Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()` by scottmcm · Pull Request #144566 · rust-lang/rust

scottmcm · 2025-07-28T07:04:22Z

I spotted this while working on the inliner (#144561). In particular, if Layout::for_value inlines, then it can be pretty easy to end up with an align_of_val::<[T]> today (demo: https://rust.godbolt.org/z/Tesnscj4a) where we can save at least a block, if not more, by using the version that's an rvalue and not a call.

rustbot · 2025-07-28T07:04:26Z

r? @davidtwco

rustbot has assigned @davidtwco.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

rustbot · 2025-07-28T07:04:29Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

oli-obk · 2025-07-28T08:16:56Z

tests/mir-opt/instsimplify/align_of_slice.rs

+pub fn of_val_slice<T>(slice: &[T]) -> usize {
+    // CHECK-LABEL: fn of_val_slice(_1: &[T])
+    // CHECK: _0 = AlignOf(T);
+    unsafe { core::intrinsics::align_of_val(slice) }
+}


Since you're explicitly mentioning this running after inlining, maybe change it to

Suggested change

pub fn of_val_slice<T>(slice: &[T]) -> usize {

// CHECK-LABEL: fn of_val_slice(_1: &[T])

// CHECK: _0 = AlignOf(T);

unsafe { core::intrinsics::align_of_val(slice) }

}

pub fn of_val_slice<T>(slice: &[T]) -> usize {

// CHECK-LABEL: fn of_val_slice(_1: &[T])

// CHECK: _0 = AlignOf(T);

something_inlined(slice)

}

#[inline(always)]

fn something_inlined<T: ?Sized>(val: &T) -> usize {

unsafe { core::intrinsics::align_of_val(slice) }

}

or use the mem wrapper

Hmm, that wouldn't work with test-mir-pass -- this folder is more unit testy, usually.

I guess I can just bring over the test from https://github.com/rust-lang/rust/pull/144572/files#diff-9e6b893e458a80c489851369ec09247ad188228d94c4c92d3ba28fea1702fdc8 though.

Just merging is fine with me, I was mostly wondering why it wasn't this way. r=me at your discretion what you think is best

oli-obk · 2025-07-29T07:22:24Z

unlikely to have a perf effect outside of microbenchmarks, but it may also unlock other opts, so let's perf it to decide whether to rollup or not

@bors try @rust-timer queue

rust-bors · 2025-07-29T07:22:28Z

⌛ Trying commit 8ef9233 with merge 045ae4f…

To cancel the try build, run the command @bors try cancel.

Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()`

scottmcm · 2025-07-29T07:27:38Z

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

+                            }
+                            scope 23 (inlined Layout::align) {
+                                scope 24 (inlined std::ptr::Alignment::as_usize) {
+                                    let mut _10: u32;


annot: this u32 is why the test needs to be split by bit width.

oli-obk

r=me with perf finished

rust-bors · 2025-07-29T09:38:28Z

☀️ Try build successful (CI)
Build commit: 045ae4f (045ae4f32f81a7bb6b048d92aad0cbad72f2761e, parent: cb6785f73df1aa3f558796a22a4ab9652cf38e26)

rust-timer · 2025-07-29T11:41:16Z

Finished benchmarking commit (045ae4f): 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)	1.0%	[1.0%, 1.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.0%	[1.0%, 1.0%]	1

Max RSS (memory usage)

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

Cycles

Results (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)	-	-	0
Regressions ❌ (secondary)	4.0%	[2.4%, 5.6%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.4%)

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

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.4%, 0.4%]	1

Bootstrap: 467.912s -> 468.532s (0.13%)
Artifact size: 376.82 MiB -> 376.83 MiB (0.00%)

oli-obk · 2025-07-29T12:40:14Z

Probably llvm opt noise

@bors r+ rollup

bors · 2025-07-29T12:40:17Z

📌 Commit 8ef9233 has been approved by oli-obk

It is now in the queue for this repository.

Rollup of 6 pull requests Successful merges: - #144560 (coverage: Treat `#[automatically_derived]` as `#[coverage(off)]`) - #144566 (Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()`) - #144587 (expand: Micro-optimize prelude injection) - #144589 (Account for `.yield` in illegal postfix operator message) - #144615 (Make resolve_fn_signature responsible for its own rib.) - #144634 (Fix typo in `DropGuard` doc) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #144566 - scottmcm:align-of-slice, r=oli-obk Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()` I spotted this while working on the inliner (#144561). In particular, if [`Layout::for_value`](https://doc.rust-lang.org/std/alloc/struct.Layout.html#method.for_value) inlines, then it can be pretty easy to end up with an `align_of_val::<[T]>` today (demo: <https://rust.godbolt.org/z/Tesnscj4a>) where we can save at least a block, if not more, by using the version that's an rvalue and not a call.

scottmcm added the A-mir-opt Area: MIR optimizations label Jul 28, 2025

rustbot assigned davidtwco Jul 28, 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. labels Jul 28, 2025

oli-obk reviewed Jul 28, 2025

View reviewed changes

scottmcm added 3 commits July 28, 2025 23:15

Add a mir-opt pre-codegen test for dropping a Box<[impl Copy]>

950a3b3

Add a MIR test for align_of_val on a slice

185b074

Simplify align_of_val::<[T]>(…) → align_of::<T>()

8ef9233

scottmcm force-pushed the align-of-slice branch from d679086 to 8ef9233 Compare July 29, 2025 06:19

This comment has been minimized.

Sign in to view

rust-bors bot added a commit that referenced this pull request Jul 29, 2025

Auto merge of #144566 - scottmcm:align-of-slice, r=<try>

045ae4f

Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()`

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 29, 2025

scottmcm commented Jul 29, 2025

View reviewed changes

oli-obk approved these changes Jul 29, 2025

View reviewed changes

This comment has been minimized.

Sign in to view

rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jul 29, 2025

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 Jul 29, 2025

Zalathar mentioned this pull request Jul 29, 2025

Rollup of 6 pull requests #144637

Merged

bors merged commit 826c462 into rust-lang:master Jul 29, 2025
11 checks passed

rustbot added this to the 1.90.0 milestone Jul 29, 2025

scottmcm deleted the align-of-slice branch July 29, 2025 19:09

Uh oh!

Conversation

scottmcm commented Jul 28, 2025 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Jul 28, 2025

Uh oh!

rustbot commented Jul 28, 2025

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

oli-obk commented Jul 29, 2025

Uh oh!

This comment has been minimized.

rust-bors bot commented Jul 29, 2025

Uh oh!

Choose a reason for hiding this comment

Uh oh!

oli-obk left a comment

Choose a reason for hiding this comment

Uh oh!

rust-bors bot commented Jul 29, 2025

Uh oh!

This comment has been minimized.

rust-timer commented Jul 29, 2025

Overall result: ❌ regressions - please read the text below

Uh oh!

oli-obk commented Jul 29, 2025

Uh oh!

bors commented Jul 29, 2025

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

6 participants

scottmcm commented Jul 28, 2025 •

edited by rustbot

Loading