[Buildroot] [PATCH] core: Use of percent_defconfig seems to impact performance

[Nevo Hed]

Noticed a significant slowdown with rise of number of external trees
in our env.  This slowdown seemed to be related to invocations if the
percent_defconfig function (GNU Make 4.3).

While I have not do a deep dive in analyzing the performance issue, it
felt like redefining the %_defconfig rule N times impact performance.

This patch makes %_defconfig a single rule which combines uses the
first return of a wildcard expression.

Timing (seconds) of `make pc_x86_64_bios_defconfig` with 1-8 external
trees:

    #Trees    Before    After
         1      0.38     0.37
         2      0.32     0.31
         3      0.31     0.33
         4      0.36     0.32
         5      0.45     0.35
         6      1.26     0.36
         7      9.10     0.36
         8     85.93     0.42

Signed-off-by: Nevo Hed <nhed+buildroot at starry.com>
---
 Makefile | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/Makefile b/Makefile
index ec7c034ac1..a8298dd5fd 100644
--- a/Makefile
+++ b/Makefile
@@ -1010,13 +1010,12 @@ oldconfig syncconfig olddefconfig: $(BUILD_DIR)/buildroot-config/conf outputmake
 defconfig: $(BUILD_DIR)/buildroot-config/conf outputmakefile
 	@$(COMMON_CONFIG_ENV) $< --defconfig$(if $(DEFCONFIG),=$(DEFCONFIG)) $(CONFIG_CONFIG_IN)
 
-define percent_defconfig
-# Override the BR2_DEFCONFIG from COMMON_CONFIG_ENV with the new defconfig
-%_defconfig: $(BUILD_DIR)/buildroot-config/conf $(1)/configs/%_defconfig outputmakefile
-	@$$(COMMON_CONFIG_ENV) BR2_DEFCONFIG=$(1)/configs/$$@ \
-		$$< --defconfig=$(1)/configs/$$@ $$(CONFIG_CONFIG_IN)
-endef
-$(eval $(foreach d,$(call reverse,$(TOPDIR) $(BR2_EXTERNAL_DIRS)),$(call percent_defconfig,$(d))$(sep)))
+%_defconfig: $(BUILD_DIR)/buildroot-config/conf  outputmakefile
+	@defconfig=$(firstword $(foreach d,\
+		$(call reverse,$(TOPDIR) $(BR2_EXTERNAL_DIRS)),$(wildcard $(d)/configs/$@))); \
+	if [ -z "$${defconfig}" ]; then echo "Can't find $@" >&2; exit 1; fi; \
+	$(COMMON_CONFIG_ENV) BR2_DEFCONFIG=$${defconfig} \
+		$< --defconfig=$${defconfig} $(CONFIG_CONFIG_IN)
 
 update-defconfig: savedefconfig
 
-- 
2.37.3