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
The same __multi3 problem also occurred on SPARC64, and they fixed it
by adding a __multi3 implementation in the kernel. See:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/sparc?id=1b4af13ff2cc6897557bb0b8d9e2fad4fa4d67aa.
Best regards,
Thomas
--
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
