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undefined reference to `__multi3' when building with gcc 7.x

To: Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org, Waldemar Brodkorb <wbx@openadk.org>
Subject: undefined reference to `__multi3' when building with gcc 7.x
From: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Date: Thu, 3 Aug 2017 22:55:47 +0200
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Hello,

When trying to build the current Linux master with a gcc 7.x toolchain
for mips64r6-n32, I'm getting the following build failure:

crypto/scompress.o: In function `.L31':
scompress.c:(.text+0x2a0): undefined reference to `__multi3'
drivers/base/component.o: In function `.L97':
component.c:(.text+0x4a4): undefined reference to `__multi3'
drivers/base/component.o: In function `component_master_add_with_match':
component.c:(.text+0x8c4): undefined reference to `__multi3'
net/core/ethtool.o: In function `ethtool_set_per_queue_coalesce':
ethtool.c:(.text+0x1ab0): undefined reference to `__multi3'
Makefile:1000: recipe for target 'vmlinux' failed
make[2]: *** [vmlinux] Error 1

Taking the example from net/core/ethtool.o, objdump says:

    1aac:       00408025        move    s0,v0
    1ab0:       e8000000        balc    1ab4 
<ethtool_set_per_queue_coalesce+0x7c>
    1ab4:       14600000        bnez    v1,1ab8 
<ethtool_set_per_queue_coalesce+0x80>

And readelf tells us:

Relocation section '.rela.text' at offset 0xaa00 contains 1189 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
[...]
000000001ab0  023a0000003d R_MIPS_PC26_S2    0000000000000000 __multi3 - 4
                    Type2: R_MIPS_NONE      
                    Type3: R_MIPS_NONE      
[...]
Symbol table '.symtab' contains 586 entries:
[...]
   570: 0000000000000000     0 OBJECT  GLOBAL DEFAULT  UND __multi3

__multi3() is normally provided by libgcc, but of course the kernel
doesn't link with libgcc.

The bug can be reproduced by building with the toolchain available at
http://toolchains.free-electrons.com/downloads/2017.08-rc1-fix-binutils/toolchains/mips64r6el-n32/tarballs/mips64r6el-n32--glibc--bleeding-edge-2017.05-1453-ga703fdd-1.tar.bz2
and building with the attached kernel configuration file.

It is not clear to me if this is a kernel issue (lack of __multi3 in
arch/mips/lib/), or a gcc bug in that it shouldn't emit a call to this
function.

FWIW, sparc64 had a similar issue, and they added __multi3 in their
libgcc replacement, see commit
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/sparc/lib?id=1b4af13ff2cc6897557bb0b8d9e2fad4fa4d67aa.

Best regards,

Thomas Petazzoni
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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