[PATCH 10/23] kbuild: cache list, composite object state per object

[Lorenzo Stoakes (ARM)]

When each object's command line is expanded, kbuild has to figure out on
multiple occasions whether it's built-in or part of a module and which
composite object it belongs to.

This causes the time spent on each directory in the kernel tree to grow
O(n^2) with its object count, which is especially problematic for instance
in drivers/gpu/drm/amd/amdgpu with 310 objects.

No-op builds (i.e. make -j $(nproc) when nothing has changed) are
particularly impacted by this.

Fix the issue by caching this data and looking it up instead of getting it
over and over again.

This has a particularly large impact on allmodconfig builds.

Whole build, 128-thread Threadripper 9980X, best of N runs:

                                         before   after     delta
                                         -------------------------------
  x86 defconfig, no-op make, gcc           0.94s    0.92s    -0.02s (-2%)
  x86 defconfig, no-op make, clang          1.1s     1.1s    -0.01s (-1%)
  x86 allmodconfig, no-op make, gcc        13.0s    12.3s    -0.64s (-5%)
  x86 allmodconfig, no-op make, clang      13.8s    13.2s    -0.64s (-5%)
  x86 allmodconfig, touch mm/vma.c, gcc    41.6s    40.9s    -0.70s (-2%)
  x86 allmodconfig, touch mm/vma.c, clang  38.1s    37.6s    -0.52s (-1%)

Assisted-by: LLM
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 scripts/Makefile.build | 8 ++++++++
 scripts/Makefile.lib   | 7 +++----
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 4349108e75e1..2cabfe85b798 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -122,6 +122,14 @@ multi-obj-m	:= $(addprefix $(obj)/, $(multi-obj-m))
 subdir-ym	:= $(addprefix $(obj)/, $(subdir-ym))
 endif
 
+# Cache which list each object is in and which composite objects it belongs to,
+# once per object for $(part-of-builtin), $(part-of-module) and $(modname-multi).
+$(foreach o, $(real-obj-y) $(lib-y), $(eval part-of-builtin_$o := y))
+$(foreach o, $(real-obj-m), $(eval part-of-module_$o := y))
+$(foreach m, $(multi-obj-ym), \
+	$(foreach o, $(call suffix-search, $m, .o, -objs -y -m), \
+		$(eval modname-multi_$o += $(m:.o=))))
+
 ifndef obj
 $(warning kbuild: Makefile.build is included improperly)
 endif
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 0a4fdd8bd975..2f447bc25e7b 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -2,8 +2,7 @@
 
 # Finds the multi-part object the current object will be linked into.
 # If the object belongs to two or more multi-part objects, list them all.
-modname-multi = $(sort $(foreach m,$(multi-obj-ym),\
-		$(if $(filter $*.o, $(call suffix-search, $m, .o, -objs -y -m)),$(m:.o=))))
+modname-multi = $(sort $(modname-multi_$*.o))
 
 __modname = $(or $(modname-multi),$(basetarget))
 
@@ -149,8 +148,8 @@ endif
 # If $(is-kernel-object) is 'y', this object will be linked to vmlinux or modules
 is-kernel-object = $(or $(part-of-builtin),$(part-of-module))
 
-part-of-builtin = $(if $(filter $(basename $@).o, $(real-obj-y) $(lib-y)),y)
-part-of-module = $(if $(filter $(basename $@).o, $(real-obj-m)),y)
+part-of-builtin = $(part-of-builtin_$(basename $@).o)
+part-of-module = $(part-of-module_$(basename $@).o)
 quiet_modtag = $(if $(part-of-module),[M],   )
 
 modkern_cflags =                                          \

-- 
2.55.0