Re: [PATCH] Makefile: let ccache cache objects built with the precompiled header
flat view
From: Ben Knoble <hidden>
Date: 2026-10-27 15:59:02
Le 27 oct. 2026 à 08:49, Harald Nordgren via GitGitGadget [off-list ref] a écrit : From: Harald Nordgren [off-list ref] Since 8d3a673147 (Makefile: precompile "git-compat-util.h", 2026-09-15), rebuilding Git with ccache is almost as slow as building it without. ccache refuses to cache a compilation that uses a precompiled header unless its "pch_defines" and "time_macros" sloppiness is enabled, so it runs the compiler for nearly every object file, even after a "make clean" in a tree it has already built. Both settings are safe for our build. The precompiled header is only pulled in with "-include" ahead of the source file, so no "#define" in a source file can come before it, and none of our sources use __DATE__, __TIME__ or __TIMESTAMP__. Enable them for the precompiled header and the objects built with it. On my machine, "make clean && make -j8" with a warm cache goes from about 100 seconds to under 30. With Clang, switching to a branch that changes one of the headers the precompiled header includes and back again still misses the cache, because Clang records the modification times of those headers in the precompiled header. Signed-off-by: Harald Nordgren <redacted> --- Makefile: let ccache cache objects built with the precompiled header With ccache, building after switching between two feature branches gets about 6 times faster. The first build of each branch gets about 25% slower while ccache fills its cache. Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2464%2FHaraldNordgren%2Fmakefile-ccache-pch-v1 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2464/HaraldNordgren/makefile-ccache-pch-v1 Pull-Request: https://github.com/git/git/pull/2464
I can’t comment on the above, not being a ccache user myself.
quoted hunk ↗ jump to hunk
Makefile | 2 ++ 1 file changed, 2 insertions(+)diff --git a/Makefile b/Makefile index a96be506b5..43ea5e0c40 100644 --- a/Makefile +++ b/Makefile@@ -2946,6 +2946,8 @@ $(PRECOMPILED_HEADER_GCH): %.gch: % GIT-CFLAGS $(missing_dep_dirs) $(missing_com$(PRECOMPILED_HEADER_USERS): %.o: %.c $(PRECOMPILED_HEADER_GCH) GIT-CFLAGS $(missing_dep_dirs) $(missing_compdb_dir) $(QUIET_CC)$(CC) -o $*.o -c -include $(PRECOMPILED_HEADER) -Winvalid-pch $(dep_args) $(compdb_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $< +$(PRECOMPILED_HEADER_GCH) $(PRECOMPILED_HEADER_USERS): export CCACHE_SLOPPINESS += pch_defines,time_macros +
This is a little curious to me. I’m familiar with the GNU Make-ism to set variables for particular targets; how does the export work? Is it the same but with a propagation to the recipe processes? Would it be better (for some definition…) to put the export in the 2 pre-compiled header recipes directly? And, what about Meson?