Skip no-op drop glue #142508

Mark-Simulacrum · 2025-06-14T14:38:51Z

Since #122662 this no longer gets used in vtables, so we're safe to fully
drop generating functions from vtables. Those are eventually cleaned up
by LLVM, but it's wasteful to produce them in the first place.

This doesn't appear to be a significant win (and shows some slight regressions) but
seems like the right thing to do. At minimum it reduces noise in the LLVM IR we generate,
which seems like a good thing.

Mark-Simulacrum · 2025-06-14T14:42:08Z

@bors2 try @rust-timer queue

rust-bors · 2025-06-14T14:42:11Z

⌛ Trying commit c839507 with merge d28feb0…

To cancel the try build, run the command @bors2 try cancel.

Skip noop drop glue Since #122662 this no longer gets used in vtables, so we're safe to fully drop generating these empty functions. Those are eventually cleaned up by LLVM, but it's wasteful to produce them in the first place. Opening this for a perf run before asking for review.

rust-bors · 2025-06-14T17:09:09Z

☀️ Try build successful (CI)
Build commit: d28feb0 (d28feb066e95d7ce212acfb87a21d02aa98b8928, parent: 9822e3dc15b5940626a186a57f877cf784b1bfb3)

rust-timer · 2025-06-14T18:43:40Z

Finished benchmarking commit (d28feb0): 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)	1.0%	[1.0%, 1.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	5
Improvements ✅ (secondary)	-2.0%	[-2.9%, -0.6%]	5
All ❌✅ (primary)	-0.1%	[-0.4%, 1.0%]	6

Max RSS (memory usage)

Results (primary -1.5%, secondary 2.7%)

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

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.6%, 2.1%]	3
Regressions ❌ (secondary)	2.7%	[2.1%, 4.0%]	3
Improvements ✅ (primary)	-3.5%	[-5.2%, -2.6%]	5
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.5%	[-5.2%, 2.1%]	8

Cycles

Results (primary 1.1%, secondary -0.8%)

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

	mean	range	count
Regressions ❌ (primary)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	6.1%	[6.1%, 6.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.6%	[-3.3%, -2.2%]	4
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	1

Binary size

Results (primary -0.3%, 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)	0.3%	[0.1%, 1.0%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.8%, -0.0%]	44
Improvements ✅ (secondary)	-1.6%	[-5.0%, -0.0%]	17
All ❌✅ (primary)	-0.3%	[-0.8%, 1.0%]	48

Bootstrap: 757.645s -> 756.931s (-0.09%)
Artifact size: 372.28 MiB -> 372.23 MiB (-0.01%)

Mark-Simulacrum · 2025-06-15T14:31:36Z

tests/codegen-units/item-collection/instantiation-through-vtable.rs

    //~ MONO_ITEM fn <Struct<u32> as Trait>::foo
    //~ MONO_ITEM fn <Struct<u32> as Trait>::bar
    let r1 = &s1 as &Trait;
    r1.foo();
    r1.bar();

    let s1 = Struct { _a: 0u64 };
-    //~ MONO_ITEM fn std::ptr::drop_in_place::<Struct<u64>> - shim(None) @@ instantiation_through_vtable-cgu.0[External]
    //~ MONO_ITEM fn <Struct<u64> as Trait>::foo
    //~ MONO_ITEM fn <Struct<u64> as Trait>::bar
    let _ = &s1 as &Trait;


It seems to me this test is mostly a duplicate of the unsizing test, so I'm skipping updating it with its own drop glue variant -- seems like that's covered there. Maybe worth deleting one of these entirely in a follow-up PR though.

rustbot · 2025-06-15T15:36:25Z

r? @fee1-dead

rustbot has assigned @fee1-dead.
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 · 2025-06-15T15:36:28Z

Some changes occurred in tests/codegen/sanitizer

cc @rcvalle

fee1-dead · 2025-06-17T08:22:32Z

The image benchmark seems spurious given the graph, so

@bors r+ rollup

bors · 2025-06-17T08:22:35Z

📌 Commit 9549f6b has been approved by fee1-dead

It is now in the queue for this repository.

fee1-dead · 2025-06-22T11:16:58Z

@bors r+

bors · 2025-06-22T11:17:00Z

📌 Commit dcac9c3 has been approved by fee1-dead

It is now in the queue for this repository.

Skip no-op drop glue Since #122662 this no longer gets used in vtables, so we're safe to fully drop generating functions from vtables. Those are eventually cleaned up by LLVM, but it's wasteful to produce them in the first place. This doesn't appear to be a significant win (and shows some slight regressions) but seems like the right thing to do. At minimum it reduces noise in the LLVM IR we generate, which seems like a good thing.

bors · 2025-06-22T12:48:11Z

⌛ Testing commit dcac9c3 with merge 0dcd498...

bors · 2025-06-22T14:02:36Z

💔 Test failed - checks-actions

Mark-Simulacrum · 2025-06-22T14:44:07Z

@bors2 try jobs=x86_64-gnu-nopt

rust-bors · 2025-06-22T14:44:10Z

⌛ Trying commit beb9227 with merge 219abb7…

To cancel the try build, run the command @bors2 try cancel.

Skip no-op drop glue Since #122662 this no longer gets used in vtables, so we're safe to fully drop generating functions from vtables. Those are eventually cleaned up by LLVM, but it's wasteful to produce them in the first place. This doesn't appear to be a significant win (and shows some slight regressions) but seems like the right thing to do. At minimum it reduces noise in the LLVM IR we generate, which seems like a good thing. try-job: x86_64-gnu-nopt

rust-bors · 2025-06-22T15:58:06Z

💔 Test failed

CI ❌

Mark-Simulacrum · 2025-06-22T16:04:06Z

@bors r=fee1-dead

bors · 2025-06-22T16:04:08Z

📌 Commit b4568d9 has been approved by fee1-dead

It is now in the queue for this repository.

bors · 2025-06-22T20:10:10Z

⌛ Testing commit b4568d9 with merge be19eda...

bors · 2025-06-22T23:29:03Z

☀️ Test successful - checks-actions
Approved by: fee1-dead
Pushing be19eda to master...

github-actions · 2025-06-22T23:31:44Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 111e9bc (parent) -> be19eda (this PR)

Test differences

Show 8 test diffs

Stage 1

[codegen-units] tests/codegen-units/item-collection/drop-glue-noop.rs: [missing] -> pass (J0)

Stage 2

[codegen-units] tests/codegen-units/item-collection/drop-glue-noop.rs: [missing] -> pass (J1)

Additionally, 6 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard be19eda0dc4c22c5cf5f1b48fd163acf9bd4b0a6 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

x86_64-apple-1: 6784.2s -> 11062.1s (63.1%)
x86_64-apple-2: 4598.5s -> 3566.7s (-22.4%)
x86_64-rust-for-linux: 3075.6s -> 2532.9s (-17.6%)
armhf-gnu: 5093.9s -> 4328.2s (-15.0%)
i686-gnu-2: 6167.1s -> 5364.3s (-13.0%)
mingw-check-1: 1776.6s -> 1552.4s (-12.6%)
aarch64-gnu-debug: 4128.8s -> 3620.9s (-12.3%)
x86_64-gnu-llvm-19-1: 3743.2s -> 3322.4s (-11.2%)
x86_64-gnu-tools: 3589.1s -> 3246.6s (-9.5%)
x86_64-gnu-debug: 5869.2s -> 5342.4s (-9.0%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2025-06-23T03:24:40Z

Finished benchmarking commit (be19eda): 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

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	1.7%	[0.4%, 3.0%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.3%]	4
Improvements ✅ (secondary)	-1.8%	[-3.1%, -0.4%]	6
All ❌✅ (primary)	0.3%	[-0.4%, 3.0%]	6

Max RSS (memory usage)

Results (primary 1.0%)

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

	mean	range	count
Regressions ❌ (primary)	5.5%	[3.6%, 7.4%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.6%	[-3.9%, -3.2%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.0%	[-3.9%, 7.4%]	4

Cycles

Results (primary 2.5%, secondary -2.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.5%, 2.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.6%	[-3.1%, -2.1%]	3
All ❌✅ (primary)	2.5%	[2.5%, 2.5%]	1

Binary size

Results (primary -0.3%, secondary -1.7%)

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

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.1%, 1.1%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.9%, -0.0%]	45
Improvements ✅ (secondary)	-1.7%	[-5.1%, -0.0%]	17
All ❌✅ (primary)	-0.3%	[-0.9%, 1.1%]	47

Bootstrap: 689.093s -> 689.346s (0.04%)
Artifact size: 371.98 MiB -> 371.89 MiB (-0.02%)

rustbot added PG-exploit-mitigations Project group: Exploit mitigations T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 14, 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 Jun 14, 2025

Mark-Simulacrum force-pushed the skip-noop-drop-glue branch from c839507 to 1573ec4 Compare June 15, 2025 14:30

rustbot added the PG-exploit-mitigations Project group: Exploit mitigations label Jun 15, 2025

Mark-Simulacrum commented Jun 15, 2025

View reviewed changes

Mark-Simulacrum changed the title ~~Skip noop drop glue~~ Skip no-op drop glue Jun 15, 2025

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Mark-Simulacrum force-pushed the skip-noop-drop-glue branch from 1573ec4 to 9549f6b Compare June 15, 2025 15:20

Mark-Simulacrum marked this pull request as ready for review June 15, 2025 15:36

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rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 15, 2025

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fee1-dead approved these changes Jun 17, 2025

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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 Jun 17, 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 Jun 22, 2025

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bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jun 22, 2025

Mark-Simulacrum force-pushed the skip-noop-drop-glue branch from dcac9c3 to beb9227 Compare June 22, 2025 14:43

rustbot added the PG-exploit-mitigations Project group: Exploit mitigations label Jun 22, 2025

Fix tests to drop now-skipped codegen

b4568d9

Mark-Simulacrum force-pushed the skip-noop-drop-glue branch from beb9227 to b4568d9 Compare June 22, 2025 16:03

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 Jun 22, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Jun 22, 2025

bors merged commit be19eda into rust-lang:master Jun 22, 2025
11 checks passed

rustbot added this to the 1.89.0 milestone Jun 22, 2025

rustbot added the perf-regression Performance regression. label Jun 23, 2025

Skip no-op drop glue #142508

Skip no-op drop glue #142508

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

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Test differences

Stage 1

Stage 2

Job duration changes

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rust-timer commented Jun 23, 2025

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

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Mark-Simulacrum commented Jun 14, 2025 •

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