[PATCH v5 01/11] kbuild: calculate 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 calculating this data once and looking it up thereafter
instead of recomputing 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            1.1s     1.1s    -0.02s (-2%)
  x86 defconfig, no-op make, clang          1.3s     1.3s    -0.01s (-1%)
  x86 allmodconfig, no-op make, gcc        15.3s    14.5s    -0.83s (-5%)
  x86 allmodconfig, no-op make, clang      16.2s    15.5s    -0.65s (-4%)
  x86 allmodconfig, touch mm/vma.c, gcc    35.0s    34.3s    -0.77s (-2%)
  x86 allmodconfig, touch mm/vma.c, clang  33.7s    33.2s    -0.51s (-2%)

Reviewed-by: Nicolas Schier <n.schier@xxxxxxxxx>
Reviewed-by: Kees Cook <kees@xxxxxxxxxx>
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 6127feb970c6..bbe9555c47a8 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 19e96b30a122..8936925d4d50 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 =                                          \

-- 
2.56.0