On Sun, Aug 13, 2017 at 10:46:02PM +0200, Thomas Petazzoni wrote:
> Date: Sun, 13 Aug 2017 22:46:02 +0200
> From: Thomas Petazzoni <thomas.petazzoni@xxxxxxxxxxxxxxxxxx>
> To: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
> Cc: Matthew Fortune <Matthew.Fortune@xxxxxxxxxx>,
> linux-mips@xxxxxxxxxxxxxx, Waldemar Brodkorb <wbx@xxxxxxxxxxx>
> Subject: Re: undefined reference to `__multi3' when building with gcc 7.x
> Content-Type: text/plain; charset=US-ASCII
>
> Hello,
>
> On Mon, 7 Aug 2017 10:34:48 +0200, Ralf Baechle wrote:
>
> > > > Chances are it's something specific to MIPS64 R6. Before trying your
> > > > config file I also tried a number of other defconfigs and all built
> > > > well.
> > > >
> > > > Here's a test case which generates a reference to __multi3:
> > > >
> > > > unsigned long func(unsigned long a, unsigned long b)
> > > > {
> > > > return a > (~0UL) / b;
> > > > }
> > > >
> > > > GCC rearanges above statement to:
> > > >
> > > > return (unsigned __int128)a * (unsigned __int128) b > 0xffffffff;
> > >
> > > And this is normal/expected ?
> >
> > Without consideration of performance, It's certainly is valid code. And
> > with that I can't drop the issue as a GCC code generation bug.
> >
> > However it seems GCC itself doesn't seem to have a __multi3 in its
> > libgcc2 - which indeed would be a GCC issue - at least none I was easily
> > able to find with grep so I'm adding Matthew Fortune to cc in the hope he
> > can shed some light on this.
>
> Indeed, I don't see __multi3 implemented in libgcc in the source code,
> but it's probably because it's tricky to see its implementation,
> as it really is there:
>
> $ ./bin/mips64el-linux-readelf -a ./mips64el-buildroot-linux-uclibc/sysroot/lib/libgcc_s.so.1 | grep multi3
> 1747: 00011700 100 FUNC GLOBAL DEFAULT 11 __multi3@@GCC_3.0
> 5511: 00011700 100 FUNC GLOBAL DEFAULT 11 __multi3
> 000435e4 -32236(gp) 00011700 00011700 FUNC 11 __multi3
>
> Objdump says:
>
> 00011700 <__multi3>:
> 11700: 0006103e dsrl32 v0,a2,0x0
> 11704: 7c89f803 dext a5,a0,0x0,0x20
> 11708: 0004403e dsrl32 a4,a0,0x0
> 1170c: 7ccaf803 dext a6,a2,0x0,0x20
> 11710: 012a589c dmul a7,a5,a6
> 11714: 010a509c dmul a6,a4,a6
> 11718: 0122489c dmul a5,a5,v0
> 1171c: 0102409c dmul a4,a4,v0
> 11720: 012a482d daddu a5,a5,a6
> 11724: 000b103e dsrl32 v0,a7,0x0
> 11728: 0049102d daddu v0,v0,a5
> 1172c: 184a0003 bgeuc v0,a6,1173c <.L2>
> 11730: 24090001 li a5,1
> 11734: 0009483c dsll32 a5,a5,0x0
> 11738: 0109402d daddu a4,a4,a5
I happened to have a GCC build dir around so I greped for __multi3 and
found it hiding in muldi3.o.
Maybe that'obvious for those in the know, not me :) Also that .o file
contained MIPS III code and I was able to get GCC to emit a reference
to __multi3 for MIPS III or MIPS64R1 targets, so version of __multi3
even seems unused.
Ralf
