Messages in this thread | | | Date | Tue, 7 Apr 2020 12:19:23 -0700 | From | Fangrui Song <> | Subject | Re: [PATCH] kbuild: support 'LLVM' to switch the default tools to Clang/LLVM |
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On 2020-04-07, 'Nick Desaulniers' via Clang Built Linux wrote: >On Tue, Apr 7, 2020 at 10:47 AM Masahiro Yamada <masahiroy@kernel.org> wrote: >> >> On Wed, Apr 8, 2020 at 2:01 AM Nick Desaulniers <ndesaulniers@google.com> wrote: >> > >> > On Tue, Apr 7, 2020 at 9:17 AM Masahiro Yamada <masahiroy@kernel.org> wrote: >> > > >> > > On Mon, Apr 6, 2020 at 8:22 PM 'Matthias Maennich' via Clang Built >> > > Linux <clang-built-linux@googlegroups.com> 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!
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