msvc: cache C++20 named modules built with clang-cl by Kataglyphis · Pull Request #2876 · mozilla/sccache

Kataglyphis · 2026-09-28T17:35:40Z

CMake 4.4 builds C++20 modules with clang-cl and passes the module flags bare, in an @<target>.modmap file. On main, -x c++-module made interface units uncacheable, and -fmodule-file left the BMI out of the hash, so importers hit a stale object after their module changed. -x c++-module is cached only together with -fmodule-output=; the -clang:/-Xclang spellings stay refused.

CI has no clang-cl, so I checked it with CMake 4.4.3, Ninja and clang-cl 23.1.1: a clean rebuild hits and restores the BMI; after a module change the importer misses and the program prints the new value.

Refs #2629

🤖 Worked with Claude Code

codecov-commenter · 2026-09-28T17:39:36Z

Codecov Report

❌ Patch coverage is 97.19626% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.06%. Comparing base (7bdb9bd) to head (dea2018).

Files with missing lines	Patch %	Lines
src/compiler/msvc.rs	97.87%	2 Missing ⚠️
src/compiler/gcc.rs	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2876      +/-   ##
==========================================
+ Coverage   78.00%   78.06%   +0.05%     
==========================================
  Files          73       73              
  Lines       43136    43233      +97     
==========================================
+ Hits        33650    33749      +99     
+ Misses       9486     9484       -2

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A new compiler-cache-key input restores the cache into the workspace (.ci-cache), hands the container CI_COMPILER_CACHE and saves it after the build on pushes. sccache cannot write to a mounted directory on Windows, so the build script moves the cache into container-local dirs and back. Not for clang-cl module builds until mozilla/sccache#2876. Also records that a cache cap below the working set is slower than no cache (Ballett's release build: 782 s capped vs 533 s uncached). Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

…ger says sccache stores nothing on modules The hub pin moves 89824691 -> 425f4019: Invoke-CmakeConfigureAndBuild -DisableSccache now overrides a preset's COMPILER_CACHE, and the hub's sccache-on-modules page is rewritten from measurement. The other two commits in the range (Windows ROCm torch source build, the deleted :latest-cross tags) touch nothing this repo builds. AGENTS.md section 2 pointed at that page as "it stores nothing there". The released sccache does store module importers on clang-cl, without their BMIs, and serves stale objects after an interface edit until mozilla/sccache#2876 ships. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

sylvestre · 2026-10-07T16:42:10Z

            Some(Clang(s)) => clangs.push(s.clone()),
+            Some(ClangModuleFile(val)) if is_clang && !val.is_empty() => {
+                let val = val.to_string_lossy();
+                extra_hash_files.push(cwd.join(val.split_once('=').map_or(&*val, |(_, p)| p)));


the name=path split is also done twice in gcc.rs, could it be a small shared helper?

Good point. Didn't see that. I created a small helper with a proper test.

CMake passes clang-cl's module flags bare: `-x c++-module` made interface units uncacheable, and `-fmodule-file` left the BMI out of the hash, so an importer could hit a stale object after its module changed. Cache `-x c++-module` only together with `-fmodule-output=`, with the BMI as an output, and hash the BMI of each `-fmodule-file`. A `/Tc` or `/Tp` input stays uncached with `-x c++-module`, since clang-cl then drops both module flags. Refs mozilla#2629 Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

gcc.rs split `-fmodule-file`'s `name=path` value in two places and msvc.rs in a third. Move it into one gcc::module_file_path helper, with a unit test. A value without `=` now keeps its original OsStr on the clang-cl path too, as gcc.rs already did, instead of going through to_string_lossy. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

sylvestre · 2026-10-14T20:19:21Z

    msvc_take_arg!("we", OsString, Concatenated, PassThroughWithSuffix),
    msvc_take_arg!("winsysroot", PathBuf, CanBeSeparated, PassThroughWithPath),
    msvc_take_arg!("wo", OsString, Concatenated, PassThroughWithSuffix),
+    msvc_take_arg!("x", OsString, Separated, ClangLanguage),


what about -xc++-module? it would pass through as an unknown flag and the BMI wouldn't be tracked
could it be CanBeSeparated, or refused?

Good catch! -x is now CanBeSeparated, as in gcc.rs (didn't see that before) so -xc++-module is cached only together with -fmodule-output=, like -x c++-module. The test covers both spellings.

`-xc++-module` went through as an unknown flag, and clang-cl writes an implicit BMI when `-fmodule-output=` is absent, so `-xc++-module m.cpp` was cached with the object only. `-x` is CanBeSeparated, as in gcc.rs, so both spellings are cached only together with `-fmodule-output=`. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

sylvestre · 2026-10-17T10:13:22Z

Thanks for your PR

Kataglyphis · 2026-10-17T12:43:38Z

Thanks for your PR

Always happy to contribute. Thanks for the review!

…X-only nvcc fix No sccache release carries the two fixes that hold back caching here: mozilla/sccache#2876 (clang-cl C++20 named modules, merged to main today) and #2809, the open fix for #2862 (a PTX-only gencode fails with `Missing "cubin" file output`). - versions.env pins the source (SCCACHE_SOURCE_REV, main at #2876's merge) and SCCACHE_BUILD_ID; shared/upstream/sccache/patches/ holds #2809 as its author wrote it. - build-sccache.sh builds one target in upstream's archive layout and stamps the version (`sccache 0.18.0+1bd7432.pr2809`). - sccache-release.yml builds x86_64/aarch64/riscv64gc musl in upstream's own musl cross containers, pinned by digest, plus x86_64-pc-windows-msvc, and publishes release sccache-v<id> with SHA256SUMS. Verified on CUDA 13.2 and an RTX 2080: unpatched main reproduces #2862. With the patch, cold and warm (14/14 hits) objects of an arch- and define-guarded kernel equal the bare build's host symbols, SASS and PTX, and run correctly on the GPU. The installers still take mozilla's 0.18.0; they move once the release exists. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_013nDuz5yv57sH4oVNMjMUNK

…e's sccache keys clang-cl modules right - windows-x64.yml's Release job restores .ci-cache, hands the container CI_COMPILER_CACHE and SCCACHE_CACHE_SIZE=2500M, and saves it from develop when the build filled it. A prune job (require-saved) keeps one entry, and both callers grant actions: write for it. - Build-Windows.ps1 moves the cache into the container's SCCACHE_DIR, restarts the server so its disk level indexes it, and moves it back in finally. - Test-SccacheModuleSafe gates it: the hub's build (0.18.0+1bd7432.pr2809, with mozilla/sccache#2876) or 0.19.0 on. :winamd64's 0.18.0 serves stale clang-cl module importers, so until that image is rebuilt the run says so and neither uses nor saves the cache. - Hub -> 0eba4bb5 (both lanes install that build; nvcc, hipcc and Rust go through sccache), AccelerANTgine -> 733fbce, OxidANT -> 9ee491f. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_013nDuz5yv57sH4oVNMjMUNK

…X-only nvcc fix No sccache release carries the two fixes that hold back caching here: mozilla/sccache#2876 (clang-cl C++20 named modules, merged to main today) and #2809, the open fix for #2862 (a PTX-only gencode fails with `Missing "cubin" file output`). - versions.env pins the source (SCCACHE_SOURCE_REV, main at #2876's merge) and SCCACHE_BUILD_ID; shared/upstream/sccache/patches/ holds #2809 as its author wrote it. - build-sccache.sh builds one target in upstream's archive layout and stamps the version (`sccache 0.18.0+1bd7432.pr2809`). - sccache-release.yml builds x86_64/aarch64/riscv64gc musl in upstream's own musl cross containers, pinned by digest, plus x86_64-pc-windows-msvc, and publishes release sccache-v<id> with SHA256SUMS. Verified on CUDA 13.2 and an RTX 2080: unpatched main reproduces #2862. With the patch, cold and warm (14/14 hits) objects of an arch- and define-guarded kernel equal the bare build's host symbols, SASS and PTX, and run correctly on the GPU. The installers still take mozilla's 0.18.0; they move once the release exists. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_013nDuz5yv57sH4oVNMjMUNK

…e's sccache keys clang-cl modules right - windows-x64.yml's Release job restores .ci-cache, hands the container CI_COMPILER_CACHE and SCCACHE_CACHE_SIZE=2500M, and saves it from develop when the build filled it. A prune job (require-saved) keeps one entry, and both callers grant actions: write for it. - Build-Windows.ps1 moves the cache into the container's SCCACHE_DIR, restarts the server so its disk level indexes it, and moves it back in finally. - Test-SccacheModuleSafe gates it: the hub's build (0.18.0+1bd7432.pr2809, with mozilla/sccache#2876) or 0.19.0 on. :winamd64's 0.18.0 serves stale clang-cl module importers, so until that image is rebuilt the run says so and neither uses nor saves the cache. - Hub -> b9e451da (both lanes install that build; nvcc, hipcc and Rust go through sccache), AccelerANTgine -> 8ed2fcf, OxidANT -> 1f9a7b4. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_013nDuz5yv57sH4oVNMjMUNK

Kataglyphis marked this pull request as draft September 28, 2026 17:39

Kataglyphis force-pushed the fix/clang-cl-cxx20-modules branch 2 times, most recently from 7f2cb36 to cfb2932 Compare September 28, 2026 21:46

Kataglyphis marked this pull request as ready for review September 28, 2026 23:08

sylvestre reviewed Oct 7, 2026

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Kataglyphis and others added 2 commits October 14, 2026 19:46

Kataglyphis force-pushed the fix/clang-cl-cxx20-modules branch from 9cf612d to 87b1d31 Compare October 14, 2026 18:26

sylvestre reviewed Oct 14, 2026

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sylvestre merged commit 1bd7432 into mozilla:main Oct 17, 2026
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BrewTestBot mentioned this pull request Oct 24, 2026

sccache 0.19.0 Homebrew/homebrew-core#320947

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msvc: cache C++20 named modules built with clang-cl - #2876

msvc: cache C++20 named modules built with clang-cl#2876
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