On Fri, Aug 04, 2017 at 05:41:51PM +0200, Thomas Petazzoni wrote:
> > > > The bug can be reproduced by building with the toolchain available at
> > > > http://toolchains.free-electrons.com/downloads/2017.08-rc1-fix-binutils/toolchains/mips64r6el-n32/tarballs/mips64r6el-n32--glibc--bleeding-edge-2017.05-1453-ga703fdd-1.tar.bz2
> > > > and building with the attached kernel configuration file.
> > > >
> > > > It is not clear to me if this is a kernel issue (lack of __multi3 in
> > > > arch/mips/lib/), or a gcc bug in that it shouldn't emit a call to this
> > > > function.
> > > >
> > > > FWIW, sparc64 had a similar issue, and they added __multi3 in their
> > > > libgcc replacement, see commit
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/sparc/lib?id=1b4af13ff2cc6897557bb0b8d9e2fad4fa4d67aa.
> > > >
> > >
> > > I think these days we've given up the stuborn resistance of the old days
> > > against adding new libgcc1 functions to the kernel, so we should probably
> > > just add it.
> > >
> > > I'm looking into this but a small wrench into the gear is that I'm still
> > > on GCC 6 so I'm off to building myself a cross-gcc first ...
> > I now can reproduce the issue with vanilla FSF binutils 2.28 and GCC 7.1.0.
> Great! However, looking at the functions that end up calling __multi3,
> I'm wondering why suddenly gcc 7.x needs to call such a function, while
> the same code was compiling without __multi3 in libgcc with gcc 6.x.
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
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;
computing which requires 128 bit intermediate results. This is the
code generated for MIPS64 R2:
And this is for R6:
move $7,$5 # $6/$7 contain the second op
move $5,$2 # $4/$5 contain the first op