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Date: Mon, 7 Nov 2016 16:19:43 +0000
From: "Richard W.M. Jones" <rich@annexia.org>
To: Xavier Leroy <xavier.leroy@inria.fr>
Cc: caml-list@inria.fr
Message-ID: <20161107161943.GI27249@annexia.org>
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Subject: Re: [Caml-list] Zarith problems on ppc64le

On Mon, Nov 07, 2016 at 04:58:24PM +0100, Xavier Leroy wrote:
> On 07/11/2016 16:48, Richard W.M. Jones wrote:
> > On Mon, Nov 07, 2016 at 01:22:01PM +0000, Richard W.M. Jones wrote:
> >>
> >> https://bugzilla.redhat.com/show_bug.cgi?id=1392247
> >>
> >> Zarith fails tests on OCaml 4.04.0 + ppc64le with some fairly gross
> >> errors, eg:
> >>
> >>  of_float -1.
> >> - = -1
> >> + = 0
> >>
> >> (It passes fine on OCaml 4.04.0 + ppc64)
> >>
> >> The implementation of Zarith is quite complex and I've so far failed
> >> to understand what's going on.
> > 
> > FWIW this seems like it is invoking undefined behaviour in C.  See
> > my analysis here:
> > 
> > https://bugzilla.redhat.com/show_bug.cgi?id=1392247#c3
> 
> Yes, something is weird here.  You can try to put
> 
>    return Val_long((intnat) x);
> 
> on line 524: it should fix the issue that you suspect.
> 
> If this is the source of the problem, I'm surprised that it doesn't
> break on other platforms as well.

Yup that's exactly the fix I came up with:

https://bugzilla.redhat.com/show_bug.cgi?id=1392247#c6

There may be other places which make this assumption, but with just
that fix the tests pass successfully on ppc64le.

Thanks,

Rich.