Floating point constant folding bug?

[Johannes Pfau]

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...)