skip codegen for intrinsics with big fallback bodies if backend does not need them by RalfJung · Pull Request #150605 · rust-lang/rust

RalfJung · 2026-01-02T17:40:51Z

This hopefully fixes the perf regression from #148478. I only added the intrinsics with big fallback bodies to the list; it doesn't seem worth the effort of going through the entire list.

Fixes #149945
Cc @scottmcm @bjorn3

rustbot · 2026-01-02T17:40:55Z

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RalfJung · 2026-01-02T17:41:52Z

compiler/rustc_session/src/session.rs

+
+    /// The names of intrinsics that the current codegen backend replaces
+    /// with its own implementations.
+    pub replaced_intrinsics: Vec<Symbol>,


It seems there is no way to get the current codegen backend from a tcx. I wasn't sure what the best way is to make this list of symbols available to monomorphization, and went for a new field in Session -- does that make sense?

I don't know enough about how all this should be structured to know what the best option is here.

This seems at least plausible, since at worst it stays empty and that doesn't hurt anything (other than perf).

@bjorn3 do you have any suggestions for how to deal with this?

I am not the biggest fan of another Session field, but don't have any other suggestions either.

RalfJung · 2026-01-02T17:42:07Z

@bors try
@rust-timer queue

skip codegen for intrinsics with big fallback bodies if backend does not need them

rust-bors · 2026-01-02T20:06:03Z

☀️ Try build successful (CI)
Build commit: 4763a83 (4763a83f81ae539aaa6f6e5e773ba1fc73de0a10, parent: 8a24a202aa02f677fc2a3e0e1a69af7545803952)

rust-timer · 2026-01-02T21:13:45Z

Finished benchmarking commit (4763a83): 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)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-1.8%	[-2.8%, -0.8%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.0%	[-2.8%, 0.6%]	3

Max RSS (memory usage)

Results (primary -1.5%, secondary 3.5%)

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

	mean	range	count
Regressions ❌ (primary)	1.3%	[0.7%, 1.9%]	2
Regressions ❌ (secondary)	3.5%	[3.5%, 3.5%]	1
Improvements ✅ (primary)	-4.3%	[-7.2%, -1.4%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.5%	[-7.2%, 1.9%]	4

Cycles

Results (primary -3.9%)

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)	-	-	0
Improvements ✅ (primary)	-3.9%	[-3.9%, -3.9%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.9%	[-3.9%, -3.9%]	1

Binary size

Results (primary 0.2%)

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

	mean	range	count
Regressions ❌ (primary)	1.4%	[1.4%, 1.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.1%, -0.0%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[-0.1%, 1.4%]	8

Bootstrap: 473.485s -> 474.195s (0.15%)
Artifact size: 390.77 MiB -> 390.79 MiB (0.01%)

…not need them

RalfJung · 2026-01-02T22:14:16Z

@bors try
@rust-timer queue

skip codegen for intrinsics with big fallback bodies if backend does not need them

rust-bors · 2026-01-03T00:43:41Z

☀️ Try build successful (CI)
Build commit: c75310a (c75310a5c412df8835187dd0ef37361b2f00d085, parent: 5497a36a7faf3d2af37beebcff7008e493202902)

rust-timer · 2026-01-03T01:24:45Z

Finished benchmarking commit (c75310a): 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)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-1.8%	[-2.9%, -0.8%]	2
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	1
All ❌✅ (primary)	-1.0%	[-2.9%, 0.7%]	3

Max RSS (memory usage)

Results (primary -4.1%)

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)	-4.1%	[-7.3%, -1.7%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-4.1%	[-7.3%, -1.7%]	3

Cycles

Results (primary -3.9%, secondary 15.2%)

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)	15.2%	[15.0%, 15.4%]	2
Improvements ✅ (primary)	-3.9%	[-3.9%, -3.9%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.9%	[-3.9%, -3.9%]	1

Binary size

Results (primary 0.2%)

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

	mean	range	count
Regressions ❌ (primary)	1.4%	[1.4%, 1.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.1%, -0.0%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[-0.1%, 1.4%]	8

Bootstrap: 471.287s -> 473.923s (0.56%)
Artifact size: 390.83 MiB -> 390.83 MiB (-0.00%)

jieyouxu · 2026-01-03T06:17:50Z

@rustbot reroll

RalfJung · 2026-01-31T14:15:38Z

Seems like I had no luck with that reroll.
@rustbot reroll

@scottmcm or could you review this?

mati865 · 2026-01-31T15:48:59Z

Cool idea!

I'll wait a few days to give @scottmcm time to respond respond as the much more knowledgeable person.

Do you know if there is a list of similarly optimised intrinsics somewhere?

RalfJung · 2026-02-02T14:19:21Z

In principle one could go over all the intrinsics that have fallback bodies, and then check whether the LLVM backend has implementations for them.

But most fallback bodies are small so the cost of monomorphizing them is tiny. Not sure if it's worth going through the entire list. I think I got all the ones that have big fallback bodies where we really don't want to pay the monomorphization cost.

mati865 · 2026-02-04T11:21:13Z

Fair enough, thanks for the explanation.

@bors r+

rust-bors · 2026-02-04T11:21:17Z

📌 Commit 57e44f5 has been approved by mati865

It is now in the queue for this repository.

@scottmcm

skip codegen for intrinsics with big fallback bodies if backend does not need them This hopefully fixes the perf regression from #148478. I only added the intrinsics with big fallback bodies to the list; it doesn't seem worth the effort of going through the entire list. Fixes #149945 Cc @scottmcm @bjorn3

@scottmcm

…r=mati865 skip codegen for intrinsics with big fallback bodies if backend does not need them This hopefully fixes the perf regression from rust-lang#148478. I only added the intrinsics with big fallback bodies to the list; it doesn't seem worth the effort of going through the entire list. Fixes rust-lang#149945 Cc @scottmcm @bjorn3

JonathanBrouwer · 2026-02-04T12:10:24Z

@bors yield
Yielding to enclosing rollup

rust-bors · 2026-02-04T12:10:28Z

Auto build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/21670890591

The next pull request likely to be tested is #152099.

…uwer Rollup of 11 pull requests Successful merges: - #150605 (skip codegen for intrinsics with big fallback bodies if backend does not need them) - #150992 (link modifier `export-symbols`: export all global symbols from selected uptream c static libraries) - #151534 (target: fix destabilising target-spec-json) - #152088 (rustbook/README.md: add missing `)`) - #151526 (Fix autodiff codegen tests) - #151810 (citool: report debuginfo test statistics) - #152065 (Convert to inline diagnostics in `rustc_ty_utils`) - #152068 (Convert to inline diagnostics in `rustc_resolve`) - #152070 (Convert to inline diagnostics in `rustc_pattern_analysis`) - #152072 (Convert to inline diagnostics in `rustc_monomorphize`) - #152083 (Fix set_times_nofollow for directory on windows) Failed merges: - #152069 (Convert to inline diagnostics in `rustc_privacy`)

RalfJung · 2026-02-04T12:24:37Z

This has perf impact, should it really be rolled up? It is marked rollup=never.

RalfJung · 2026-02-04T12:25:34Z

Oh I guess that mark got lost in the bors transition?
@bors rollup=never

rust-bors · 2026-02-04T20:30:46Z

☀️ Test successful - CI
Approved by: mati865
Duration: 3h 17m 40s
Pushing db3e99b to main...

github-actions · 2026-02-04T20:33:41Z

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 8bccf12 (parent) -> db3e99b (this PR)

Test differences

Show 16 test diffs

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

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard db3e99bbab28c6ca778b13222becdea54533d908 --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

dist-aarch64-llvm-mingw: 1h 34m -> 1h 50m (+17.0%)
dist-aarch64-apple: 2h 17m -> 2h (-12.5%)
dist-x86_64-apple: 2h 11m -> 1h 59m (-8.7%)
dist-various-1: 1h 11m -> 1h 5m (-8.4%)
armhf-gnu: 1h 33m -> 1h 27m (-7.2%)
dist-armhf-linux: 1h 26m -> 1h 31m (+6.7%)
aarch64-gnu-llvm-20-1: 59m 31s -> 1h 3m (+6.7%)
dist-apple-various: 1h 13m -> 1h 18m (+6.2%)
x86_64-msvc-1: 2h 25m -> 2h 34m (+6.2%)
x86_64-msvc-2: 2h 29m -> 2h 38m (+5.9%)

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 · 2026-02-04T21:12:55Z

Finished benchmarking commit (db3e99b): 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%	[-2.3%, -0.4%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.3%	[-2.3%, -0.4%]	2

Max RSS (memory usage)

Results (primary 1.9%, secondary 2.4%)

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

	mean	range	count
Regressions ❌ (primary)	5.3%	[1.3%, 9.3%]	2
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-1.5%	[-1.8%, -1.1%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.9%	[-1.8%, 9.3%]	4

Cycles

Results (primary -2.6%)

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)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.6%	[-2.6%, -2.6%]	1

Binary size

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

Bootstrap: 472.482s -> 472.94s (0.10%)
Artifact size: 398.10 MiB -> 398.12 MiB (0.01%)

rustbot assigned jieyouxu Jan 2, 2026

RalfJung commented Jan 2, 2026

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Auto merge of #150605 - RalfJung:fallback-intrinsic-skip, r=<try>

4763a83

skip codegen for intrinsics with big fallback bodies if backend does not need them

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RalfJung force-pushed the fallback-intrinsic-skip branch from 4ca06da to a170604 Compare January 2, 2026 19:29

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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 Jan 2, 2026

skip codegen for intrinsics with big fallback bodies if backend does …

57e44f5

…not need them

RalfJung force-pushed the fallback-intrinsic-skip branch from a170604 to 57e44f5 Compare January 2, 2026 22:14

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Auto merge of #150605 - RalfJung:fallback-intrinsic-skip, r=<try>

c75310a

skip codegen for intrinsics with big fallback bodies if backend does not need them

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 3, 2026

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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 Feb 4, 2026

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Rollup of 11 pull requests #152099

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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 Feb 4, 2026

rust-bors bot merged commit db3e99b into rust-lang:main Feb 4, 2026
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rustbot added this to the 1.95.0 milestone Feb 4, 2026

rustbot removed the perf-regression Performance regression. label Feb 4, 2026

RalfJung deleted the fallback-intrinsic-skip branch February 5, 2026 07:51

RalfJung mentioned this pull request Feb 5, 2026

implement carryless_mul #152132

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

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

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Test differences

Job duration changes

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Overall result: ✅ improvements - no action needed

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