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Subject[RFC PATCH 4/4] kbuild: evaluate cc-option and friends only when building kernel
Date
$(call cc-option,...) is costly because it invokes the C compiler.

Given that only some targets need compiler flags, it is pointless
to compute them all the time.

The variable no-dot-config-targets lists the targets we can run
without the .config file, such as "make clean", "make help", etc.

My idea is similar here. We can add no-compiler-targets to list the
targets we can run without the target compiler information. This
includes no-dot-config-targets + config targets + misc.

When we run only targets listed in the no-compiler-targets, we can
set cc-option and friends to no-op. The hostcc-option is an exception
since the host compiler is needed for building fixdep, kconfig, etc.

I did not add "dtbs" and "%.dtb" to no-compiler-targets. This is
intentional. Theoretically, we can build device tree blobs without
the C compiler. It it true that in-kernel DT files are pre-processed
by CPP before DTC, but KBUILD_CPPFLAGS is unused. However, for the
reason of scripts/dtc/ location, Kbuild descends into scripts/mod/
before the DT build. I do not want to trigger the unrelated re-build
of modpost when building DT. Perhaps, we can fix this with further
refactoring, but not now.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
---

Makefile | 10 ++++++++++
scripts/Kbuild.include | 14 +++++++++-----
2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/Makefile b/Makefile
index a4fd682..80e1a38 100644
--- a/Makefile
+++ b/Makefile
@@ -224,9 +224,13 @@ no-dot-config-targets := clean mrproper distclean \
$(version_h) headers_% archheaders archscripts \
kernelversion %src-pkg

+no-compiler-targets := $(no-dot-config-targets) config %config \
+ kernelrelease image_name
+
config-targets := 0
mixed-targets := 0
dot-config := 1
+need-compiler := 1

ifneq ($(filter $(no-dot-config-targets), $(MAKECMDGOALS)),)
ifeq ($(filter-out $(no-dot-config-targets), $(MAKECMDGOALS)),)
@@ -234,6 +238,12 @@ ifneq ($(filter $(no-dot-config-targets), $(MAKECMDGOALS)),)
endif
endif

+ifneq ($(filter $(no-compiler-targets), $(MAKECMDGOALS)),)
+ ifeq ($(filter-out $(no-compiler-targets), $(MAKECMDGOALS)),)
+ need-compiler := 0
+ endif
+endif
+
ifeq ($(KBUILD_EXTMOD),)
ifneq ($(filter config %config,$(MAKECMDGOALS)),)
config-targets := 1
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 9ffd3dd..222d0a2 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -96,6 +96,13 @@ try-run = $(shell set -e; \
fi; \
rm -f "$$TMP" "$$TMPO")

+# hostcc-option
+# Usage: cflags-y += $(call hostcc-option,-march=winchip-c6,-march=i586)
+hostcc-option = $(call __cc-option, $(HOSTCC),\
+ $(HOSTCFLAGS) $(HOST_EXTRACFLAGS),$(1),$(2))
+
+ifeq ($(need-compiler),1)
+
# as-option
# Usage: cflags-y += $(call as-option,-Wa$(comma)-isa=foo,)

@@ -123,11 +130,6 @@ CC_OPTION_CFLAGS = $(filter-out $(GCC_PLUGINS_CFLAGS),$(KBUILD_CFLAGS))
cc-option = $(call __cc-option, $(CC),\
$(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS),$(1),$(2))

-# hostcc-option
-# Usage: cflags-y += $(call hostcc-option,-march=winchip-c6,-march=i586)
-hostcc-option = $(call __cc-option, $(HOSTCC),\
- $(HOSTCFLAGS) $(HOST_EXTRACFLAGS),$(1),$(2))
-
# cc-option-yn
# Usage: flag := $(call cc-option-yn,-march=winchip-c6)
cc-option-yn = $(call try-run,\
@@ -180,6 +182,8 @@ ld-version = $(shell $(LD) --version | $(srctree)/scripts/ld-version.sh)
# Usage: $(call ld-ifversion, -ge, 22252, y)
ld-ifversion = $(shell [ $(ld-version) $(1) $(2) ] && echo $(3) || echo $(4))

+endif
+
######

###
--
2.7.4
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