linux-kernel - Re: mips64-linux-ld: div64.c:undefined reference to `_

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Message-ID: <20260113215808.135dc8b6@pumpkin>
Date: Tue, 13 Jan 2026 21:58:08 +0000
From: David Laight <david.laight.linux@...il.com>
To: kernel test robot <lkp@...el.com>
Cc: oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org, Andrew
 Morton <akpm@...ux-foundation.org>, Linux Memory Management List
 <linux-mm@...ck.org>, Nicolas Pitre <npitre@...libre.com>,
 linux-mips@...r.kernel.org, tsbogend@...ha.franken.de
Subject: Re: mips64-linux-ld: div64.c:undefined reference to `__multi3'

On Tue, 13 Jan 2026 20:04:55 +0000
David Laight <david.laight.linux@...il.com> wrote:

Resend fixing Thomas's email

> On Wed, 14 Jan 2026 01:59:24 +0800
> kernel test robot <lkp@...el.com> wrote:
> 
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > head:   b71e635feefc852405b14620a7fc58c4c80c0f73
> > commit: d10bb374c41e4c4dced04ae7d2fe2d782a5858a0 lib: mul_u64_u64_div_u64(): optimise the divide code
> > date:   8 weeks ago
> > config: mips-randconfig-r113-20260113 (https://download.01.org/0day-ci/archive/20260114/202601140146.hMLODc6v-lkp@intel.com/config)
> > compiler: mips64-linux-gcc (GCC) 8.5.0
> > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260114/202601140146.hMLODc6v-lkp@intel.com/reproduce)
> > 
> > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > the same patch/commit), kindly add following tags
> > | Reported-by: kernel test robot <lkp@...el.com>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202601140146.hMLODc6v-lkp@intel.com/
> > 
> > All errors (new ones prefixed by >>):
> > 
> >    mips64-linux-ld: lib/math/div64.o: in function `mul_u64_add_u64_div_u64':
> >    div64.c:(.text+0x84): undefined reference to `__multi3'  
> > >> mips64-linux-ld: div64.c:(.text+0x11c): undefined reference to `__multi3'    
> >   
> 
> This looks like a bug in the mips 'port'.
> arch/mips/lib/multi3.c has the comment:
> 
> /*
>  * GCC 7 & older can suboptimally generate __multi3 calls for mips64r6, so for
>  * that specific case only we implement that intrinsic here.
>  *
>  * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82981
>  */
> #if defined(CONFIG_64BIT) && defined(CONFIG_CPU_MIPSR6) && (__GNUC__ < 8)
> 
> So this code is excluded for gcc 8.5 but the compiler is generating the call.
> 
> Looking at the git log for that file there is a comment that includes:
> 	"we wouldn't expect any calls to __multi3 to be generated from
> 	 kernel code".
> Not true....
> Not sure why the link didn't fail before though, something subtle must
> have changed.
> 
> I think the fix is just to remove the gcc version check.
> The code itself just adds the results of four multiply instructions together.
> 
> 	David