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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