[PATCH v2] 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 whenever we build with the precompiled header.
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.
    
    Changes in v2:
    
     * Move the CCACHE_SLOPPINESS export from the precompiled header targets
       into the ifndef NO_PRECOMPILED_HEADER block.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2464%2FHaraldNordgren%2Fmakefile-ccache-pch-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2464/HaraldNordgren/makefile-ccache-pch-v2
Pull-Request: https://github.com/git/git/pull/2464

Range-diff vs v1:

 1:  1984b13246 ! 1:  e61cca8cca Makefile: let ccache cache objects built with the precompiled header
     @@ Commit message
          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.
     +    Enable them whenever we build with the precompiled header.
          On my machine, "make clean && make -j8" with a warm cache goes from
          about 100 seconds to under 30.
      
     @@ Commit message
          Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
      
       ## Makefile ##
     -@@ Makefile: $(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) $<
     +@@ Makefile: PRECOMPILED_HEADER_GCH := $(addsuffix .gch,$(PRECOMPILED_HEADER))
       
     -+$(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) $<
     + ifndef NO_PRECOMPILED_HEADER
     + PRECOMPILED_HEADER_USERS := $(filter-out $(COMPAT_OBJS) $(REFTABLE_OBJS) $(patsubst %.c,%.o,$(THIRD_PARTY_SOURCES)),$(OBJECTS))
     ++export CCACHE_SLOPPINESS += pch_defines,time_macros
     + endif
       
     + dep_files := $(foreach f,$(OBJECTS) $(PRECOMPILED_HEADER_GCH),$(dir $f).depend/$(notdir $f).d)


 Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Makefile b/Makefile
index a96be506b5..80653bb22c 100644
--- a/Makefile
+++ b/Makefile
@@ -2903,6 +2903,7 @@ PRECOMPILED_HEADER_GCH := $(addsuffix .gch,$(PRECOMPILED_HEADER))
 
 ifndef NO_PRECOMPILED_HEADER
 PRECOMPILED_HEADER_USERS := $(filter-out $(COMPAT_OBJS) $(REFTABLE_OBJS) $(patsubst %.c,%.o,$(THIRD_PARTY_SOURCES)),$(OBJECTS))
+export CCACHE_SLOPPINESS += pch_defines,time_macros
 endif
 
 dep_files := $(foreach f,$(OBJECTS) $(PRECOMPILED_HEADER_GCH),$(dir $f).depend/$(notdir $f).d)

base-commit: 37b3475fb530c14109211faca33716a173db61ed
-- 
gitgitgadget