Re: [PATCH v2] Makefile.llvm: simplify LLVM build

[Nick Desaulniers <ndesaulniers@google.com>]

On Mon, Mar 30, 2020 at 11:25 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> 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.

$ sudo apt install clang-8
$ which clang-8
/usr/bin/clang-8
$ ls -l `!!`
/usr/bin/clang-8 -> ../lib/llvm-8/bin/clang
$ ls /usr/lib/llvm-8/bin
<non suffixed versions>

Ok, so Nathan, it looks like we don't need the version suffixes.
Instead, we can be more explicit with our $PATH, and only add the
above (and bintutils).  I was thinking supporting the suffix was
required for our CI, but it seems like maybe not.

> 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.

Respectfully, I strongly disagree.  This should be handled by
modifications to $PATH, either by your shell's .rc file when you
always want it, or exported for a session when you want it, or
prefixed to an invocation for the duration of that command.  We should
not have a new variable just for the path of a few tools.

Rather than `make LLVM_DIR=~/llvm-project LLVM=1`, you can do
`PATH=$PATH:~/llvm-project make LLVM=1`. (or export it manually or via
your shell .rc, depending on how comfortable you are with that
version).

> Having both LLVM_DIR and LLVM_SUFFIX seems verbose.

I agree, so maybe just LLVM=y, and we can support both non-standard
locations and debian suffixes via modifications to PATH.

>
> 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.

Or no one was testing specific versions of gcc with more than one
installed.  I can ask the KernelCI folks next week if this is an issue
they face or have faced.
-- 
Thanks,
~Nick Desaulniers