Re: [PATCH v3] kbuild: Add support to generate LLVM bitcode files
From: Matthias Kaehlcke
Date: Fri Apr 21 2017 - 15:55:11 EST
Hi Masahiro,
El Fri, Apr 21, 2017 at 02:02:46PM +0900 Masahiro Yamada ha dit:
> 2017-04-05 2:27 GMT+09:00 Matthias Kaehlcke <mka@xxxxxxxxxxxx>:
> > From: VinÃcius Tinti <viniciustinti@xxxxxxxxx>
> >
> > 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?
Yes, the terminology should be changed to talk about 'LLVM assembly'.
> > # 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
Will change
> > # 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)
Indeed, unsurprisingly the content of a .ll file generated from a .S
is the same as the corresponding .s.
Besides the Makefile rules it isn't clear to me how assembly would be
converted to LLVM IR. I suggest to remove the rules for assembly.
Cheers
Matthias