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

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Message-ID: <alpine.DEB.2.21.2601140453090.6421@angie.orcam.me.uk>
Date: Wed, 14 Jan 2026 06:19:02 +0000 (GMT)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: David Laight <david.laight.linux@...il.com>
cc: kernel test robot <lkp@...el.com>, 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, 
    Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Subject: Re: mips64-linux-ld: div64.c:undefined reference to `__multi3'

On Tue, 13 Jan 2026, David Laight wrote:

> > 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.

 Or rather fix the version check.  The GCC fix went in with GCC 10:

$ git log -1 --pretty=oneline 48b2123f6336
48b2123f6336ba6c06846d7c8b60bd14eaeae7ec re PR target/82981 (unnecessary __multi3 call for mips64r6 linux kernel)
$ git show 48b2123f6336:gcc/BASE-VER
10.0.0
$ 

I don't know why the PR got it all wrong; I've fixed it now.

  Maciej