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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Tue, 31 Mar 2020 15:24:50 +0900
Message-ID: <CAK7LNAT1HoV5wUZRdeU0+P1nYAm2xQ4tpOG+7UtT4947QByakg@mail.gmail.com>
Subject: Re: [PATCH v2] Makefile.llvm: simplify LLVM build
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Sandeep Patil <sspatil@google.com>
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Sender: linux-kernel-owner@vger.kernel.org
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On Tue, Mar 31, 2020 at 4:03 AM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> On Mon, Mar 30, 2020 at 11:58:19AM -0700, Nick Desaulniers wrote:
> > On Sat, Mar 28, 2020 at 6:57 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
> > >
> > > I also had planned to provide a single switch to change
> > > all the tool defaults to LLVM.
> > >
> > > So, supporting 'LLVM' is fine, but I'd rather want this
> > > look symmetrical, and easy to understand.
> > >
> > > CPP        = $(CC) -E
> > > ifneq ($(LLVM),)
> >
> > Yes, a simple if statement is much simpler than the overly complex patch I had.
> >
> > > CC         = $(LLVM_DIR)clang
> >
> > Do we need $LLVM_DIR? Shouldn't users just have that in their $PATH?
> >
> > Also, I think we need to support suffixed binaries, as debian
> > distributes these with version suffixes, as Nathan points out.  Or do
> > the debian packages install suffixed binaries AND path versioned
> > non-suffixed binaries?
>
> I think the idea here is that ultimately, the suffixed versions of clang
> that Debian has in /usr/bin are symlinks to binaries in
> /usr/lib/llvm-#/bin; as a result, a user could say
> LLVM_DIR=/usr/lib/llvm-#/bin/ and all of those tools would be picked up
> automatically. I am not really sure what is better.


I periodically build the latest llvm from the trunk,
and install it under my home directory.

So, I just thought it would be useful to
allow a user to specify the llvm directory.

Of course, I can do the equivalent by tweaking PATH, but
I hesitate to make the non-released version my default.


Having both LLVM_DIR and LLVM_SUFFIX seems verbose.

In fact, the debian provides multiple versions of GCC.
For example, my machine has

masahiro@pug:~$ ls -1 /usr/bin/gcc-*
/usr/bin/gcc-4.8
/usr/bin/gcc-5
/usr/bin/gcc-7
/usr/bin/gcc-ar
/usr/bin/gcc-ar-4.8
/usr/bin/gcc-ar-5
/usr/bin/gcc-ar-7
/usr/bin/gcc-nm
/usr/bin/gcc-nm-4.8
/usr/bin/gcc-nm-5
/usr/bin/gcc-nm-7
/usr/bin/gcc-ranlib
/usr/bin/gcc-ranlib-4.8
/usr/bin/gcc-ranlib-5
/usr/bin/gcc-ranlib-7

But, nobody has suggested GCC_SUFFIX.

So, I guess CROSS_COMPILE was enough to
choose a specific tool version.


> I'll try to have some other comments by later today/tonight.
>
> > > LD         = $(LLVM_DIR)ld.lld
> > > AR         = $(LLVM_DIR)llvm-ar
> > > NM         = $(LLVM_DIR)llvm-nm
> > > OBJCOPY    = $(LLVM_DIR)llvm-objcopy
> > > OBJDUMP    = $(LLVM_DIR)llvm-objdump
> > > READELF    = $(LLVM_DIR)llvm-readelf
> > > OBJSIZE    = $(LLVM_DIR)llvm-size
> > > STRIP      = $(LLVM_DIR)llvm-strip
> > > else
> > > CC         = $(CROSS_COMPILE)gcc
> > > LD         = $(CROSS_COMPILE)ld
> > > AR         = $(CROSS_COMPILE)ar
> > > NM         = $(CROSS_COMPILE)nm
> > > OBJCOPY    = $(CROSS_COMPILE)objcopy
> > > OBJDUMP    = $(CROSS_COMPILE)objdump
> > > READELF    = $(CROSS_COMPILE)readelf
> > > OBJSIZE    = $(CROSS_COMPILE)size
> > > STRIP      = $(CROSS_COMPILE)strip
> > > endif
> > >
> > >
> > >
> > > I attached two patches.
> > > Comments appreciated.
> >
> > I'm not sure the second one that recommends changing cc/c++ is the way
> > to go; I think it might harm hermeticity.
>
> Agreed. I do not modify my host system at all for this project, just
> relying on PATH modification. In theory, we can still override HOSTCC
> and HOSTCXX but that would defeat the purpose of that patch.
>
> Cheers,
> Nathan
>
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-- 
Best Regards
Masahiro Yamada