path: root/Makefile
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authorMasahiro Yamada <>2021-03-16 01:12:56 +0900
committerMasahiro Yamada <>2021-04-25 05:14:41 +0900
commitba64beb17493a4bfec563100c86a462a15926f24 (patch)
tree878793a9636595b9185b2c4e4abacbb2ce4aa7bc /Makefile
parente24b3ffcf4216d819b52618b6f17ba7410d1d845 (diff)
kbuild: check the minimum assembler version in Kconfig
Documentation/process/changes.rst defines the minimum assembler version (binutils version), but we have never checked it in the build time. Kbuild never invokes 'as' directly because all assembly files in the kernel tree are *.S, hence must be preprocessed. I do not expect raw assembly source files (*.s) would be added to the kernel tree. Therefore, we always use $(CC) as the assembler driver, and commit aa824e0c962b ("kbuild: remove AS variable") removed 'AS'. However, we are still interested in the version of the assembler acting behind. As usual, the --version option prints the version string. $ as --version | head -n 1 GNU assembler (GNU Binutils for Ubuntu) 2.35.1 But, we do not have $(AS). So, we can add the -Wa prefix so that $(CC) passes --version down to the backing assembler. $ gcc -Wa,--version | head -n 1 gcc: fatal error: no input files compilation terminated. OK, we need to input something to satisfy gcc. $ gcc -Wa,--version -c -x assembler /dev/null -o /dev/null | head -n 1 GNU assembler (GNU Binutils for Ubuntu) 2.35.1 The combination of Clang and GNU assembler works in the same way: $ clang -no-integrated-as -Wa,--version -c -x assembler /dev/null -o /dev/null | head -n 1 GNU assembler (GNU Binutils for Ubuntu) 2.35.1 Clang with the integrated assembler fails like this: $ clang -integrated-as -Wa,--version -c -x assembler /dev/null -o /dev/null | head -n 1 clang: error: unsupported argument '--version' to option 'Wa,' For the last case, checking the error message is fragile. If the proposal for -Wa,--version support [1] is accepted, this may not be even an error in the future. One easy way is to check if -integrated-as is present in the passed arguments. We did not pass -integrated-as to CLANG_FLAGS before, but we can make it explicit. Nathan pointed out -integrated-as is the default for all of the architectures/targets that the kernel cares about, but it goes along with "explicit is better than implicit" policy. [2] With all this in my mind, I implemented scripts/ to check the assembler version in Kconfig time. $ scripts/ gcc GNU 23501 $ scripts/ clang -no-integrated-as GNU 23501 $ scripts/ clang -integrated-as LLVM 0 [1]: [2]: Signed-off-by: Masahiro Yamada <> Reviewed-by: Nathan Chancellor <>
Diffstat (limited to 'Makefile')
1 files changed, 3 insertions, 1 deletions
diff --git a/Makefile b/Makefile
index cc5b7e39fde4..2b161f5a5a66 100644
--- a/Makefile
+++ b/Makefile
@@ -580,7 +580,9 @@ ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
ifneq ($(CROSS_COMPILE),)
CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))
-ifneq ($(LLVM_IAS),1)
+ifeq ($(LLVM_IAS),1)
+CLANG_FLAGS += -integrated-as
CLANG_FLAGS += -no-integrated-as
GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))