[perf] Revert FastISel patch#154511

nikic · 2026-03-28T16:45:01Z

This caused a significant compile-time regression for debug builds.

There is another change (llvm/llvm-project#186723) that mitigates that regression, but not fully. Revert it for now.

nikic · 2026-03-28T16:45:13Z

[perf] Revert FastISel patch

rust-bors · 2026-03-28T18:53:49Z

☀️ Try build successful (CI)
Build commit: af18a79 (af18a79249ebb7cdcf8e745d19085bf1b3c46242, parent: 7e28c7438a7b0c79a09724ba799d32ed461beb72)

rust-timer · 2026-03-28T19:34:22Z

Finished benchmarking commit (af18a79): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.3%	[-3.2%, -0.2%]	27
Improvements ✅ (secondary)	-1.2%	[-5.0%, -0.2%]	14
All ❌✅ (primary)	-1.3%	[-3.2%, -0.2%]	27

Max RSS (memory usage)

Results (primary -1.1%)

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)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

Cycles

Results (primary -2.5%, secondary -3.2%)

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)	-2.5%	[-3.4%, -2.2%]	4
Improvements ✅ (secondary)	-3.2%	[-4.4%, -2.4%]	3
All ❌✅ (primary)	-2.5%	[-3.4%, -2.2%]	4

Binary size

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

Bootstrap: 484.417s -> 484.146s (-0.06%)
Artifact size: 394.97 MiB -> 396.98 MiB (0.51%)

This change caused a significant compile-time regression for debug builds.

rustbot · 2026-03-28T20:47:06Z

r? @cuviper

rustbot has assigned @cuviper.
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

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: @cuviper

rustbot · 2026-03-28T20:47:07Z

⚠️ Warning ⚠️

Some commits in this PR modify submodules.

If this was not intentional, see I changed a submodule on accident in the rustc dev guide.

nikic · 2026-03-28T20:47:24Z

r? @dianqk

dianqk · 2026-03-28T21:46:53Z

@bors r+

rust-bors · 2026-03-28T21:46:56Z

📌 Commit 1a32061 has been approved by dianqk

It is now in the queue for this repository.

[perf] Revert FastISel patch This caused a significant compile-time regression for debug builds. There is another change (llvm/llvm-project#186723) that mitigates that regression, but not fully. Revert it for now.

rust-log-analyzer · 2026-03-29T06:48:46Z

The job armhf-gnu failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

rust-bors · 2026-03-29T06:49:11Z

💔 Test for 1481366 failed: CI. Failed job:

auto - armhf-gnu (web logs, enhanced plaintext logs)

mati865 · 2026-03-29T10:53:37Z

Spurious?

@bors retry

rust-bors · 2026-03-30T05:23:54Z

☀️ Test successful - CI
Approved by: dianqk
Duration: 3h 20m 48s
Pushing 116458d to main...

github-actions · 2026-03-30T05:26:35Z

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 80ad557 (parent) -> 116458d (this PR)

Test differences

Show 3 test diffs

Stage 2

[run-make] tests/run-make/compressed-debuginfo-zstd: pass -> ignore (ignored if LLVM wasn't build with zstd for ELF section compression or LLVM is not the default codegen backend) (J0)

Additionally, 2 doctest diffs were found. These are ignored, as they are noisy.

Job group index

J0: x86_64-gnu-nopt

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 116458d0a5ae01cd517cabd2d1aee7f5457018ab --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-rust-for-linux: 53m 54s -> 1h 8m (+28.0%)
pr-check-1: 35m 51s -> 45m 32s (+27.0%)
test-various: 2h 2m -> 2h 32m (+23.7%)
x86_64-gnu-tools: 1h 6m -> 1h 20m (+19.9%)
x86_64-gnu-miri: 1h 32m -> 1h 51m (+19.7%)
armhf-gnu: 1h 33m -> 1h 52m (+19.7%)
optional-x86_64-gnu-parallel-frontend: 2h 43m -> 3h 14m (+18.9%)
x86_64-gnu-distcheck: 2h 9m -> 2h 31m (+16.9%)
pr-check-2: 46m 45s -> 54m 2s (+15.6%)
x86_64-gnu: 2h 26m -> 2h 48m (+15.2%)

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 · 2026-03-30T06:05:51Z

Finished benchmarking commit (116458d): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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)	-1.3%	[-3.2%, -0.1%]	26
Improvements ✅ (secondary)	-1.2%	[-5.1%, -0.2%]	13
All ❌✅ (primary)	-1.3%	[-3.2%, -0.1%]	26

Max RSS (memory usage)

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

Cycles

Results (primary -2.9%, secondary -1.2%)

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)	4.9%	[4.9%, 4.9%]	1
Improvements ✅ (primary)	-2.9%	[-3.4%, -2.3%]	2
Improvements ✅ (secondary)	-4.2%	[-4.4%, -4.1%]	2
All ❌✅ (primary)	-2.9%	[-3.4%, -2.3%]	2

Binary size

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

Bootstrap: 490.149s -> 496.876s (1.37%)
Artifact size: 394.89 MiB -> 394.82 MiB (-0.02%)

cuviper · 2026-03-30T17:48:39Z

Nominating as a perf fix to go along with the accepted #154344.

@rustbot label beta-nominated

rustbot · 2026-04-02T14:07:20Z

beta backport approved as per compiler team on Zulip. A backport PR will be authored by the release team at the end of the current development cycle. Backport labels are handled by them.

[beta] backports - stdarch subtree update #153336 (partial) - aarch64: fix UB in non-power-of-two reads and writes rust-lang/stdarch#2042 - add neon load/store assembly test #154094 - don't drop arguments' temporaries in `dbg!` #154074 - Init self_decl with a correct vis #154313 - Update LLVM to 22.1.2 #154344 - [perf] Revert FastISel patch #154511 - core: Destabilize beta-stable `RangeInclusiveIter::remainder` #154459 - Revert "Fix: On wasm targets, call `panic_in_cleanup` if panic occurs in cleanup" #154700 - core: Update the feature gate on `TryFrom<integer> for bool` #154691 r? cuviper

rustbot added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 28, 2026

This comment has been minimized.

Sign in to view

rust-bors bot pushed a commit that referenced this pull request Mar 28, 2026

Auto merge of #154511 - nikic:revert-fastisel, r=<try>

af18a79

[perf] Revert FastISel patch

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Mar 28, 2026

This comment has been minimized.

Sign in to view

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Mar 28, 2026

[perf] Revert FastISel patch

1a32061

This change caused a significant compile-time regression for debug builds.

nikic force-pushed the revert-fastisel branch from 7eebcdb to 1a32061 Compare March 28, 2026 20:45

nikic marked this pull request as ready for review March 28, 2026 20:47

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Mar 28, 2026

rustbot assigned cuviper Mar 28, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Mar 28, 2026

rustbot assigned dianqk and unassigned cuviper Mar 28, 2026

nikic mentioned this pull request Mar 28, 2026

Revert "Rollup merge of #154344 - dianqk:update-llvm, r=nikic" #154470

Closed

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-review Status: Awaiting review from the assignee but also interested parties. labels Mar 28, 2026

This comment has been minimized.

Sign in to view

rust-bors bot 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 Mar 29, 2026

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-review Status: Awaiting review from the assignee but also interested parties. labels Mar 29, 2026

This comment has been minimized.

Sign in to view

rust-bors bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Mar 30, 2026

rust-bors bot merged commit 116458d into rust-lang:main Mar 30, 2026
12 checks passed

rustbot added this to the 1.96.0 milestone Mar 30, 2026

nikic mentioned this pull request Mar 30, 2026

Update LLVM to 22.1.2 #154344

Merged

rustbot added the beta-nominated Nominated for backporting to the compiler in the beta channel. label Mar 30, 2026

cuviper added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. beta-nominated Nominated for backporting to the compiler in the beta channel. and removed beta-nominated Nominated for backporting to the compiler in the beta channel. labels Mar 30, 2026

Kobzol mentioned this pull request Mar 31, 2026

Rollup of 22 pull requests #154384

Merged

rustbot added the beta-accepted Accepted for backporting to the compiler in the beta channel. label Apr 2, 2026

cuviper mentioned this pull request Apr 2, 2026

[beta] backports #154725

Merged

cuviper removed the beta-nominated Nominated for backporting to the compiler in the beta channel. label Apr 2, 2026

cuviper modified the milestones: 1.96.0, 1.95.0 Apr 2, 2026

Uh oh!

Conversation

nikic commented Mar 28, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

nikic commented Mar 28, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

rust-bors bot commented Mar 28, 2026

Uh oh!

This comment has been minimized.

rust-timer commented Mar 28, 2026

Overall result: ✅ improvements - no action needed

Uh oh!

rustbot commented Mar 28, 2026

Uh oh!

rustbot commented Mar 28, 2026

Uh oh!

nikic commented Mar 28, 2026

Uh oh!

dianqk commented Mar 28, 2026

Uh oh!

rust-bors bot commented Mar 28, 2026

Uh oh!

This comment has been minimized.

rust-log-analyzer commented Mar 29, 2026

Uh oh!

rust-bors bot commented Mar 29, 2026

Uh oh!

mati865 commented Mar 29, 2026

Uh oh!

This comment has been minimized.

rust-bors bot commented Mar 30, 2026

Uh oh!

Uh oh!

github-actions bot commented Mar 30, 2026

Test differences

Stage 2

Job duration changes

Uh oh!

rust-timer commented Mar 30, 2026

Overall result: ✅ improvements - no action needed

Uh oh!

cuviper commented Mar 30, 2026

Uh oh!

rustbot commented Apr 2, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

7 participants

nikic commented Mar 28, 2026 •

edited by rustbot

Loading