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

To: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Subject: Re: undefined reference to `__multi3' when building with gcc 7.x
From: Ralf Baechle <ralf@linux-mips.org>
Date: Fri, 4 Aug 2017 02:05:57 +0200
Cc: linux-mips@linux-mips.org, Waldemar Brodkorb <wbx@openadk.org>
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On Thu, Aug 03, 2017 at 10:55:47PM +0200, Thomas Petazzoni wrote:

> 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.

I think these days we've given up the stuborn resistance of the old days
against adding new libgcc1 functions to the kernel, so we should probably
just add it.

I'm looking into this but a small wrench into the gear is that I'm still
on GCC 6 so I'm off to building myself a cross-gcc first ...

  Ralf

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