Re: [Caml-list] ppc64le behaviour of Int64.min_int ÷ -1

[Xavier Leroy <Xavier.Leroy@inria.fr>]

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Hi Richard,

You need to put "let division_crashes_on_overflow = true" in asmcomp/$ARCH/
arch.ml, just like in asmcomp/power/arch.ml.  The name of the variable is
perhaps counterintuitive but means "min_int / -1 doesn't do what we want,
so add tests (in cmmgen) for this case".

- Xavier


2015-06-29 19:50 GMT+02:00 Richard W.M. Jones <rich@annexia.org>:

> Downstream bug: https://bugzilla.redhat.com/show_bug.cgi?id=1236615
>
> OCaml expects Int64.min_int ÷ -1 to overflow and wrap around
> (resulting in Int64.min_int).  There is also a test case in the test
> suite for this.  See this previously fixed bug:
>
> http://caml.inria.fr/mantis/view.php?id=5513
>
> The compiler seems to cater for the case where an overflow in the
> division throws an exception.
>
> Unfortunately ppc64le does neither of these things: On ppc64le the
> result of the computation is explicitly undefined, it sets a flag bit
> (OV), but it doesn't throw an exception.
>
> [See page 76 of
> https://www.power.org/wp-content/uploads/2013/05/PowerISA_V2.07_PUBLIC.pdf
> ]
>
> I wonder if there is a way to tell the compiler about this?  I could
> add a conditional branch, but that would make the expansion of every
> Int64.div and Int64.rem quite large (but maybe that doesn't matter as
> these functions are going to be slow anyway?)
>
> Suggestions welcome ...
>
> Rich.
>
> --
> Richard Jones
> Red Hat
>
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