Re: [RFC PATCH] kbuild: support llvm-ar

[Jordan Rupprecht]

I have a patch up for review that fixes the second case of chopping
off the directory when nesting thin archives:
https://reviews.llvm.org/D57842. I'll commit it tomorrow if there are
no more comments.

I was looking at the first case of supporting the P modifier, and
found that, as implemented (with D57842 patched in), llvm-ar now seems
to work as if P is the default in some cases, e.g. with your initial
test case:
  $ gcc -c -x c /dev/null -o foo.o
  $ mkdir a
  $ gcc -c -x c /dev/null -o a/foo.o
  $ ar rcST a/built-in.a a/foo.o
  $ rm -f built-in.a && ar rcST built-in.a a/built-in.a foo.o && cat built-in.a
!<thin>
//                                              8         `
foo.o/

/0              0           0     0     644     920       `
  $ rm -f built-in.a && llvm-ar rcST built-in.a a/built-in.a foo.o &&
cat built-in.a
!<thin>
//                                              16        `
a/foo.o/
foo.o/
/0              0           0     0     644     920       `
/9              0           0     0     644     920       `

It didn't work like that universally -- deleting archive members
doesn't work as expected -- so I'll have to do some more
investigation.
Anyway, once D57842 is committed (or if you want to patch it locally),
you *should* be able to reduce the llvm-ar special fix to just not
pass the P modifier, until we accept it.

On Thu, Jan 17, 2019 at 1:55 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> On Wed, Jan 16, 2019 at 10:34 PM Masahiro Yamada
> <yamada.masahiro@xxxxxxxxxxxxx> wrote:
> >
> > I want to avoid applying this patch because this patch is ugly, and
> > people are trying to fix llvm-ar. I tried the latest llvm-ar, and I
> > saw some improvement, but still cannot build the kernel with it.
> >
> > The main reason of this post is for the record, and suggest how llvm-ar
> > should work.
> >
> > As you may have noticed, there are two issues:
> >
> >  [1] llvm-ar does not recognize the 'P' option
> >  [2] llvm-ar does not flatten thin archives (not fixed yet)
> >
> > Let me explain each of them.
> >
> > [1] Why is 'P' option needed for GNU ar?
> >
> > It may not be so clear the reason of commit 9a6cfca4f413 ("kbuild:
> > thin archives use P option to ar").
> >
> > If two objects from different directories have the same file name,
> > GNU ar may drop the one when flattening thin archives.
> >
> > Here is a simple test case:
> >
> >   $ gcc -c -x c /dev/null -o foo.o
> >   $ mkdir a
> >   $ gcc -c -x c /dev/null -o a/foo.o
> >   $ ar rcST a/built-in.a a/foo.o
> >   $ ar rcST built-in.a a/built-in.a foo.o
> >   $ cat built-in.a
> >   !<thin>
> >   //                                              8         `
> >   foo.o/
> >
> >   /0              0           0     0     644     944       `
> >
> > We expect built-in.a contains both a/foo.o and foo.o, but the former
> > is lost. The 'P' option solves this.
> >
> >   $ rm -f built-in.a
> >   $ ar rcSTP built-in.a a/built-in.a foo.o
> >   $ cat built-in.a
> >   !<thin>
> >   //                                              16        `
> >   a/foo.o/
> >   foo.o/
> >   /0              0           0     0     644     944       `
> >   /9              0           0     0     644     944       `
> >
> > Interestingly, GNU ar works as expected without 'P' when the order of
> > a/built-in.a and foo.o are opposite:
> >
> >   $ rm built-in.a
> >   $ ar rcST built-in.a foo.o a/built-in.a
> >   $ cat built-in.a
> >   !<thin>
> >   //                                              16        `
> >   foo.o/
> >   a/foo.o/
> >   /0              0           0     0     644     944       `
> >   /7              0           0     0     644     944       `
> >
> > Anyway, even the latest GNU toolchain works like that, so
> > Kbuild does need the 'P' option.
> >
> > The real world example is sh.
> >
> >   arch/sh/kernel/cpu/fpu.c
> >   arch/sh/kernel/cpu/sh2a/fpu.c
> >   arch/sh/kernel/cpu/sh4/fpu.c
> >   arch/sh/kernel/cpu/sh5/fpu.c
> >
> > [2] flattening thin archives
> >
> > llvm-ar cannot flatten archives.
> >
> > This issue was filed in the following:
> >   https://bugs.llvm.org/show_bug.cgi?id=37530
> >
> > Its status was marked as "RESOLVED FIXED", but actually not fixed
> > as of writing (Jan 17, 2019).
> >
> > The older version of llvm-ar worked like this:
> >
> >   $ rm -f built-in.a a/built-in.a
> >   $ llvm-ar rcST a/built-in.a a/foo.o
> >   $ llvm-ar rcST built-in.a a/built-in.a
> >   $ cat built-in.a !<thin>
> >   //                                              14        `
> >   a/built-in.a/
> >   /0              0           0     0     644     136       `
> >
> > Recently, two commits were pushed to the trunk.
> >   [llvm-ar] Flatten thin archives.
> >   [llvm-ar] Resubmit recursive thin archive test with fix for full path names and better error messages
> >
> > As far as I tested, the latest llvm-ar works as follows:
> >
> >   $ llvm-ar rcST a/built-in.a a/foo.o
> >   $ llvm-ar rcST built-in.a a/built-in.a
> >   $ cat built-in.a
> >   !<thin>
> >   //                                              8         `
> >   foo.o/
> >
> >   /0              0           0     0     644     800       `
> >
> > OK, it flattens the thin archive, but the problem is it rips off
> > the directory path.
> >
> > GNU ar works as follows:
> >
> >   $ ar rcST   a/built-in.a   a/foo.o
> >   $ ar rcST    built-in.a a/built-in.a
> >   $ cat built-in.a
> >   !<thin>
> >   //                                              10        `
> >   a/foo.o/
> >
> >   /0              0           0     0     644     800       `
> >
> > If you use the latest llvm-ar for building the kernel, you will see
> > a mysterious 'llvm-ar: error: No such file or directory.' error.
> > (I think the error message could be improved because it is unclear
> > what is missing.)
> >
> > [Workaround in this patch]
> >
> > Currently, llvm-ar cannot flatten thin archives correctly. So, Kbuild
> > uses '$(AR) t' to expand thin archives from subdirectories, then repack
> > all objects into the current directory built-in.a.
> >
> > The 'P' for cmd_link_l_target is unneeded because Kbuild does not
> > support subdir objects for lib-y in the first place.
> >
> > [How to use]
> >
> > Pass AR=llvm-ar in the configuration and build command:
> >
> >  $ make AR=llvm-ar defconfig
> >  $ make AR=llvm-ar
> >
> > Or, simply in one line:
> >
> >  $ make AR=llvm-ar defconfig all
>
> Thank you Masahiro for the excellent feedback on what's missing with
> llvm-ar.  I will work with Jordan (cc'ed) to sort out these remaining
> issues.  I think we should do our best to fix these in llvm-ar to the
> extent possible, before modifying Kbuild to support it.
>
> >
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
>
> For the above reason;
> Nacked-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> to prevent this from going in before we've had time to address the
> feedback from this patch in llvm-ar.  There's no rush to support
> llvm-ar, so I'd like to not burden the kernel with these changes.  But
> we will keep this patch in our backpocket should we determine that we
> cannot meet compatibility with binutils ar.
>
> > ---
> >
> > This patch can be cleanly applied to Linux 5.0-rc2 + the following:
> > https://patchwork.kernel.org/patch/10761625/
> > https://patchwork.kernel.org/patch/10767193/
> > https://patchwork.kernel.org/patch/10767195/
> > https://patchwork.kernel.org/patch/10767503/
> > https://patchwork.kernel.org/patch/10767509/
> >
> >
> >  init/Kconfig           | 3 +++
> >  scripts/Makefile.build | 8 +++++++-
> >  2 files changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 513fa54..185274ac 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -23,6 +23,9 @@ config CLANG_VERSION
> >         int
> >         default $(shell,$(srctree)/scripts/clang-version.sh $(CC))
> >
> > +config AR_IS_LLVM
> > +       def_bool $(success,$(AR) --version | head -n 1 | grep -q LLVM)
> > +
> >  config CC_HAS_ASM_GOTO
> >         def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))
> >
> > diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> > index f8e2794..b866bb5 100644
> > --- a/scripts/Makefile.build
> > +++ b/scripts/Makefile.build
> > @@ -399,7 +399,13 @@ $(sort $(subdir-obj-y)): $(subdir-ym) ;
> >  ifdef builtin-target
> >
> >  quiet_cmd_ar_builtin = AR      $@
> > +ifdef CONFIG_AR_IS_LLVM
> > +      cmd_ar_builtin = rm -f $@; $(AR) rcST$(KBUILD_ARFLAGS) $@ \
> > +                      $(foreach f, $(real-prereqs), \
> > +                      $(if $(filter $(subdir-obj-y), $(f)), $(shell $(AR) t $(f)), $(f)))
> > +else
> >        cmd_ar_builtin = rm -f $@; $(AR) rcSTP$(KBUILD_ARFLAGS) $@ $(real-prereqs)
> > +endif
> >
> >  $(builtin-target): $(real-obj-y) FORCE
> >         $(call if_changed,ar_builtin)
> > @@ -427,7 +433,7 @@ ifdef lib-target
> >  quiet_cmd_link_l_target = AR      $@
> >
> >  # lib target archives do get a symbol table and index
> > -cmd_link_l_target = rm -f $@; $(AR) rcsTP$(KBUILD_ARFLAGS) $@ $(real-prereqs)
> > +cmd_link_l_target = rm -f $@; $(AR) rcsT$(KBUILD_ARFLAGS) $@ $(real-prereqs)
> >
> >  $(lib-target): $(lib-y) FORCE
> >         $(call if_changed,link_l_target)
> > --
> > 2.7.4
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers

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