Re: [PATCH] kbuild: support 'LLVM' to switch the default tools to Clang/LLVM

[Fangrui Song]

On 2020-04-06, Masahiro Yamada wrote:
> On Mon, Apr 6, 2020 at 8:55 AM 'Fangrui Song' via Clang Built Linux
> <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> > On 2020-04-06, Masahiro Yamada wrote:
> > >On Sat, Apr 4, 2020 at 3:24 AM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> > >>
> > >> On Thu, Apr 2, 2020 at 10:17 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
> > >> >
> > >> > As Documentation/kbuild/llvm.rst implies, building the kernel with a
> > >> > full set of LLVM tools gets very verbose and unwieldy.
> > >> >
> > >> > Provide a single switch 'LLVM' to use Clang and LLVM tools instead of
> > >> > GCC and Binutils. You can pass LLVM=1 from the command line or as an
> > >> > environment variable. Then, Kbuild will use LLVM toolchains in your
> > >> > PATH environment.
> > >> >
> > >> > Please note LLVM=1 does not turn on the LLVM integrated assembler.
> > >> > You need to explicitly pass AS=clang to use it. When the upstream
> > >> > kernel is ready for the integrated assembler, I think we can make
> > >> > it default.
> > >>
> > >> Having this behavior change over time may be surprising.  I'd rather
> > >> that if you want to not use the integrated assembler, you explicitly
> > >> negate it, or just don't use the LLVM=1 syntax, ie. `make CC=clang
> > >> LD=ld.lld ...`.
> > >>
> > >> We could modify how `-no-integrated-as` is chosen when LLVM=1.
> > >>
> > >> make LLVM=1 LLVMIA=0 ... # add `-no-integrated-as`
> > >> # what the flag is doesn't really matter to me, something shorter might be nice.
> > >> make LLVM=1 # use all LLVM tools
> > >>
> > >> Since we got rid of $(AS), it would be appropriate to remove/change it
> > >> there, since no one really relies on AS=clang right now. (We do have 1
> > >> of our 60+ CI targets using it, but we can also change that trivially.
> > >> So I think we have a lot of freedom to change how `-no-integrated-as`
> > >> is set.
> > >>
> > >> This could even be independent of this patch.
> > >
> > >
> > >I also thought a boolean flag is preferred.
> > >
> > >AS=clang will not live long anyway, and
> > >I hesitated to break the compatibility
> > >for the short-term workaround.
> > >
> > >But, if this is not a big deal, I can
> > >replace AS=clang with LLVMIA=1.
> >
> > My mere complaint is that it may be difficult to infer the intention (integrated
> > assembler) from the abbreviation "IA" in "LLVMIA" :/
> >
> > Something with "AS" in the name may be easier for a user to understand,
> > e.g. CLANG_AS or LLVM_AS.
>
> I see 'llvm-as' in my PATH,
> but it is a different kind of tool, right?
> (converter from LLVM assembler *.ll to LLVM bit code *.bc)
>
> So, I thought "LLVM_AS" might be confusing.

You are right. llvm-as converts a textual form of LLVM IR (.ll) to
a binary form bitcode (.bc). LLVM_AS is confusing. CLANG_AS/CLANGAS might be
suitable.

  clang a.c '-###' => clang -cc1     # like gcc invokes cc1
  clang a.s '-###' => clang -cc1as   # this invokes the integrated assembler