[RFC PATCH] kbuild: support llvm-ar
From: Masahiro Yamada
Date: Thu Jan 17 2019 - 01:35:08 EST
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
Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
---
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