Introduce -Zsplit-metadata option #120855

bjorn3 · 2024-02-09T18:48:03Z

This will split the crate metadata out of library files. Instead only the svh and a bit of extra metadata is preserved to allow for loading the right rmeta file. This significantly reduces library size. In addition it allows for cheaper checks if different library files are the same crate.

A fair amount of the complexity in this PR is to work around the fact that cargo doesn't directly support this option yet.

Fixes #23366
Closes #29511
Fixes #57076

Revives #93945

rustbot · 2024-02-09T18:48:11Z

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bjorn3 · 2024-02-09T18:49:19Z

compiler/rustc_metadata/src/locator.rs

                    rlibs.insert(loc_canon, PathKind::ExternFlag);
                } else if loc.file_name().unwrap().to_str().unwrap().ends_with(".rmeta") {
                    rmetas.insert(loc_canon, PathKind::ExternFlag);
                } else {
+                    rmetas.insert(loc_canon.with_extension("rmeta"), PathKind::ExternFlag);


This is a hack. It should probably be replaced with proper lookup in the library search path, or with cargo passing --extern twice for both the rlib/dylib and the rmeta file.

bjorn3 · 2024-02-09T18:51:31Z

src/bootstrap/src/core/build_steps/compile.rs

+            if extension == "so" || extension == "dylib" {
+                // FIXME workaround for the fact that cargo doesn't understand `-Zsplit-metadata`
+                toplevel.push((file_stem.clone(), "rmeta".to_owned(), None));
+            }


This together with the rustc wrapper passing --emit metadata is a big hack working around the fact that cargo doesn't support -Zsplit-metadata and as such doesn't cause a .rmeta file to be emitted and also doesn't track it in the compiler-artifact json message. This also causes issues with recompilations as the loop below may copy outdated rmeta files into the sysroot, which would then result in compiler errors.

bjorn3 · 2024-02-09T18:52:18Z

tests/ui/duplicate_entry_error.rs

@@ -1,4 +1,4 @@
-// normalize-stderr-test "loaded from .*libstd-.*.rlib" -> "loaded from SYSROOT/libstd-*.rlib"
+// normalize-stderr-test "loaded from .*libstd-.*.rmeta" -> "loaded from SYSROOT/libstd-*.rmeta"


The .rmeta file is now considered the canonical one as the .rlib doesn't contain any crate metadata beyond the header.

bjorn3 · 2024-02-09T18:52:35Z

@bors try @rust-timer queue

bors · 2024-02-09T18:53:44Z

⌛ Trying commit 7770002 with merge 8fb65ac...

Introduce -Zsplit-metadata option This will split the crate metadata out of library files. Instead only the svh and a bit of extra metadata is preserved to allow for loading the right rmeta file. This significantly reduces library size. In addition it allows for cheaper checks if different library files are the same crate. A fair amount of the complexity in this PR is to work around the fact that cargo doesn't directly support this option yet. Fixes rust-lang#23366 Fixes rust-lang#57076 Revives rust-lang#93945

bjorn3 · 2024-02-09T18:54:55Z

What would be the best way to handle the necessary cargo changes? We can't use them until the bootstrap compiler includes the cargo changes, but cargo would depend on -Zsplit-metadata which doesn't exist yet. Would unconditionally creating a .rmeta file for dylibs like we do for rlibs be fine? Or should I add a -Zsplit-metadata option to cargo without it getting tested and then once the bootstrap bump happens, change this PR to use this option? Or land -Zsplit-metadata first without actually making any use of it, then update cargo and finally make use of it in the standard library?

bors · 2024-02-09T20:22:41Z

☀️ Try build successful - checks-actions
Build commit: 8fb65ac (8fb65acf1a11f547302c62c93bf83cca3ff4cd9d)

rust-timer · 2024-02-09T23:49:26Z

Finished benchmarking commit (8fb65ac): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.4%	[0.3%, 2.7%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.4%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	7.1%	[5.4%, 8.4%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.2%	[-5.2%, -5.2%]	1
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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

Bootstrap: 666.509s -> 669.311s (0.42%)
Artifact size: 307.99 MiB -> 301.34 MiB (-2.16%)

bjorn3 · 2024-02-10T14:17:08Z

This should save about 4MB on the download size of a toolchain without any extra targets other than the host:

component	`e28fae5` (no split metadata)	`8fb65ac` (split metadata)	diff
rustc	70.54MB	68.39MB	-2.15MB
rust-std	27.63MB	25.61MB	-2MB
rustc-dev	110.84MB	110.90MB	+0.06MB

For Bevy the size of the target dir shrinks a lot:

mode	normal build	`--emit metadata -Zsplit-metadata`	diff	relative diff
debug mode	4.6GiB	4.3GiB	-300MiB	-6.5%
debug mode (no incr comp)	2.8GiB	2.6GiB	-200MiB	-7%
release mode	843.6MiB	614.3MiB	-230MiB	-27%

bors · 2024-02-16T09:54:33Z

☔ The latest upstream changes (presumably #120486) made this pull request unmergeable. Please resolve the merge conflicts.

petrochenkov · 2024-03-07T15:10:42Z

@bjorn3
Could you

Split 2eb24d9 into a separate PR in case it causes regressions
Split all preparatory refactorings like 1065559 into a separate PR
Rebase the remaining changes, make CI green, cleanup history, and add FIXME comments to logic that only exists due to lacking cargo support

(I'll be able to review the first two PRs quickly.)
@rustbot author

bors · 2024-12-27T15:58:48Z

☔ The latest upstream changes (presumably #134822) made this pull request unmergeable. Please resolve the merge conflicts.

This will split the crate metadata out of library files. Instead only the svh is preserved to allow for loading the right rmeta file. This significicantly reduces library size. In addition it allows for cheaper checks if different library files are the same crate.

petrochenkov · 2025-02-19T15:39:53Z

Status is not very clear, still waiting on author.
@bjorn3 Do you plan to work on completing it this year?

bjorn3 · 2025-02-19T15:43:32Z

I do still plan on working on this.

petrochenkov · 2025-03-13T13:23:56Z

Closing in favor of #137535.

Introduce `-Zembed-metadata` to allow omitting full metadata from rlibs and dylibs This is a continuation of rust-lang#120855 (I was mentored by `@bjorn3` to move it forward). Most of the original code was written by bjorn3, I tried to clean it up a bit and add some documentation and tests. This PR introduces a new unstable compiler flag called `-Zembed-metadata=[no|yes]`, with the default being `yes` (see rust-lang#57076 for context). When set to `no`, rustc will only store a small metadata stub inside rlibs/dylibs instead of the full metadata, to keep their size smaller. It should be used in combination with `--emit=metadata`, so that the users of such a compiled library can still read the metadata from the corresponding `.rmeta` file. [This comment](rust-lang#120855 (comment)) shows an example of binary/artifact size wins that can be achieved using this approach. Contrary to rust-lang#120855, this PR only introduces the new flag, along with a couple of run-make tests and documentation, but does not yet use it in bootstrap to actually compile rustc. I plan to do that as a follow-up step (along with integration in Cargo, which should ideally just always pass this flag to reduce the size of target directories). Fixes rust-lang#23366 Closes rust-lang#29511 Fixes rust-lang#57076 Another attempt of rust-lang#93945 and rust-lang#120855. r? `@petrochenkov`

Introduce `-Zembed-metadata` to allow omitting full metadata from rlibs and dylibs This is a continuation of rust-lang/rust#120855 (I was mentored by `@bjorn3` to move it forward). Most of the original code was written by bjorn3, I tried to clean it up a bit and add some documentation and tests. This PR introduces a new unstable compiler flag called `-Zembed-metadata=[no|yes]`, with the default being `yes` (see rust-lang/rust#57076 for context). When set to `no`, rustc will only store a small metadata stub inside rlibs/dylibs instead of the full metadata, to keep their size smaller. It should be used in combination with `--emit=metadata`, so that the users of such a compiled library can still read the metadata from the corresponding `.rmeta` file. [This comment](rust-lang/rust#120855 (comment)) shows an example of binary/artifact size wins that can be achieved using this approach. Contrary to rust-lang/rust#120855, this PR only introduces the new flag, along with a couple of run-make tests and documentation, but does not yet use it in bootstrap to actually compile rustc. I plan to do that as a follow-up step (along with integration in Cargo, which should ideally just always pass this flag to reduce the size of target directories). Fixes rust-lang/rust#23366 Closes rust-lang/rust#29511 Fixes rust-lang/rust#57076 Another attempt of rust-lang/rust#93945 and rust-lang/rust#120855. r? `@petrochenkov`

@bjorn3

### What does this PR try to resolve? This PR adds Cargo integration for the new unstable `-Zembed-metadata` rustc flag, which was implemented in rust-lang/rust#137535 ([tracking issue](rust-lang/rust#139165)). The new behavior has to be enabled explicitly using a new unstable CLI flag `-Zno-embed-metadata`. The `-Zembed-metadata=no` rustc flag can reduce disk usage of compiled artifacts, and also the size of Rust dynamic library artifacts shipped to users. However, it is not enough to just pass this flag through `RUSTFLAGS`; it needs to be integrated within Cargo, because it interacts with how the `--emit` flag is passed to rustc, and also how `--extern` args are passed to the final linked artifact build by Cargo. Furthermore, using the flag for all crates in a crate graph compiled by Cargo would be suboptimal (this will all be described below). When you pass `-Zembed-metadata=no` to rustc, it will not store Rust metadata into the compiled artifact. This is important when compiling libs/rlibs/dylibs, since it reduces their size on disk. However, this also means that everytime we use this flag, we have to make sure that we also: - Include `metadata` in the `--emit` flag to generate a `.rmeta` file, otherwise no metadata would be generated whatsoever, which would mean that the artifact wouldn't be usable as a dependency. - Pass also `--extern <dep>=<path>.rmeta` when compiling the final linkable artifact. Before, Cargo would only pass `--extern <dep>=<path>.[rlib|so|dll]`. Since with `-Zembed-metadata=no`, the metadata is only in the `.rmeta` file and not in the rlib/dylib, this is needed to help rustc find out where the metadata lies. - Note: this essentially doubles the cmdline length when compiling the final linked artifact. Not sure if that is a concern. The two points above is what this PR implements, and why this rustc flag needs Cargo integration. The `-Zembed-metadata` flag is only passed to libs, rlibs and dylibs. It does not seem to make sense for other crate types. The one situation where it might make sense are proc macros, but according to @bjorn3 (who initially came up with the idea for `-Zembed-metadata`, it isn't really worth it). Here is a table that summarizes the changes in passed flags and generated files on disk for rlibs and dylibs: | **Crate type** | **Flags** | **Generated files** | **Disk usage** | |--|--|--|--| | Rlib/Lib (before) | `--emit=dep-info,metadata,link` | `.rlib` (with metadata), `.rmeta` (for pipelining) | - | | Rlib/Lib (after) | `--emit=dep-info,metadata,link -Zembed-metadata=no` | `.rlib` (without metadata), `.rmeta` (for metadata/pipelining) | Reduced (metadata no longer duplicated) | | Dylib (before) | `--emit=dep-info,link` | `[.so\|.dll]` (with metadata) | - | | Dylib (after) | `--emit=dep-info,metadata,link -Zembed-metadata=no` | `[.so\|.dll]` (without metadata), `.rmeta` | Unchanged, but split between two files | Behavior for other target kinds/crate types should be unchanged. From the table above, we can see two benefits of using `-Zembed-metadata=no`: - For rlibs/dylibs, we no longer store their metadata twice in the target directory, thus reducing target directory size. - For dylibs, we store esssentially the same amount of data on disk, but the benefit is that the metadata is now in a separate .rmeta file. This means that you can ship the dylib (`.so`/`.dll`) to users without also shipping the metadata. This would slightly reduce e.g. the [size](rust-lang/rust#120855 (comment)) of the shipped rustc toolchains (note that the size reduction here is after the toolchain has been already heavily compressed). Note that if this behavior ever becomes the default, it should be possible to simplify the code quite a bit, and essentially merge the `requires_upstream_objects` and `benefits_from_split_metadata` functions. I did a very simple initial benchmark to evaluate the space savings on cargo itself and [hyperqueue](https://github.com/It4innovations/hyperqueue) (a mid-size crate from my work) using `cargo build` and `cargo build --release` with and without `-Zembed-metadata=no`: ![image](https://github.com/user-attachments/assets/a26994a2-156f-4863-a823-1042ebe03bf0) For debug/incremental builds, the effect is smaller, as the artifact disk usage is dwarfed by incremental artifacts and debuginfo. But for (non-incremental) release builds, the disk savings (and also performed I/O operations) are significantly reduced. ### How should we test and review this PR? I wrote two basic tests. The second one tests a situation where a crate depends on a dylib dependency, which is quite rare, but the behavior of this has actually changed in this PR (see comparison table above). Testing this on various real-world projects (or even trying to enable it by default across the whole Cargo suite?) might be beneficial. ## Unresolved questions ### Is this a breaking change? With this new behavior, dylibs and rlibs will no longer contain metadata. If they are compiled with Cargo, that shouldn't matter, but other build systems might have to adapt. ### Should this become the default? I think that in terms of disk size usage and performed I/O operations, it is a pure win. It should either generate less disk data (for rlibs) or the ~same amount of data for dylibs (the data will be a bit larger, because the dylib will still contain a metadata stub header, but that's like 50 bytes and doesn't scale with the size of the dylib, so it's negligible). So I think that eventually, we should just do this by default in Cargo, unless some concerns are found. I suppose that before stabilizing we should also benchmark the effect on compilation performance.

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Introduce -Zsplit-metadata option #93945

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Remove a workaround for a bug #122186

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Simplify find_commandline_library

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Fix stdlib building

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Rustbuild: enable -Zsplit-metadata for stage != 0

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Fix cg_clif tests

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Kobzol mentioned this pull request Feb 24, 2025

Introduce -Zembed-metadata to allow omitting full metadata from rlibs and dylibs #137535

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Add -Zsplit-metadata flag to reduce disk usage rust-lang/compiler-team#851

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Add support for -Zembed-metadata rust-lang/cargo#15378

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Introduce -Zsplit-metadata option #120855

Introduce -Zsplit-metadata option #120855

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		@@ -1,4 +1,4 @@
		// normalize-stderr-test "loaded from .libstd-..rlib" -> "loaded from SYSROOT/libstd-*.rlib"
		// normalize-stderr-test "loaded from .libstd-..rmeta" -> "loaded from SYSROOT/libstd-*.rmeta"

Introduce -Zsplit-metadata option #120855

Introduce -Zsplit-metadata option #120855

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rust-timer commented Feb 9, 2024

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

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