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

[Masahiro Yamada]

On Wed, Apr 8, 2020 at 4:19 AM Fangrui Song <maskray@xxxxxxxxxx> wrote:
>
>
> On 2020-04-07, 'Nick Desaulniers' via Clang Built Linux wrote:
> >On Tue, Apr 7, 2020 at 10:47 AM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
> >>
> >> On Wed, Apr 8, 2020 at 2:01 AM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> >> >
> >> > On Tue, Apr 7, 2020 at 9:17 AM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
> >> > >
> >> > > On Mon, Apr 6, 2020 at 8:22 PM 'Matthias Maennich' via Clang Built
> >> > > Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> >> > > >
> >> > > > On Fri, Apr 03, 2020 at 02:17:09PM +0900, Masahiro Yamada 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.
> >> > > > >
> >> > > > >We discussed what we need, and we agreed to go with a simple boolean
> >> > > > >switch (https://lkml.org/lkml/2020/3/28/494).
> >> > > > >
> >> > > > >Some items in the discussion:
> >> > > > >
> >> > > > >- LLVM_DIR
> >> > > > >
> >> > > > >  When multiple versions of LLVM are installed, I just thought supporting
> >> > > > >  LLVM_DIR=/path/to/my/llvm/bin/ might be useful.
> >> > > > >
> >> > > > >  CC      = $(LLVM_DIR)clang
> >> > > > >  LD      = $(LLVM_DIR)ld.lld
> >> > > > >    ...
> >> > > > >
> >> > > > >  However, we can handle this by modifying PATH. So, we decided to not do
> >> > > > >  this.
> >> > > > >
> >> > > > >- LLVM_SUFFIX
> >> > > > >
> >> > > > >  Some distributions (e.g. Debian) package specific versions of LLVM with
> >> > > > >  naming conventions that use the version as a suffix.
> >> > > > >
> >> > > > >  CC      = clang$(LLVM_SUFFIX)
> >> > > > >  LD      = ld.lld(LLVM_SUFFIX)
> >> > > > >    ...
> >> > > > >
> >> > > > >  will allow a user to pass LLVM_SUFFIX=-11 to use clang-11 etc.,
> >> > > > >  but the suffixed versions in /usr/bin/ are symlinks to binaries in
> >> > > > >  /usr/lib/llvm-#/bin/, so this can also be handled by PATH.
> >> > > > >
> >> > > > >- HOSTCC, HOSTCXX, etc.
> >> > > > >
> >> > > > >  We can switch the host compilers in the same way:
> >> > > > >
> >> > > > >  ifneq ($(LLVM),)
> >> > > > >  HOSTCC       = clang
> >> > > > >  HOSTCXX      = clang++
> >> > > > >  else
> >> > > > >  HOSTCC       = gcc
> >> > > > >  HOSTCXX      = g++
> >> > > > >  endif
> >> > > > >
> >> > > > >  This may the right thing to do, but I could not make up my mind.
> >> > > > >  Because we do not frequently switch the host compiler, a counter
> >> > > > >  solution I had in my mind was to leave it to the default of the
> >> > > > >  system.
> >> > > > >
> >> > > > >  HOSTCC       = cc
> >> > > > >  HOSTCXX      = c++
> >> > > >
> >> > > > What about HOSTLD ? I saw recently, that setting HOSTLD=ld.lld is not
> >> > > > yielding the expected result (some tools, like e.g. fixdep still require
> >> > > > an `ld` to be in PATH to be built). I did not find the time to look into
> >> > > > that yet, but I would like to consistently switch to the llvm toolchain
> >> > > > (including linker and possibly more) also for hostprogs.
> >> > >
> >> > >
> >> > > HOSTLD=ld.lld worked for me, but HOSTCC=clang did not.
> >> > >
> >> > >
> >> > >
> >> > > HOSTCC=clang without CC=clang fails to build objtool.
> >> > >
> >> > > The build system of objtool is meh.  :(
> >> >
> >> > Let's tackle that in a follow up, with the goal of build hermiticity
> >> > in mind.  I think there's good feedback in this thread to inform the
> >> > design of a v2:
> >> > 1. CLANG_AS=0 to disable integrated as.  Hopefully we won't need this
> >> > much longer, so we don't need to spend too much time on this, Masahiro
> >> > please just choose a name for this.  llvm-as naming conventions
> >> > doesn't follow the rest of binutils.
> >>
> >> I am not so familiar with the terminology in LLVM,
> >> but I feel 'integrated' is a keyword IMHO.
> >> I prefer LLVM_IA=1.  (or  LLVM_INTEGRATED_AS=1)
> >
> >I'm happy with either, and I trust your judgement.  You choose.
> >Hopefully we will fix all our assembler bugs soon and won't need the
> >flag much longer.
>
> Maybe "IAS", e.g. LLVM_IAS=1 or CLANG_IAS=1
>
> IAS is referred to in a few places. IA is not a common abbreviation.
>
> I don't have strong opinion here and thank Masahiro a lot for the
> improvement!
>

OK, I will rename it to LLVM_IAS.

Thanks for the advice.

-- 
Best Regards
Masahiro Yamada