Re: mips64-linux-ld: div64.c:undefined reference to `__multi3'

[David Laight <david.laight.linux@gmail.com>]

On Wed, 14 Jan 2026 06:19:02 +0000 (GMT)
"Maciej W. Rozycki" <macro@orcam.me.uk> wrote:

> 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:

Does that mean the GCC 10 generates the multiply instructions and never calls
__multi3?
(Rather than just not using __multi3() for that specific example.)

In this case gcc knows the high bits are all zero - so just needs the two
instructions to generate the high and low parts.

	David

> 
> $ 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
>