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SubjectRe: [PATCH v3] kbuild: Add support to generate LLVM bitcode files
Hi Matthias,

2017-04-05 2:27 GMT+09:00 Matthias Kaehlcke <>:
> From: Vinícius Tinti <>
> Add rules to kbuild in order to generate LLVM bitcode files with the .ll
> extension when using clang.

First, I'd like to be sure about the terminology "LLVM bitcode"
because "bitcode" sounds like human-unreadable binary.

For example, 'man llvm-as' says:
llvm-as is the LLVM assembler. It reads a file containing
human-readable LLVM assembly language, translates it to LLVM
bitcode, and writes the result into a file or to standard output.

As far as I understood:

*.ll - LLVM assembly (human readable file)
*.bc - LLVM bitcode (binary file)

Is this correct?

> # from c code
> CC=clang make kernel/pid.ll

This does not work because CC is overridden in the top-level Makefile.
It should be
make CC=clang kernel/pid.ll

> # from asm code
> CC=clang make arch/x86/kernel/preempt.ll

arch/x86/kernel/preempt.* does not exist
(at least in the latest tree).

> +
> +quiet_cmd_as_ll_S = CPP $(quiet_modtag) $@
> + cmd_as_ll_S = $(CPP) $(a_flags) -o $@ $<
> +
> +$(obj)/%.ll: $(src)/%.S FORCE
> + $(call if_changed_dep,as_ll_S)
> +

I could not understand how this rule can convert
architecture-specific assembly to LLVM intermediate expression.

This is just pre-processing *.S file.

Actually, this is completely the same as the rule *.S -> *.s

quiet_cmd_cpp_s_S = CPP $(quiet_modtag) $@
cmd_cpp_s_S = $(CPP) $(a_flags) -o $@ $<

$(obj)/%.s: $(src)/%.S FORCE
$(call if_changed_dep,cpp_s_S)

Best Regards
Masahiro Yamada

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