perf: build optimization (P0/P1/P2)#26
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…nged On a successful build, writes target/.build_cache with the output dir and ninja binary path. On the next invocation, if build.ninja in that dir is newer than all source files and mcpp.toml, invokes ninja directly without re-running toolchain resolve, scanner, make_plan, or emit. Reduces no-change builds from ~10s to <0.5s.
Replace the global build.ninja.dd (one dyndep file for all modules) with per-file .dd files. Each .cppm gets its own .ddi → .dd conversion via the new cxx_dyndep rule, and each compile edge references only its own .dd file. Before: touching one file → all 39 modules dirty → full rebuild (~21s) After: touching one file → only that file's .dd dirty → incremental New `mcpp dyndep --single --output <file.dd> <file.ddi>` mode added for the per-file conversion. Legacy multi-file mode still available.
GCC always updates the .gcm (BMI) file's timestamp even when the module interface hasn't changed. This causes all downstream modules to be recompiled unnecessarily. Fix: the cxx_module rule now backs up the BMI before compilation, and if the new BMI is byte-identical to the backup, restores the old file (preserving its timestamp). Combined with restat = 1 in the per-file dyndep entries, ninja skips downstream modules when only the implementation changed. This means modifying a function body without changing the module interface no longer triggers a cascade rebuild of all importers.
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Summary
Three build performance optimizations for incremental compilation:
P0: Frontend dirty check — skip
prepare_build(toolchain resolve, scanner, make_plan) whenbuild.ninjais newer than all source files +mcpp.toml. Reduces no-change builds from ~10s to <0.5s.P1: Per-file dyndep — replace the global
build.ninja.ddwith per-file.ddfiles. Each.cppmgets its own.ddi → .ddconversion. Modifying one file no longer marks all 39 modules dirty.P2: BMI copy_if_different — backup
.gcmbefore compilation, restore old file if byte-identical. Combined withrestat = 1, prevents cascade rebuilds when only implementation changes (not interface).Test plan
mcpp buildsucceeds (full build)mcpp test— all 86 unit tests pass.ddfiles