[PATCH] Makefile: let ccache cache objects built with the precompiled header

["Harald Nordgren via GitGitGadget" <gitgitgadget@gmail.com>]

From: Harald Nordgren <haraldnordgren@gmail.com>

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 <haraldnordgren@gmail.com>
---
    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

 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
+
 $(filter-out $(PRECOMPILED_HEADER_USERS),$(OBJECTS)): %.o: %.c GIT-CFLAGS $(missing_dep_dirs) $(missing_compdb_dir)
 	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(compdb_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
 

base-commit: 7e34d8174dcda3435d1404c9a30bef46afe6c13e
-- 
gitgitgadget