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

To: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>, Matthew Fortune <Matthew.Fortune@imgtec.com>
Subject: Re: undefined reference to `__multi3' when building with gcc 7.x
From: Ralf Baechle <ralf@linux-mips.org>
Date: Mon, 7 Aug 2017 10:34:48 +0200
Cc: linux-mips@linux-mips.org, Waldemar Brodkorb <wbx@openadk.org>
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On Sat, Aug 05, 2017 at 01:56:49PM +0200, Thomas Petazzoni wrote:

> On Sat, 5 Aug 2017 00:25:00 +0200, Ralf Baechle wrote:
> 
> > > 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;
> 
> 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.

  Ralf

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