Hi,

Miroslav Lichvar <mlichvar@redhat.com> writes:

> I was looking at a problem with guile-1.8.8 when compiled with
> gcc-6.0. Two of the tests from the test suite were failing with
> strange "out of range" errors [1]. After some investigation I think
> the bug is that the code in libguile/conv-integer.i.c relies on
> overflow of signed integers in the following code (starting on line
> 77), specifically -TYPE_MIN being less than zero. Adding -fwrapv to
> CFLAGS worked as a workaround for me.
>
>           if (mpz_sgn (SCM_I_BIG_MPZ (val)) >= 0)
>             {
>               if (n < 0)
>                 goto out_of_range;
>             }
>           else
>             {
>               n = -n;
>               if (n >= 0)
>                 goto out_of_range;
>             }

Thanks for bringing this to our attention.  I've attached a preliminary
patch to address these issues on the 'stable-2.0' branch.

> Looking at the current guile code, conv-integer.i.c is identical to
> what it was in 1.8.8, but interestingly the tests didn't fail for me.
> Maybe something else is preventing gcc from using the optimization?

The build system of recent Guile 2.0.x automatically adds -fwrapv to
CFLAGS where supported.  However, I hope to remove -fwrapv in the
future, when we gain confidence that no code in Guile depends on it.

> I'm not sure what would be the best way to fix it. Maybe n should
> really be unsigned and compared to the maximum values, but what would
> be the absolute value of TYPE_MIN if it should work also with other
> integer representations than two's complement?

My approach was to compare (abs_n - 1) to -(TYPE_MIN + 1) in the case
where n is negative.

    Thanks,
      Mark