`transmute` should also assume non-null pointers #136735

scottmcm · 2025-02-08T08:24:28Z

Previously it only did integer-ABI things, but this way it does data pointers too. That gives more information in general to the backend, and allows slightly simplifying one of the helpers in slice iterators.

rustbot · 2025-02-08T08:24:36Z

r? @oli-obk

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scottmcm · 2025-02-08T08:27:19Z

tests/codegen/slice-iter-len-eq-zero.rs

+    // CHECK: %[[A:.+]] = load ptr
+    // CHECK-SAME: !nonnull
+    // CHECK: %[[B:.+]] = load ptr
+    // CHECK-SAME: !nonnull


annot: this now goes through the transmute instead of the load-as-nonnull, but still ends up getting the !nonnull on the load as desired. (And all the https://github.com/rust-lang/rust/blob/master/tests/codegen/slice-iter-nonnull.rs tests still pass as well, no updates needed.)

scottmcm · 2025-02-08T09:58:44Z

.../mir-opt/pre-codegen/slice_iter.slice_iter_generic_is_empty.PreCodegen.after.panic-abort.mir

-        let mut _2: *const *const T;
-        let mut _3: *const std::ptr::NonNull<T>;
-        let mut _8: *const T;
+        let mut _2: *const T;
+        let mut _7: *const T;


Really not that substantial a difference, but saved a local and means it no longer has the pointer-to-pointer types.

scottmcm · 2025-02-08T10:01:43Z

@bors try @rust-timer queue

bors · 2025-02-08T10:02:53Z

⌛ Trying commit 48c0083 with merge 6ae82df...

`transmute` should also assume non-null pointers Previously it only did integer-ABI things, but this way it does data pointers too. That gives more information in general to the backend, and allows slightly simplifying one of the helpers in slice iterators.

the8472 · 2025-02-08T10:33:40Z

library/core/src/slice/iter.rs

 /// [`iter_mut`]: slice::iter_mut
 /// [slices]: slice
 #[stable(feature = "rust1", since = "1.0.0")]
+#[repr(C)] // *Not* a guarantee, but keeps the codegen tests consistent


Is this about layout randomization? If so then using the //@ needs-deterministic-layouts test annotation should be better, that way people can still get randomized iters.

Ah, that does sound better.

@rustbot author

#[cfg_attr(not(doc), repr(C))] is also used in some places.

Thanks, the test annotation worked to keep it passing.

@rustbot ready

bors · 2025-02-08T11:47:26Z

☀️ Try build successful - checks-actions
Build commit: 6ae82df (6ae82df21229ba9bca0dd9144cc5781c1875c6df)

rust-timer · 2025-02-08T13:55:31Z

Finished benchmarking commit (6ae82df): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	1.2%	[0.2%, 3.1%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.9%, -0.1%]	12
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.9%, 3.1%]	16

Max RSS (memory usage)

Results (primary -0.4%, secondary 2.4%)

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)	3.0%	[2.1%, 4.3%]	3
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-3.9%	[-6.6%, -2.1%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-6.6%, 4.3%]	6

Cycles

Results (primary 0.9%, secondary 2.8%)

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)	2.7%	[2.7%, 2.7%]	1
Regressions ❌ (secondary)	2.8%	[2.8%, 2.8%]	1
Improvements ✅ (primary)	-0.9%	[-0.9%, -0.9%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.9%	[-0.9%, 2.7%]	2

Binary size

Results (primary -0.2%, secondary -0.0%)

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)	0.1%	[0.0%, 0.4%]	16
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	34
Improvements ✅ (primary)	-0.3%	[-1.8%, -0.0%]	48
Improvements ✅ (secondary)	-0.2%	[-0.5%, -0.1%]	8
All ❌✅ (primary)	-0.2%	[-1.8%, 0.4%]	64

Bootstrap: 781.002s -> 777.406s (-0.46%)
Artifact size: 329.08 MiB -> 328.44 MiB (-0.19%)

lqd · 2025-02-08T13:58:16Z

(syn is currently being noisy)

scottmcm · 2025-02-09T00:05:25Z

Interesting, saved 630K off librustc_driver.so somehow, and a nice improvement in bootstrap time.

Simplify `slice::Iter::next` enough that it inlines Inspired by this zulip conversation: <https://rust-lang.zulipchat.com/#narrow/channel/189540-t-compiler.2Fwg-mir-opt/topic/Feedback.20on.20a.20MIR.20optimization.20idea/near/498579990> Draft for now because it needs rust-lang#136735 to get the codegen tests to pass.

oli-obk · 2025-02-11T09:33:03Z

library/core/src/slice/iter/macros.rs

        } else {
            // SAFETY: for non-ZSTs, the type invariant ensures it cannot be null
-            let $end = unsafe { *(&raw const $this.end_or_len).cast::<NonNull<T>>() };
+            let $end = unsafe { mem::transmute::<*const T, NonNull<T>>($this.end_or_len) };


Would it make sense/work to use NonNull::new_unchecked here and make the body of that use a transmute instead?

In general I would like to move NonNull to using transmutes.

But in this specific case I really don't want to do that, because it adds a UbCheck which would then have major impact on perf because of just how critical next is.

(Though maybe after #136771, once the super-critical methods aren't using this helper macro any more, it could be worth trying.)

scottmcm · 2025-02-13T06:50:12Z

Rebased to fix the conflict with the //@ compile-flags change.

@bors r=oli-obk

bors · 2025-02-13T06:50:15Z

📌 Commit 083672b has been approved by oli-obk

It is now in the queue for this repository.

Previously it only did integer-ABI things, but this way it does data pointers too. That gives more information in general to the backend, and allows slightly simplifying one of the helpers in slice iterators.

scottmcm · 2025-02-13T07:02:35Z

Weird, somehow the rebase got the indent wrong and broke tidy. Well, fixed 🤷

@bors r=oli-obk

bors · 2025-02-13T07:02:38Z

📌 Commit 0cc14b6 has been approved by oli-obk

It is now in the queue for this repository.

bors · 2025-02-14T09:06:20Z

⌛ Testing commit 0cc14b6 with merge d88ffcd...

bors · 2025-02-14T11:59:31Z

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing d88ffcd to master...

rust-timer · 2025-02-14T13:17:00Z

Finished benchmarking commit (d88ffcd): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.3%, 0.8%]	4
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-0.5%	[-1.0%, -0.2%]	7
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	2
All ❌✅ (primary)	-0.2%	[-1.0%, 0.8%]	11

Max RSS (memory usage)

Results (primary -0.5%, secondary -0.0%)

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)	4.9%	[4.9%, 4.9%]	1
Regressions ❌ (secondary)	2.4%	[2.4%, 2.5%]	3
Improvements ✅ (primary)	-3.2%	[-3.7%, -2.7%]	2
Improvements ✅ (secondary)	-3.7%	[-4.0%, -3.3%]	2
All ❌✅ (primary)	-0.5%	[-3.7%, 4.9%]	3

Cycles

Results (primary -1.3%)

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)	-	-	0
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.3%	[-1.3%, -1.3%]	1

Binary size

Results (primary -0.2%, secondary -0.0%)

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)	0.1%	[0.0%, 0.3%]	21
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	34
Improvements ✅ (primary)	-0.3%	[-1.8%, -0.0%]	43
Improvements ✅ (secondary)	-0.2%	[-0.5%, -0.1%]	8
All ❌✅ (primary)	-0.2%	[-1.8%, 0.3%]	64

Bootstrap: 787.772s -> 789.359s (0.20%)
Artifact size: 348.36 MiB -> 347.77 MiB (-0.17%)

Kobzol · 2025-02-18T08:51:40Z

Performance is a wash.

@rustbot label: +perf-regression-triaged

rustbot assigned oli-obk Feb 8, 2025

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 Feb 8, 2025

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Simplify slice::Iter::next enough that it inlines #136771

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transmute should also assume non-null pointers

0cc14b6

Previously it only did integer-ABI things, but this way it does data pointers too. That gives more information in general to the backend, and allows slightly simplifying one of the helpers in slice iterators.

scottmcm force-pushed the transmute-nonnull branch from 083672b to 0cc14b6 Compare February 13, 2025 07:01

bors added the merged-by-bors This PR was explicitly merged by bors. label Feb 14, 2025

bors merged commit d88ffcd into rust-lang:master Feb 14, 2025
7 checks passed

rustbot added this to the 1.86.0 milestone Feb 14, 2025

rustbot added the perf-regression-triaged The performance regression has been triaged. label Feb 18, 2025

scottmcm deleted the transmute-nonnull branch February 19, 2025 01:29

eddyb mentioned this pull request Jul 4, 2025

rustup: update to nightly-2025-05-09 (~1.88) and Rust 2024 edition. Rust-GPU/rust-gpu#249

Merged

delan mentioned this pull request Jul 30, 2025

cargo: Bump rustc to 1.89 servo/servo#36818

Merged

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transmute should also assume non-null pointers #136735

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Overall result: ❌✅ regressions and improvements - please read the text below

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`transmute` should also assume non-null pointers #136735

`transmute` should also assume non-null pointers #136735

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