On Fri, Apr 26, 2013 at 12:27:53PM +0200, Ondřej Surý wrote:
> This code from libgd2:src/gd.c:clip_1d:
> *y1 -= m * (*x1 - mindim);
> where
> m = (double) -0.050000
> *x1 = -200
> mindim = 0
> *y1 = 15
> results in *y1 = 4, which is incorrect value, since it should be 5.
Nope. The result of "m * (*x1 - mindim)" is not 10, it is a floating
point value near 10, as 10 can't be expressed in double. So this is:
15 - 10.00000001 = 4.9999999. This converted to int is 4.
> Most simple workaround, which allows gcc to produce correct value:
> *y1 -= (int)(m * (*x1 - mindim));
Here you force the later part to be 10.
> Assigning to some other variable also works ok:
> int t;
> t = m * (*x1 - mindim);
> *y1 -= t;
The same.
> gcc-4.7 is unfortunatelly also affected.
> I just hope we don't compile the nuclear reactor controls with gcc :)
Just don't convert floating point to fixed point.
Bastian
--
Virtue is a relative term.
-- Spock, "Friday's Child", stardate 3499.1
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