[PATCH] lib/clz_tab.c: put in lib-y rather than obj-y

[PATCH] lib/clz_tab.c: put in lib-y rather than obj-y

[Chris Metcalf]

The clz table (__clz_tab) in lib/clz_tab.c is also provided as part
of libgcc.a, and many architectures link against libgcc.  To allow
the linker to avoid a multiple-definition link failure, clz_tab.o
has to be in lib/lib.a rather than lib/builtin.o.  The specific
issue is that libgcc.a comes before lib/builtin.o on vmlinux.o's
link command line, so its _clz.o is pulled to satisfy __clz_tab,
and then when the remainder of lib/builtin.o is pulled in to satisfy
all the other dependencies, the __clz_tab symbols conflict.  By
putting clz_tab.o in lib.a, the linker can simply avoid pulling
it into vmlinux.o when this situation arises.

The definitions of __clz_tab are the same in libgcc.a and in the
kernel; arguably we could also simply rename the kernel version,
but it's unlikely the libgcc version will ever change to become
incompatible, so just using it seems reasonably safe.

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
---
I have this in the tile tree for linux-next since it broke tilepro,
but perhaps it makes more sense for it to get pushed through
Andrew's tree or somewhere else?

 lib/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/Makefile b/lib/Makefile
index 7f1de26613d2..8487893b4deb 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -153,7 +153,7 @@ obj-$(CONFIG_GLOB) += glob.o
 obj-$(CONFIG_MPILIB) += mpi/
 obj-$(CONFIG_SIGNATURE) += digsig.o
 
-obj-$(CONFIG_CLZ_TAB) += clz_tab.o
+lib-$(CONFIG_CLZ_TAB) += clz_tab.o
 
 obj-$(CONFIG_DDR) += jedec_ddr_data.o
 
-- 
2.1.2

Re: [PATCH] lib/clz_tab.c: put in lib-y rather than obj-y

[David Miller]

From: Chris Metcalf <cmetcalf@ezchip.com>
Date: Tue, 22 Dec 2015 12:41:24 -0500

> The clz table (__clz_tab) in lib/clz_tab.c is also provided as part
> of libgcc.a, and many architectures link against libgcc.  To allow
> the linker to avoid a multiple-definition link failure, clz_tab.o
> has to be in lib/lib.a rather than lib/builtin.o.  The specific
> issue is that libgcc.a comes before lib/builtin.o on vmlinux.o's
> link command line, so its _clz.o is pulled to satisfy __clz_tab,
> and then when the remainder of lib/builtin.o is pulled in to satisfy
> all the other dependencies, the __clz_tab symbols conflict.  By
> putting clz_tab.o in lib.a, the linker can simply avoid pulling
> it into vmlinux.o when this situation arises.
> 
> The definitions of __clz_tab are the same in libgcc.a and in the
> kernel; arguably we could also simply rename the kernel version,
> but it's unlikely the libgcc version will ever change to become
> incompatible, so just using it seems reasonably safe.
> 
> Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>

No objections from me:

Acked-by: David S. Miller <davem@davemloft.net>