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) > 2. HOSTCC=clang HOSTLD=ld.lld set by LLVM=1 for helping with build hermiticity. > -- Best Regards Masahiro Yamada
