Have arrays' `drop_glue` just unsize and call the slice version by scottmcm · Pull Request #155184 · rust-lang/rust

scottmcm · 2026-04-12T05:42:21Z

It's silly to emit two loops (because of the drop ladder -- just one in panic=abort) for every array length that's dropped when we can just polymorphize to the slice version.

Built atop #154327 to avoid conflicts later, so draft for now.

r? @WaffleLapkin

rustbot · 2026-04-12T05:42:28Z

WaffleLapkin is not on the review rotation at the moment.
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scottmcm · 2026-04-12T05:47:45Z

+
+// RAW: ; core::ptr::drop_glue::<[array_drop_glue::NeedsDrop; [[N:7|13|42]]]>
+// RAW-NEXT: inlinehint
+// RAW: call core::ptr::drop_glue::<[array_drop_glue::NeedsDrop]>


View changes since the review

Compare nightly https://rust.godbolt.org/z/5Wv1q86ja with a loop in each of the three.

WaffleLapkin · 2026-04-14T10:40:31Z

💭 does it make sense to keep the array case separate for N <= 1 or something?

scottmcm · 2026-04-15T01:20:10Z

My understanding is that LLVM will very effectively inline those -- just like how we don't special case the array iterator for N ≤ 1 and still use the polymorphic version.

Does make me ponder if we should have a separate general "hey, there's no Drop so we can delegate to the one for the one field" kind of check, but I'd leave all these for a different PR if that's ok.

rustbot · 2026-05-09T08:08:14Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

scottmcm · 2026-05-09T08:11:05Z

@bors try @rust-timer queue

Have arrays' `drop_glue` just unsize and call the slice version

rust-bors · 2026-05-09T10:21:17Z

☀️ Try build successful (CI)
Build commit: ec33dfc (ec33dfc30f643cd46f52d91e53fe4fe66cd8d655, parent: 4a997eeefbb8cf41617324bb6d08c72171c614e2)

rust-timer · 2026-05-09T11:14:00Z

Finished benchmarking commit (ec33dfc): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.3%	[-0.3%, -0.3%]	1

Max RSS (memory usage)

Results (primary -5.2%)

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)	-5.2%	[-5.2%, -5.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-5.2%	[-5.2%, -5.2%]	1

Cycles

Results (secondary -2.8%)

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)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.8%	[-5.4%, -3.8%]	3
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.3%)

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

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.2%, 1.1%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-0.5%, -0.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[-0.5%, 1.1%]	4

Bootstrap: 497.359s -> 500.627s (0.66%)
Artifact size: 397.13 MiB -> 397.30 MiB (0.04%)

WaffleLapkin

I have a few minor questions, but the change looks good to me, so r=me, unless you think it would be valuable to add comments to code/test.

View changes since this review

WaffleLapkin · 2026-05-11T11:21:13Z

+        start.terminator = Some(Terminator {
+            source_info,
+            kind: TerminatorKind::Call {
+                func: Operand::function_handle(tcx, def_id, [ty::GenericArg::from(slice_ty)], span),


(question) What's the def_id a def id of?

It's the def id of the drop_glue itself. Basically the same as require_lang_item(LangItem::DropGlue), but it was already passed in to generate the drop glue we're currently making, so I didn't need to look it up -- the existing parameter on https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/shim/fn.build_drop_shim.html.

WaffleLapkin · 2026-05-11T11:24:50Z

+// fragility from that changing we don't check any particular order.
+
+// RAW: ; core::ptr::drop_glue::<[array_drop_glue::NeedsDrop; [[N:7|13|42]]]>
+// RAW-NOT: inlinehint


(question) Why are you checking for lack of inlinehint here?

Oh, because I'd not un-pushed the second commit where I experimented with not touching the inlining check.

I went back to the original which I think got overall-better perf results where the delegating ones are inlinehint but the one with the actual loop(s) isn't, and thus these tests check that.

WaffleLapkin · 2026-05-11T11:29:21Z

+
+// CHECK-LABEL: ; core::ptr::drop_glue::<[array_drop_glue::NeedsDrop]>
+// CHECK-NOT: inlinehint
+// OPT: add nuw nsw {{.+}} 1


What is this checking for?

That it's the actual loop which got optimized to add nuw nsw despite just being Add in the code generating the loop.

scottmcm · 2026-05-11T18:02:50Z

@bors r=WaffleLapkin

rust-bors · 2026-05-11T18:02:54Z

📌 Commit 05f52c7 has been approved by WaffleLapkin

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

JonathanBrouwer · 2026-05-11T19:36:17Z

@bors rollup
Perf seems to be noise

@WaffleLapkin

…, r=WaffleLapkin Have arrays' `drop_glue` just unsize and call the slice version It's silly to emit two loops (because of the drop ladder -- just one in panic=abort) for every array length that's dropped when we can just polymorphize to the slice version. Built atop rust-lang#154327 to avoid conflicts later, so draft for now. r? @WaffleLapkin

…uwer Rollup of 11 pull requests Successful merges: - #156448 (miri subtree update) - #155023 (Introduce move expressions (`move($expr)`) ) - #156429 (Simplify `intrinsic::raw_eq` in MIR when possible) - #147672 (LLBC-linker: Do not strip debug symbols for the nvptx target anymore) - #155169 (jsondoclint: simplify code using idiomatic Rust) - #155184 ( Have arrays' `drop_glue` just unsize and call the slice version) - #156022 (rustdoc: Fix cosmetic issues when reporting unresolved paths in `broken_intra_doc_links`) - #156442 (Show intrinsics::gpu in docs) - #156461 (LLVM 23: Specify `returnaddress` intrinsic return type) - #156462 (LLVM 23: Accept float (instead of hex) literals in codegen tests) - #156466 (Refactor `CheckAttrVisitor` so rustfmt can format it.)

…uwer Rollup of 11 pull requests Successful merges: - rust-lang/rust#156448 (miri subtree update) - rust-lang/rust#155023 (Introduce move expressions (`move($expr)`) ) - rust-lang/rust#156429 (Simplify `intrinsic::raw_eq` in MIR when possible) - rust-lang/rust#147672 (LLBC-linker: Do not strip debug symbols for the nvptx target anymore) - rust-lang/rust#155169 (jsondoclint: simplify code using idiomatic Rust) - rust-lang/rust#155184 ( Have arrays' `drop_glue` just unsize and call the slice version) - rust-lang/rust#156022 (rustdoc: Fix cosmetic issues when reporting unresolved paths in `broken_intra_doc_links`) - rust-lang/rust#156442 (Show intrinsics::gpu in docs) - rust-lang/rust#156461 (LLVM 23: Specify `returnaddress` intrinsic return type) - rust-lang/rust#156462 (LLVM 23: Accept float (instead of hex) literals in codegen tests) - rust-lang/rust#156466 (Refactor `CheckAttrVisitor` so rustfmt can format it.)

rust-lang/rust#155184 lowers array drop glue to call slice drop glue, which we cannot always check. We have no need to check the lowering, however.

rustbot assigned WaffleLapkin Apr 12, 2026

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change the type of the argument of drop_in_place lang item to &mut _ #154327

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scottmcm force-pushed the intercept-array-drop-shim branch from d1bdee8 to 2ae092a Compare April 15, 2026 01:23

rustbot added A-run-make Area: port run-make Makefiles to rmake.rs A-test-infra-minicore Area: `minicore` test auxiliary and `//@ add-core-stubs` PG-exploit-mitigations Project group: Exploit mitigations labels Apr 15, 2026

scottmcm removed PG-exploit-mitigations Project group: Exploit mitigations A-run-make Area: port run-make Makefiles to rmake.rs A-test-infra-minicore Area: `minicore` test auxiliary and `//@ add-core-stubs` labels Apr 15, 2026

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scottmcm added the S-blocked Status: Blocked on something else such as an RFC or other implementation work. label Apr 28, 2026

scottmcm force-pushed the intercept-array-drop-shim branch 2 times, most recently from 8a17db9 to bf354cd Compare May 7, 2026 03:22

rustbot added the A-run-make Area: port run-make Makefiles to rmake.rs label May 7, 2026

Have arrays' drop_glue just unsize and call the slice version

05f52c7

scottmcm force-pushed the intercept-array-drop-shim branch from 1f28062 to b0020c9 Compare May 9, 2026 08:08

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label May 9, 2026

rust-bors Bot pushed a commit that referenced this pull request May 9, 2026

Auto merge of #155184 - scottmcm:intercept-array-drop-shim, r=<try>

ec33dfc

Have arrays' `drop_glue` just unsize and call the slice version

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rustbot removed S-waiting-on-perf Status: Waiting on a perf run to be completed. perf-regression Performance regression. labels May 9, 2026

WaffleLapkin approved these changes May 11, 2026

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scottmcm force-pushed the intercept-array-drop-shim branch from b0020c9 to 05f52c7 Compare May 11, 2026 16:36

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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 11, 2026

This was referenced May 11, 2026

Rollup of 8 pull requests #156470

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Rollup of 11 pull requests #156473

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rust-bors Bot merged commit aeffd15 into rust-lang:main May 12, 2026
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rust-timer commented May 9, 2026

Overall result: ✅ improvements - no action needed

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