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Prehash visibilities in resolver #143371

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@cjgillot cjgillot commented Jul 3, 2025

Based on #143247
r? @ghost for perf

@rustbot rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Jul 3, 2025
@cjgillot cjgillot force-pushed the prehash-visibilities branch from aaaefd1 to 17c75cb Compare July 3, 2025 14:06
@petrochenkov petrochenkov self-assigned this Jul 4, 2025
@petrochenkov petrochenkov added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jul 4, 2025
@cjgillot cjgillot force-pushed the prehash-visibilities branch from 17c75cb to 11751ce Compare July 4, 2025 14:01
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cjgillot commented Jul 4, 2025

@bors try @rust-timer queue

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@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 4, 2025
bors added a commit that referenced this pull request Jul 4, 2025
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Auto merge of #143371 - cjgillot:prehash-visibilities, r=<try>
556648c 
Prehash visibilities in resolver

Based on #143247
r? `@ghost` for perf
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bors commented Jul 4, 2025

⌛ Trying commit 11751ce with merge 556648c...

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bors commented Jul 4, 2025

☀️ Try build successful - checks-actions
Build commit: 556648c (556648c0fadc2403580e32dad35144435e964fcc)

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Finished benchmarking commit (556648c): 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)		 3.0% [3.0%, 3.0%] 1
Regressions ❌
(secondary)		 0.4% [0.3%, 0.4%] 2
Improvements ✅
(primary)		 -0.3% [-0.3%, -0.3%] 1
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) 1.4% [-0.3%, 3.0%] 2

Max RSS (memory usage)

Results (primary -1.9%, secondary -2.0%)

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)		 -1.9% [-2.7%, -1.3%] 4
Improvements ✅
(secondary)		 -2.0% [-3.3%, -0.5%] 4
All ❌✅ (primary) -1.9% [-2.7%, -1.3%] 4

Cycles

Results (secondary -0.9%)

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)		 1.0% [1.0%, 1.0%] 1
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 -1.8% [-3.0%, -0.6%] 2
All ❌✅ (primary) - - 0

Binary size

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

Bootstrap: 461.146s -> 462.46s (0.28%)
Artifact size: 372.20 MiB -> 372.16 MiB (-0.01%)

@rustbot rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jul 4, 2025
@cjgillot cjgillot force-pushed the prehash-visibilities branch from 11751ce to 2a49c4c Compare July 20, 2025 13:26
@petrochenkov petrochenkov added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 21, 2025
petrochenkov
petrochenkov reviewed Jul 21, 2025
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compiler/rustc_resolve/src/lib.rs
let visibilities_hash = {
let mut hasher = StableHasher::new();
let mut hcx = self.create_stable_hashing_context();
self.visibilities_for_hashing.hash_stable(&mut hcx, &mut hasher);
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I thought about doing this operation in fn feed_visibility and keeping only the hash in Resolver instead of the whole visibilities_for_hashing table.

Is creating the hashing context a more or less expensive operation than adding a table element?

petrochenkov
petrochenkov reviewed Jul 21, 2025
View reviewed changes
compiler/rustc_middle/src/hir/map.rs
// the fly in the resolver, storing only their accumulated hash in `ResolverGlobalCtxt`,
// and combining it with other hashes here.
resolutions.visibilities_for_hashing.hash_stable(&mut hcx, &mut stable_hasher);
resolutions.visibilities_hash.hash_stable(&mut hcx, &mut stable_hasher);
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I was also wondering if this operation (including visibilities, and maybe some other parts of resolutions, into the crate hash) needs to be performed at all, if we are hashing the whole resolutions as a query result now.

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No, we can't. resolutions is hashed for the current crate. But downstream crates only have the crate hash to decide whether load metadata from file or consider it unchanged. As metadata contains visibility, we either need to have resolutions's own hash or this more targeted version.

@petrochenkov petrochenkov marked this pull request as ready for review July 21, 2025 15:07
@petrochenkov petrochenkov added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 21, 2025
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