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Re: undefined reference to `__multi3' when building with gcc 7.x

To: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Subject: Re: undefined reference to `__multi3' when building with gcc 7.x
From: Ralf Baechle <ralf@linux-mips.org>
Date: Sat, 5 Aug 2017 00:25:00 +0200
Cc: linux-mips@linux-mips.org, Waldemar Brodkorb <wbx@openadk.org>
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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
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;

computing which requires 128 bit intermediate results.  This is the
code generated for MIPS64 R2:

        dmultu  $4,$5
        mfhi    $2
        sltu    $2,$0,$2

And this is for R6:

        move    $2,$4
        move    $7,$5                   # $6/$7 contain the second op
        move    $6,$0
        move    $4,$0
        move    $5,$2                   # $4/$5 contain the first op
        sd      $31,8($sp)
        balc    __multi3
        sltu    $2,$0,$2

  Ralf

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