feat: parameterize BMI paths for Clang module pipeline parity#34
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Introduce BmiTraits abstraction (model.cppm) that encapsulates the
differences between GCC and Clang module compilation:
- BMI directory name (gcm.cache vs pcm.cache)
- BMI file extension (.gcm vs .pcm)
- Whether -fmodule-output= is needed (Clang: yes)
- Whether -fprebuilt-module-path= is needed (Clang: yes)
- Whether -fmodules is needed for P1689 scan (GCC: yes, Clang: no)
Changes across the pipeline:
- ninja_backend: remove is_gcc gate on dyndep, parameterize bmi_path
lambda, add -fmodule-output=$bmi_out for Clang in cxx_module rule,
conditionally emit -fmodules in cxx_scan, pass --bmi-dir/--bmi-ext
to cxx_dyndep and cxx_collect rules
- flags: add -fprebuilt-module-path=<bmiDir> for Clang
- dyndep: add DyndepOptions, parameterize bmi_basename extension and
directory in emit_dyndep/emit_dyndep_single/emit_dyndep_from_files
- cli: cmd_dyndep parses --bmi-dir/--bmi-ext and passes DyndepOptions
Rename gcmFiles→bmiFiles, gcmDir()→bmiDir(), add bmiDirName/manifestTag fields to CacheKey. Manifest format now uses the tag from CacheKey (gcm: or pcm:) and parsing accepts both prefixes for forward compat. Callers set bmiDirName/manifestTag via BmiTraits from PR1.
- Restore is_gcc gate on dyndep (Clang lacks -fdeps-format=p1689r5; needs clang-scan-deps for P1689, future work) - Fix static-deps path: use implicit output (|) for BMI files so $out only contains the .o file; set $bmi_out for all compilers (GCC ignores it, Clang uses it for -fmodule-output) - Add E2E test 38_llvm_modules.sh: multi-module Clang project with export module + cross-module import, verifying pcm.cache/ layout
Clang lacks GCC's -fdeps-format=p1689r5 compiler flag but ships
clang-scan-deps which produces identical P1689 JSON output per-file.
Changes:
- plan.cppm: add scanDepsPath field to BuildPlan
- clang.cppm: add find_scan_deps() helper
- cli.cppm: discover clang-scan-deps in compiler's bin/ directory
- ninja_backend.cppm: enable dyndep when scanDepsPath is available,
emit Clang-specific cxx_scan rule using clang-scan-deps with
-format=p1689 and stdout redirection to .ddi file
This gives Clang the same per-file incremental rebuild capability
that GCC has had via the dyndep pipeline.
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Summary
BmiTraitsabstraction inmodel.cppmthat encapsulates GCC vs Clang module compilation differences (BMI dir/ext, flags needed)is_gccgate on dyndep — Clang 18+ supports P1689 with the same flagsbmi_pathlambda,bmi_basename(), and dyndep emit functions to useBmiTraitsinstead of hardcodedgcm.cache/*.gcm-fmodule-output=$bmi_outtocxx_modulerule for Clang (GCC writes BMI implicitly, Clang needs explicit flag)-fprebuilt-module-path=<bmiDir>to cxxflags for Clang--bmi-dir/--bmi-extthroughcxx_dyndep/cxx_collectNinja rules tomcpp dyndepsubcommandThis is the first of 3 PRs to bring Clang module support to GCC parity on Linux. Design doc:
.agents/docs/2026-05-15-clang-parity-and-toolchain-abstraction.mdTest plan
mcpp buildcompiles successfullymcpp test— all 12 test suites, 104 tests pass