Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()` #144566

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

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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

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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

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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

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oli-obk approved these changes Jul 29, 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 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

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Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()` #144566

Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()` #144566

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Simplify align_of_val::<[T]>(…) → align_of::<T>() #144566

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Overall result: ❌ regressions - please read the text below

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Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()` #144566

Simplify `align_of_val::<[T]>(…)` → `align_of::<T>()` #144566

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