Print CGU reuse statistics in `-Zprint-mono-items` #144899

Kobzol · 2025-08-04T11:30:29Z

I'm trying to expose more information about incremental profiling from rustc (https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Profiling.2Fanalysis.20of.20incremental.20builds/with/531383501). One of the things that would be quite useful to expose is the CGU reuse state, so that when you do a rebuild, you can see all the CGUs (and all the functions) that had to be recompiled.

Currently, we have (AFAIK) two ways of outputting monomorphization statistics:

-Zdump-mono-stats outputs statistics about number of instantiations and expected compilation cost of individual functions in the local crate being compiled. It can be outputted either as Markdown or JSON.
-Zprint-mono-items outputs a pair (item, CGU) for each monomorphized item.

I was thinking about recording CGU statistics in the self-profile output, but I realized that as a simpler step, we could just add CGU reuse data to -Zprint-mono-items, as an additional output. That is what this PR does.

rustbot · 2025-08-04T11:30:33Z

r? @jieyouxu

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rustbot · 2025-08-04T11:30:35Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

jieyouxu · 2025-08-04T11:35:32Z

Rerolling a codegen reviewer for vibes on the approach; even though this is an perma-unstable flag.
r? codegen

klensy · 2025-08-04T12:25:40Z

-Zdump-mono-stats
-Zprint-mono-items

Will be nice to name it -Zprint-mono-stats :-)

bjorn3 · 2025-08-04T13:18:47Z

compiler/rustc_codegen_ssa/src/base.rs

@@ -735,6 +735,11 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
    crate::assert_module_sources::assert_module_sources(tcx, &|cgu_reuse_tracker| {
        for (i, cgu) in codegen_units.iter().enumerate() {
            let cgu_reuse = cgu_reuse[i];
+
+            if tcx.sess.opts.unstable_opts.print_mono_items {
+                println!("CGU_REUSE {} {cgu_reuse}", cgu.name());


Maybe do this inside the cgu_reuse_tracker instead? The current changes in this PR don't apply to cg_clif.

Good point, moved it to assert_module_sources.

saethlin · 2025-08-07T23:03:14Z

@bors r+ rollup

bors · 2025-08-07T23:03:17Z

📌 Commit d857d54 has been approved by saethlin

It is now in the queue for this repository.

Print CGU reuse statistics in `-Zprint-mono-items` I'm trying to expose more information about incremental profiling from rustc (https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Profiling.2Fanalysis.20of.20incremental.20builds/with/531383501). One of the things that would be quite useful to expose is the CGU reuse state, so that when you do a rebuild, you can see all the CGUs (and all the functions) that had to be recompiled. Currently, we have (AFAIK) two ways of outputting monomorphization statistics: 1) `-Zdump-mono-stats` outputs statistics about number of instantiations and expected compilation cost of individual functions in the local crate being compiled. It can be outputted either as Markdown or JSON. 2) `-Zprint-mono-items` outputs a pair (item, CGU) for each monomorphized item. I was thinking about recording CGU statistics in the self-profile output, but I realized that as a simpler step, we could just add CGU reuse data to `-Zprint-mono-items`, as an additional output. That is what this PR does.

Rollup of 19 pull requests Successful merges: - #144400 (`tests/ui/issues/`: The Issues Strike Back [3/N]) - #144764 ([codegen] assume the tag, not the relative discriminant) - #144807 (Streamline config in bootstrap) - #144899 (Print CGU reuse statistics in `-Zprint-mono-items`) - #144909 (Add new `test::print_merged_doctests_times` used by rustdoc to display more detailed time information) - #144912 (Resolver: introduce a conditionally mutable Resolver for (non-)speculative resolution.) - #144914 (Add support for `ty::Instance` path shortening in diagnostics) - #144931 ([win][arm64ec] Fix msvc-wholearchive for Arm64EC) - #144999 (coverage: Remove all unstable support for MC/DC instrumentation) - #145009 (A couple small changes for rust-analyzer next-solver work) - #145030 (GVN: Do not flatten derefs with ProjectionElem::Index. ) - #145042 (stdarch subtree update) - #145047 (move `type_check` out of `compute_regions`) - #145051 (Prevent name collisions with internal implementation details) - #145053 (Add a lot of NLL `known-bug` tests) - #145055 (Move metadata symbol export from exported_non_generic_symbols to exported_symbols) - #145057 (Clean up some resolved test regressions of const trait removals in std) - #145068 (Readd myself to review queue) - #145070 (Add minimal `armv7a-vex-v5` tier three target) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #144899 - Kobzol:cgu-reuse, r=saethlin Print CGU reuse statistics in `-Zprint-mono-items` I'm trying to expose more information about incremental profiling from rustc (https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Profiling.2Fanalysis.20of.20incremental.20builds/with/531383501). One of the things that would be quite useful to expose is the CGU reuse state, so that when you do a rebuild, you can see all the CGUs (and all the functions) that had to be recompiled. Currently, we have (AFAIK) two ways of outputting monomorphization statistics: 1) `-Zdump-mono-stats` outputs statistics about number of instantiations and expected compilation cost of individual functions in the local crate being compiled. It can be outputted either as Markdown or JSON. 2) `-Zprint-mono-items` outputs a pair (item, CGU) for each monomorphized item. I was thinking about recording CGU statistics in the self-profile output, but I realized that as a simpler step, we could just add CGU reuse data to `-Zprint-mono-items`, as an additional output. That is what this PR does.

Rollup of 19 pull requests Successful merges: - rust-lang/rust#144400 (`tests/ui/issues/`: The Issues Strike Back [3/N]) - rust-lang/rust#144764 ([codegen] assume the tag, not the relative discriminant) - rust-lang/rust#144807 (Streamline config in bootstrap) - rust-lang/rust#144899 (Print CGU reuse statistics in `-Zprint-mono-items`) - rust-lang/rust#144909 (Add new `test::print_merged_doctests_times` used by rustdoc to display more detailed time information) - rust-lang/rust#144912 (Resolver: introduce a conditionally mutable Resolver for (non-)speculative resolution.) - rust-lang/rust#144914 (Add support for `ty::Instance` path shortening in diagnostics) - rust-lang/rust#144931 ([win][arm64ec] Fix msvc-wholearchive for Arm64EC) - rust-lang/rust#144999 (coverage: Remove all unstable support for MC/DC instrumentation) - rust-lang/rust#145009 (A couple small changes for rust-analyzer next-solver work) - rust-lang/rust#145030 (GVN: Do not flatten derefs with ProjectionElem::Index. ) - rust-lang/rust#145042 (stdarch subtree update) - rust-lang/rust#145047 (move `type_check` out of `compute_regions`) - rust-lang/rust#145051 (Prevent name collisions with internal implementation details) - rust-lang/rust#145053 (Add a lot of NLL `known-bug` tests) - rust-lang/rust#145055 (Move metadata symbol export from exported_non_generic_symbols to exported_symbols) - rust-lang/rust#145057 (Clean up some resolved test regressions of const trait removals in std) - rust-lang/rust#145068 (Readd myself to review queue) - rust-lang/rust#145070 (Add minimal `armv7a-vex-v5` tier three target) r? `@ghost` `@rustbot` modify labels: rollup

Rollup of 19 pull requests Successful merges: - rust-lang#144400 (`tests/ui/issues/`: The Issues Strike Back [3/N]) - rust-lang#144764 ([codegen] assume the tag, not the relative discriminant) - rust-lang#144807 (Streamline config in bootstrap) - rust-lang#144899 (Print CGU reuse statistics in `-Zprint-mono-items`) - rust-lang#144909 (Add new `test::print_merged_doctests_times` used by rustdoc to display more detailed time information) - rust-lang#144912 (Resolver: introduce a conditionally mutable Resolver for (non-)speculative resolution.) - rust-lang#144914 (Add support for `ty::Instance` path shortening in diagnostics) - rust-lang#144931 ([win][arm64ec] Fix msvc-wholearchive for Arm64EC) - rust-lang#144999 (coverage: Remove all unstable support for MC/DC instrumentation) - rust-lang#145009 (A couple small changes for rust-analyzer next-solver work) - rust-lang#145030 (GVN: Do not flatten derefs with ProjectionElem::Index. ) - rust-lang#145042 (stdarch subtree update) - rust-lang#145047 (move `type_check` out of `compute_regions`) - rust-lang#145051 (Prevent name collisions with internal implementation details) - rust-lang#145053 (Add a lot of NLL `known-bug` tests) - rust-lang#145055 (Move metadata symbol export from exported_non_generic_symbols to exported_symbols) - rust-lang#145057 (Clean up some resolved test regressions of const trait removals in std) - rust-lang#145068 (Readd myself to review queue) - rust-lang#145070 (Add minimal `armv7a-vex-v5` tier three target) r? `@ghost` `@rustbot` modify labels: rollup

Rollup of 19 pull requests Successful merges: - rust-lang/rust#144400 (`tests/ui/issues/`: The Issues Strike Back [3/N]) - rust-lang/rust#144764 ([codegen] assume the tag, not the relative discriminant) - rust-lang/rust#144807 (Streamline config in bootstrap) - rust-lang/rust#144899 (Print CGU reuse statistics in `-Zprint-mono-items`) - rust-lang/rust#144909 (Add new `test::print_merged_doctests_times` used by rustdoc to display more detailed time information) - rust-lang/rust#144912 (Resolver: introduce a conditionally mutable Resolver for (non-)speculative resolution.) - rust-lang/rust#144914 (Add support for `ty::Instance` path shortening in diagnostics) - rust-lang/rust#144931 ([win][arm64ec] Fix msvc-wholearchive for Arm64EC) - rust-lang/rust#144999 (coverage: Remove all unstable support for MC/DC instrumentation) - rust-lang/rust#145009 (A couple small changes for rust-analyzer next-solver work) - rust-lang/rust#145030 (GVN: Do not flatten derefs with ProjectionElem::Index. ) - rust-lang/rust#145042 (stdarch subtree update) - rust-lang/rust#145047 (move `type_check` out of `compute_regions`) - rust-lang/rust#145051 (Prevent name collisions with internal implementation details) - rust-lang/rust#145053 (Add a lot of NLL `known-bug` tests) - rust-lang/rust#145055 (Move metadata symbol export from exported_non_generic_symbols to exported_symbols) - rust-lang/rust#145057 (Clean up some resolved test regressions of const trait removals in std) - rust-lang/rust#145068 (Readd myself to review queue) - rust-lang/rust#145070 (Add minimal `armv7a-vex-v5` tier three target) r? `@ghost` `@rustbot` modify labels: rollup

rustbot assigned jieyouxu Aug 4, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 4, 2025

rustbot assigned saethlin and unassigned jieyouxu Aug 4, 2025

bjorn3 reviewed Aug 4, 2025

View reviewed changes

Print CGU reuse statistics when -Zprint-mono-items is enabled

d857d54

Kobzol force-pushed the cgu-reuse branch from 25d8e13 to d857d54 Compare August 4, 2025 13:43

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 Aug 7, 2025

Zalathar mentioned this pull request Aug 8, 2025

Rollup of 19 pull requests #145077

Merged

bors merged commit 2b7659c into rust-lang:master Aug 8, 2025
10 checks passed

rustbot added this to the 1.91.0 milestone Aug 8, 2025

Kobzol deleted the cgu-reuse branch August 8, 2025 09:12

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