Am Fri, 15 Mar 2013 20:20:14 +0000 schrieb Iain Buclaw <[email protected]>:
> On 15 March 2013 18:35, Johannes Pfau <[email protected]> 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...)
