Makefile: let ccache cache objects built with the precompiled header

PATCH Superseded Harald Nordgren via GitGitGadget 4 participants 8 messages Oct 27 – Oct 30 Kbuild

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Harald Nordgren via GitGitGadget

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2026-10-27 12:40 UTC #0 lore raw

PATCH 1/1 Makefile: let ccache cache objects built with the precompiled header

From: Harald Nordgren <[email protected]>

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 <[email protected]>

---
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(+)

Makefile +2 -0

index a96be506b5..43ea5e0c40 100644
@@ -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

Signature · 1 line

gitgitgadget

Ben Knoble

2026-10-27 15:59 UTC #1 lore raw

Quoted 30 lines · Harald Nordgren via GitGitGadget

From: Harald Nordgren <[email protected]>
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 <[email protected]>
---
Makefile: let ccache cache objects built with the precompiled header

Quoted 7 lines

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.

Makefile $(PRECOMPILED_HEADER_GCH): %.gch: % GIT-CFLAGS $(missing_dep_dirs) $(missing_com · line 2950

Quoted 13 lines

Makefile | 2 ++

1 file changed, 2 insertions(+)

Makefile +2 -0

index a96be506b5..43ea5e0c40 100644
@@ -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?

Harald Nordgren

2026-10-27 20:00 UTC #2 lore raw

Quoted 11 lines

Makefile $(PRECOMPILED_HEADER_GCH): %.gch: % GIT-CFLAGS $(missing_dep_dirs) $(missing_com · line 2950
Quoted 6 lines
Makefile +2 -0
@@ -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?

Quoted 2 lines

Would it be better (for some definition…) to put the export in the 2 pre-compiled header recipes directly?

Yeah, that could be a good idea.

Quoted 1 line

And, what about Meson?

I think it's not touched by this? My goal is just to speed up my local builds.

Harald

D. Ben Knoble

2026-10-27 21:57 UTC #3 lore raw

Quoted 4 lines · Harald Nordgren

Quoted 2 lines
And, what about Meson?
I think it's not touched by this? My goal is just to speed up my local builds.

Yeah, it'd generally be nice to keep the 2 on par though ;)

I don't know enough about our pch infra, though, so I'll bow out from here. Just thought maybe we should make Meson do the same?

Signature · 1 line

D. Ben Knoble

Patrick Steinhardt

2026-10-28 07:35 UTC #4 lore raw

Quoted 11 lines · D. Ben Knoble

Quoted 4 lines · Harald Nordgren
Quoted 2 lines
And, what about Meson?
I think it's not touched by this? My goal is just to speed up my local builds.
Yeah, it'd generally be nice to keep the 2 on par though ;)
I don't know enough about our pch infra, though, so I'll bow out from here. Just thought maybe we should make Meson do the same?

As Meson auto-detects ccache I'd think that stuff like this should rather be handled in Meson itself. I e.g. found [1], which feels like the right direction to me.

But if somebody wants to implement this in our instructions meanwhile I won't complain.

Patrick

[1]: https://github.com/mesonbuild/meson/issues/14662

Harald Nordgren

2026-10-28 09:59 UTC #5 lore raw

Quoted 13 lines

Quoted 8 lines
Quoted 3 lines
Quoted 1 line
And, what about Meson?
I think it's not touched by this? My goal is just to speed up my local builds.
Yeah, it'd generally be nice to keep the 2 on par though ;)
I don't know enough about our pch infra, though, so I'll bow out from here. Just thought maybe we should make Meson do the same?
As Meson auto-detects ccache I'd think that stuff like this should rather be handled in Meson itself. I e.g. found [1], which feels like the right direction to me.

Quoted 6 lines

But if somebody wants to implement this in our instructions meanwhile I won't complain.
Patrick
[1]: https://github.com/mesonbuild/meson/issues/14662

Ok, so I'll leave Meson out of this. Maybe I'll look into opening a PR there instead.

Harald

Harald Nordgren via GitGitGadget

author

2026-10-29 08:33 UTC #7 lore raw

PATCH v2 1/1 Makefile: let ccache cache objects built with the precompiled header

In reply to Harald Nordgren via GitGitGadget

From: Harald Nordgren <[email protected]>

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.

Signed-off-by Harald Nordgren <[email protected]>

---
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 <[email protected]>

## 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(+)

Makefile +1 -0

index a96be506b5..80653bb22c 100644
@@ -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

Signature · 1 line

gitgitgadget

Harald Nordgren

2026-10-30 22:26 UTC #6 lore raw

In reply to Harald Nordgren

Quoted 16 lines

Quoted 16 lines
Quoted 8 lines
Quoted 3 lines
Quoted 1 line
And, what about Meson?
I think it's not touched by this? My goal is just to speed up my local builds.
Yeah, it'd generally be nice to keep the 2 on par though ;)
I don't know enough about our pch infra, though, so I'll bow out from here. Just thought maybe we should make Meson do the same?
As Meson auto-detects ccache I'd think that stuff like this should rather be handled in Meson itself. I e.g. found [1], which feels like the right direction to me.
But if somebody wants to implement this in our instructions meanwhile I won't complain.

Quoted 6 lines

Quoted 3 lines
Patrick
[1]: https://github.com/mesonbuild/meson/issues/14662
Ok, so I'll leave Meson out of this. Maybe I'll look into opening a PR there instead.

This has now been merged into Meson:
https://github.com/mesonbuild/meson/pull/16317

Harald