Bug 1392247

Are you building in a side tag?  I just tried building in mock on one of the Brno openpower hub ppc64le boxes, and even after wiping out the mock cache and building with --enablerepo=local I'm still getting ocaml 4.02.3-3.fc25.  Perhaps the mirrors haven't caught up yet.

The build succeeds and all tests pass with that version of ocaml, by the way.

Yes you need to install the f26-ocaml packages.  Try adding the
following repo to your machine:

$ cat /etc/yum.repos.d/tmp-f26-ocaml.repo
[f26-ocaml]
name=f26-ocaml
baseurl=https://kojipkgs.fedoraproject.org/repos/f26-ocaml/latest/$basearch/
enabled=1
gpgcheck=0

I still don't really understand what's going on here, but the problem
appears to happen in the Val_long(x) cast here:

https://github.com/bobot/zarith/blob/master/caml_z.c#L524

I added lots of debugging around this part of the code.  On ppc64 (thought
to be correct):

x = -1
Z_MIN_INT_FL = -4611686018427383808 Z_MAX_INT_FL = 4611686018427383808
x as Long = -1
(uintnat)(x) = 18446744073709551615
(uintnat)(x) << 1 = 18446744073709551614
(intnat) ((uintnat)(x) << 1) = -2
((intnat) ((uintnat)(x) << 1)) + 1 = -1

On ppc64le (incorrect):

x = -1
Z_MIN_INT_FL = -4611686018427383808 Z_MAX_INT_FL = 4611686018427383808
x as Long = 1
(uintnat)(x) = 0
(uintnat)(x) << 1 = 0
(intnat) ((uintnat)(x) << 1) = 0
((intnat) ((uintnat)(x) << 1)) + 1 = 1


Why on earth does (uintnat)(-1) return 0?  That makes no sense to me.
(I'm assuming that uintnat is unsigned long)

OK so as far as I can tell this is undefined behaviour in C:

https://stackoverflow.com/questions/10541200/is-the-behaviour-of-casting-a-negative-double-to-unsigned-int-defined-in-the-c-s

https://gcc.gnu.org/ml/gcc-help/2010-04/msg00140.html

and therefore the Zarith code is wrong.

Also:

https://stackoverflow.com/questions/2490600/iphone-floats-cast-to-unsigned-ints-get-set-to-0-if-they-are-negative

This patch fixes things for me.  I'm going to drop it into our
ocaml-zarith package to get things moving along.

diff --git a/caml_z.c b/caml_z.c
index b690a2a..42d3850 100644
--- a/caml_z.c
+++ b/caml_z.c
@@ -521,7 +521,7 @@ CAMLprim value ml_z_of_float(value v)
   Z_MARK_OP;
   x = Double_val(v);
 #if Z_USE_NATINT
-  if (x >= Z_MIN_INT_FL && x <= Z_MAX_INT_FL) return Val_long(x);
+  if (x >= Z_MIN_INT_FL && x <= Z_MAX_INT_FL) return Val_long((intnat)x);
 #endif
   Z_MARK_SLOW;
 #ifdef ARCH_ALIGN_INT64

Thank you for figuring this out, Richard.  I will send your patch upstream, with attribution.

Hi Jerry, there is a thread on caml-list where this was discussed:

https://sympa.inria.fr/sympa/arc/caml-list/2016-11/msg00013.html

So Xavier is aware of the problem/fix.  But it would be good to make
sure a patch gets added upstream and not forgotten about.

And here it is: http://forge.ocamlcore.org/tracker/index.php?func=detail&aid=1704&group_id=243&atid=1096

This bug appears to have been reported against 'rawhide' during the Fedora 26 development cycle.
Changing version to '26'.

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