Computing crate_hash from metadata encoding instead of HIR (implements #94878) (very draft) by Daniel-B-Smith · Pull Request #154724 · rust-lang/rust

Daniel-B-Smith · 2026-04-02T18:15:01Z

This PR converts the crate_hash/SVH to depend on metadata instead of HIR whenever metadata is needed. A trimmed down crate_hash is kept for dylib/binary cases where metadata is not present.

The change to the HIR hash is potentially safe orthogonal this change. Without that change, this PR is a performance regression. I am bundling them because I believe the HIR hash change is more likely to be safe in light of the HIR hash being less load bearing on the SVH.

nnethercote · 2026-04-09T23:26:03Z

@bors try @rust-timer queue

Computing crate_hash from metadata encoding instead of HIR (implements #94878) (very draft)

rust-bors · 2026-05-05T22:24:35Z

☀️ Try build successful (CI)
Build commit: 9180a5c (9180a5c6a9ad54fd478c2f21273a551c73f21cc9, parent: 740679e1f5554a3c1695317e084809e3fceb9825)

rust-timer · 2026-05-05T23:06:18Z

Finished benchmarking commit (9180a5c): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@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)	0.4%	[0.2%, 0.5%]	6
Regressions ❌ (secondary)	0.5%	[0.1%, 1.0%]	12
Improvements ✅ (primary)	-0.5%	[-5.5%, -0.1%]	68
Improvements ✅ (secondary)	-1.4%	[-5.0%, -0.1%]	46
All ❌✅ (primary)	-0.5%	[-5.5%, 0.5%]	74

Max RSS (memory usage)

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)	0.7%	[0.6%, 1.1%]	4
Improvements ✅ (primary)	-2.9%	[-4.3%, -2.2%]	3
Improvements ✅ (secondary)	-1.9%	[-8.2%, -0.5%]	11
All ❌✅ (primary)	-2.9%	[-4.3%, -2.2%]	3

Cycles

Results (primary -4.8%, secondary 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)	10.4%	[1.1%, 24.8%]	6
Improvements ✅ (primary)	-4.8%	[-8.7%, -2.5%]	3
Improvements ✅ (secondary)	-2.7%	[-5.8%, -0.4%]	15
All ❌✅ (primary)	-4.8%	[-8.7%, -2.5%]	3

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 495.494s -> 494.288s (-0.24%)
Artifact size: 394.42 MiB -> 394.47 MiB (0.01%)

Daniel-B-Smith · 2026-05-06T20:45:23Z

One last perf run as the numbers are already getting into the ballpark.

This run removes the DepPathHash from the HIR hash since https://github.com/rust-lang/rust/blob/4a13f36/compiler/rustc_hir/src/hir.rs#L1583 indicates that the OwnerInfo hashes already contain all of the HIR body in their hashes. opt_hir_hash is no longer used for the SVH, so it only needs to detect changes in the HIR.

I'm relatively confident at this point that there are no more (easy) ways to make any of the three hashes (HIR hash, metadata crate hash, and !metatdata crate hash) meaningfully cheaper. The next two steps are to clean up the various shortcuts I took to get this to the profiling stage faster and start a deeper test by incrementally compiling through a few larger crates/binaries including rustc.

mejrs · 2026-05-06T21:20:44Z

@bors try @rust-timer queue

Computing crate_hash from metadata encoding instead of HIR (implements #94878) (very draft)

rust-bors · 2026-05-06T23:29:50Z

☀️ Try build successful (CI)
Build commit: d83f021 (d83f0213d3b2172cfc04784f39a42736048ff7fd, parent: 365c0e1d7a614ca94cb48431dcd2bc6d3b645db1)

rust-timer · 2026-05-07T00:10:27Z

Finished benchmarking commit (d83f021): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing 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)	0.3%	[0.2%, 0.6%]	7
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	6
Improvements ✅ (primary)	-0.5%	[-5.5%, -0.1%]	82
Improvements ✅ (secondary)	-1.4%	[-5.1%, -0.1%]	47
All ❌✅ (primary)	-0.4%	[-5.5%, 0.6%]	89

Max RSS (memory usage)

Results (primary -0.5%, 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)	1.8%	[1.7%, 1.8%]	2
Regressions ❌ (secondary)	0.9%	[0.4%, 2.1%]	6
Improvements ✅ (primary)	-2.0%	[-3.6%, -0.7%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.5%	[-3.6%, 1.8%]	5

Cycles

Results (primary -4.1%, 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)	-	-	0
Regressions ❌ (secondary)	0.8%	[0.5%, 1.1%]	3
Improvements ✅ (primary)	-4.1%	[-7.8%, -2.1%]	3
Improvements ✅ (secondary)	-3.3%	[-5.3%, -0.7%]	17
All ❌✅ (primary)	-4.1%	[-7.8%, -2.1%]	3

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 501.88s -> 497.481s (-0.88%)
Artifact size: 395.21 MiB -> 395.12 MiB (-0.02%)

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 2, 2026