Floating point constant folding bug?

[Iain Buclaw]

On 15 March 2013 22:00, Johannes Pfau <nospam at example.com> wrote:

> Am Fri, 15 Mar 2013 20:20:14 +0000
> schrieb Iain Buclaw <ibuclaw at ubuntu.com>:
>
> > On 15 March 2013 18:35, Johannes Pfau <nospam at example.com> wrote:
> >
> > > Forget what I've said, I somehow read that code as a
> > > bitwise/reinterpret cast but of course the code does an
> > > integer/float conversion so it's probably OK.
> > >
> > > This is more interesting:
> > >
> > > ---
> > >     float f = float.infinity;
> > >     float f2 = float.max;
> > >     int i = cast(int) f;
> > >     int i2 = cast(int) f2;
> > >     writefln("0x%x == int.min(0x%x)", i, int.min);
> > >     writefln("0x%x == int.max(0x%x)", cast(int)float.infinity,
> > > int.max); writefln("0x%x == int.min(0x%x)", i2, int.min);
> > >     writefln("0x%x == int.max(0x%x)", cast(int)float.max, int.max);
> > > ---
> > >
> > > const folding expands to int.max, but the same code at runtime
> > > expands to int.min. C does not define what happens if too large
> > > float values are cast into ints. Do you know if D somehow defines
> > > this?
> > >
> > > (There's a failing test in the test suite which assumes that the
> > > result is int.min in all cases above)
> > >
> >
> >
> > I think the behaviour from DMD is more incidental than intended.
> > They use C's FLOAT_INFINITY, etc, macros. But because of the way it's
> > written, gcc doesn't optimise the use of the value.   Whereas with
> > gdc, because it uses gcc's real_t types, values of extreme numbers
> > may change to be either 2147483647 or -2147483648 depending on
> > whether -O is used.  As I said, in most cases you can see the same
> > behaviour in C.
> >
> > I can see if there might be some flag I can set to try and make gcc's
> > codegen more safe for floats, but that might have impact on runtime.
> >
> >
>
> I think in this case GCC might intentionally use saturating overflows.
> I just found this message:
>
> http://gcc.gnu.org/ml/gcc-patches/2003-09/msg02090.html
>
> (This doesn't explain though what the test (constfold.d:test2) in the
> dmd testsuite is actually supposed to test...)
>


It tests an incidental feature of dmd-generated code that got turned into a
test case.  Rather than something defined in the spec.

-- 
Iain Buclaw

*(p < e ? p++ : p) = (c & 0x0f) + '0';
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