: transmute adts to their fields if a field projection is immediately transmuted anyway #153085

Open

Prepare NonNull for pattern types #152702

Merged

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Mar 1, 2026

rust-bors bot pushed a commit that referenced this pull request Mar 1, 2026

Auto merge of #153251 - scottmcm:deallocate-nonnull, r=<try>

0fd2a86

Update `__rust_[rd]ealloc` to take `NonNull<u8>` instead of `*mut u8`

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Mar 1, 2026

rust-bors bot 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 Mar 1, 2026

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bjorn3 reviewed Mar 2, 2026

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rust-bors bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Mar 2, 2026

rust-bors bot merged commit 8d50bcc into rust-lang:main Mar 2, 2026
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rustbot added this to the 1.96.0 milestone Mar 2, 2026

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Overall result: ❌ regressions - no action needed

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Test differences

Job duration changes

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rust-timer commented Mar 2, 2026

Overall result: ❌ regressions - no action needed

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scottmcm commented Mar 1, 2026 •

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