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    kbuild: check the minimum assembler version in Kconfig · ba64beb1
    Masahiro Yamada authored
    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
    aa824e0c ("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/as-version.sh to
    check the assembler version in Kconfig time.
    
      $ scripts/as-version.sh gcc
      GNU 23501
      $ scripts/as-version.sh clang -no-integrated-as
      GNU 23501
      $ scripts/as-version.sh clang -integrated-as
      LLVM 0
    
    [1]: https://github.com/ClangBuiltLinux/linux/issues/1320
    [2]: https://lore.kernel.org/linux-kbuild/20210307044253.v3h47ucq6ng25iay@archlinux-ax161/
    
    
    
    Signed-off-by: default avatarMasahiro Yamada <masahiroy@kernel.org>
    Reviewed-by: default avatarNathan Chancellor <nathan@kernel.org>
    ba64beb1