Stop emitting UbChecks on every Vec→Slice #150265

scottmcm · 2025-12-22T12:33:17Z

Spotted this in #148766's test changes. It doesn't seem like this ubcheck would catch anything useful; let's see if skipping it helps perf. (After all, this is inside every [] on a vec, among other things.)

scottmcm · 2025-12-22T12:41:20Z

@bors try @rust-timer queue

Stop emitting UbChecks on every Vec→Slice

rust-bors · 2025-12-22T14:58:52Z

☀️ Try build successful (CI)
Build commit: d3405d7 (d3405d79c1b6f5617f04715f0b349c565c6a743d, parent: 000ccd651d6dfeab13f7703d92a5fd7a9ff7510f)

rust-timer · 2025-12-22T15:40:26Z

Finished benchmarking commit (d3405d7): comparison URL.

Overall result: ❌✅ regressions and improvements - 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)	1.6%	[0.6%, 2.4%]	4
Regressions ❌ (secondary)	0.4%	[0.0%, 2.3%]	7
Improvements ✅ (primary)	-1.4%	[-6.0%, -0.1%]	28
Improvements ✅ (secondary)	-0.7%	[-2.2%, -0.1%]	7
All ❌✅ (primary)	-1.0%	[-6.0%, 2.4%]	32

Max RSS (memory usage)

Results (primary 1.5%, secondary 1.9%)

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

	mean	range	count
Regressions ❌ (primary)	3.8%	[1.3%, 6.7%]	6
Regressions ❌ (secondary)	3.3%	[0.9%, 6.1%]	14
Improvements ✅ (primary)	-5.4%	[-7.2%, -3.5%]	2
Improvements ✅ (secondary)	-1.9%	[-2.6%, -1.5%]	5
All ❌✅ (primary)	1.5%	[-7.2%, 6.7%]	8

Cycles

Results (primary -1.0%, secondary -1.9%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.3%, 2.8%]	2
Regressions ❌ (secondary)	2.3%	[2.1%, 2.8%]	3
Improvements ✅ (primary)	-2.4%	[-2.7%, -2.3%]	5
Improvements ✅ (secondary)	-4.0%	[-6.5%, -2.1%]	6
All ❌✅ (primary)	-1.0%	[-2.7%, 2.8%]	7

Binary size

Results (primary -0.2%, secondary -0.5%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 1.2%]	11
Regressions ❌ (secondary)	0.6%	[0.0%, 1.2%]	2
Improvements ✅ (primary)	-0.3%	[-0.9%, -0.0%]	64
Improvements ✅ (secondary)	-0.6%	[-3.6%, -0.0%]	20
All ❌✅ (primary)	-0.2%	[-0.9%, 1.2%]	75

Bootstrap: 481.34s -> 483.129s (0.37%)
Artifact size: 390.37 MiB -> 390.63 MiB (0.07%)

Spotted this in PR148766's test changes. It doesn't seem like this ubcheck would catch anything useful; let's see if skipping it helps perf.

rustbot · 2025-12-22T22:17:51Z

r? @ibraheemdev

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scottmcm · 2025-12-22T22:19:22Z

Reconfirming after rebasing, but should be basically the same
@bors try @rust-timer queue

Stop emitting UbChecks on every Vec→Slice

scottmcm · 2025-12-22T22:21:03Z

library/core/src/slice/cmp.rs

-    fn not_equal(&self, other: &[B]) -> bool {
-        !self.equal(other)
-    }


Annot: nothing actually overrode this anywhere, so removed it in favour of the usual PartialEq::ne.

scottmcm · 2025-12-22T22:33:21Z

tests/mir-opt/jump_threading.chained_conditions.JumpThreading.panic-abort.diff

+          StorageLive(_38);
+          _36 = copy _29 as &[u8] (Transmute);
+          _38 = copy _28 as &[u8] (Transmute);
+          _7 = <[u8] as PartialEq>::eq(move _36, move _38) -> [return: bb19, unwind unreachable];


annot: note that we're still inlining the whole &String → &str → &u8 part (since it'll essentially disappear in LLVM), just stopping at <[_]>::eq which sharing at the MIR level is probably best.

rust-bors · 2025-12-23T00:41:17Z

☀️ Try build successful (CI)
Build commit: fa07ba2 (fa07ba28fd0d71635c2fcc8ab0cb7944dd5fea85, parent: 04813e4de86a5e024e71756ef092637aa862c984)

rust-timer · 2025-12-23T01:22:03Z

Finished benchmarking commit (fa07ba2): comparison URL.

Overall result: ❌✅ regressions and improvements - 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)	1.4%	[0.3%, 2.6%]	5
Regressions ❌ (secondary)	0.6%	[0.0%, 2.5%]	5
Improvements ✅ (primary)	-1.5%	[-6.0%, -0.2%]	26
Improvements ✅ (secondary)	-0.6%	[-2.0%, -0.1%]	10
All ❌✅ (primary)	-1.0%	[-6.0%, 2.6%]	31

Max RSS (memory usage)

Results (primary -0.1%, secondary 1.6%)

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

	mean	range	count
Regressions ❌ (primary)	4.3%	[2.1%, 5.4%]	3
Regressions ❌ (secondary)	3.5%	[0.8%, 5.8%]	9
Improvements ✅ (primary)	-4.6%	[-7.7%, -2.3%]	3
Improvements ✅ (secondary)	-1.7%	[-2.2%, -1.4%]	5
All ❌✅ (primary)	-0.1%	[-7.7%, 5.4%]	6

Cycles

Results (primary -1.0%, 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)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	2.5%	[2.2%, 2.9%]	3
Improvements ✅ (primary)	-2.6%	[-2.8%, -2.3%]	2
Improvements ✅ (secondary)	-3.3%	[-6.9%, -1.4%]	6
All ❌✅ (primary)	-1.0%	[-2.8%, 2.2%]	3

Binary size

Results (primary -0.2%, secondary -0.5%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 1.2%]	8
Regressions ❌ (secondary)	0.6%	[0.0%, 1.2%]	2
Improvements ✅ (primary)	-0.3%	[-0.9%, -0.0%]	57
Improvements ✅ (secondary)	-0.6%	[-3.7%, -0.0%]	20
All ❌✅ (primary)	-0.2%	[-0.9%, 1.2%]	65

Bootstrap: 481.395s -> 479.879s (-0.31%)
Artifact size: 390.31 MiB -> 390.61 MiB (0.08%)

saethlin · 2025-12-23T16:41:00Z

library/alloc/src/vec/mod.rs

-        unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
+        unsafe {
+            // normally this would use `slice::from_raw_parts`, but it's
+            // hot enough that avoiding the UB check is worth it


I wouldn't say "hot" here, I think some readers might take this comment to mean there is runtime overhead to UB checks.

saethlin · 2025-12-23T16:46:47Z

library/core/src/slice/cmp.rs

+    // It's not worth trying to inline the loops underneath here *in MIR*,
+    // and preventing it encourages more useful inlining upstream,
+    // such as in `<str as PartialEq>::eq`.
+    // The codegen backend can still inline it later if needed.
+    #[rustc_no_mir_inline]


If the problem is the loop, why not put the attribute on the generic form that actually has a loop in it, instead of covering the entire entrypoint into the specialized-upon trait?

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rust-bors bot added a commit that referenced this pull request Dec 22, 2025

Auto merge of #150265 - scottmcm:vec-less-ubchecks, r=<try>

d3405d7

Stop emitting UbChecks on every Vec→Slice

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 22, 2025

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Dec 22, 2025

Stop emitting UbChecks on every Vec→Slice

fd8744f

Spotted this in PR148766's test changes. It doesn't seem like this ubcheck would catch anything useful; let's see if skipping it helps perf.

scottmcm force-pushed the vec-less-ubchecks branch from 9ca160d to fd8744f Compare December 22, 2025 22:08

scottmcm marked this pull request as ready for review December 22, 2025 22:17

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Dec 22, 2025

rustbot assigned ibraheemdev Dec 22, 2025

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Dec 22, 2025

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rust-bors bot added a commit that referenced this pull request Dec 22, 2025

Auto merge of #150265 - scottmcm:vec-less-ubchecks, r=<try>

fa07ba2

Stop emitting UbChecks on every Vec→Slice

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 22, 2025

scottmcm commented Dec 22, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 23, 2025

scottmcm mentioned this pull request Dec 23, 2025

Replace Rvalue::NullaryOp by a variant in mir::Operand. #148766

Merged

saethlin reviewed Dec 23, 2025

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

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

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