floating-WTF
sclytrack
sclytrack at fake.com
Tue Jan 24 01:03:52 PST 2012
On 01/24/2012 04:13 AM, Caligo wrote:
> alias double Real;
> //alias float Real;
>
> // simple linear interpolation; I partitioned the internals to help me
> figure out what was happening.
> Real lerp(Real t, Real a, Real b){
> Real s1 = (1.0 - t) * a;
> Real s2 = t * b;
> Real rt1 = s1 + s2;
> Real rt2 = ((1.0 - t) * a) + (t * b);
> writefln("t=%.2f, a=%.2f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f,
> rt2=%.2f :::", t, a, b, s1, s2, rt1, rt2);
> return rt2;
> }
>
> unittest{
>
> writeln(lerp(0.75, -2.0, 2.0)); // the correct result is 1.0
> }
>
> compile and run with 'dmd -inline -unittest' and the output should be:
>
> t=0.75, a=-2.00, b=2.00, s1=-0.50, s2=1.50, rt1=1.00, rt2=1.00 :::
> 1
>
> Now, change 'Real' to float by uncommenting the second line and
> compile and run with 'dmd -unittest'. This is what I get for the
> output:
>
> t=0.75, a=0.00, b=2.00, s1=-0.50, s2=0.00, rt1=-0.50, rt2=1.50 :::
> 1.5
This: (with float)
writefln("t=%.2f, a=%.2f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f,
rt2=%.2f :::", t, a, b, s1, s2, rt1, rt2);
writefln("t=%.2f, a=%f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f, rt2=%.2f
:::", t, a, b, s1, s2, rt1, rt2);
Outputs: (with float)
t=0.75, a=0.00, b=2.00, s1=-0.50, s2=0.00, rt1=-0.50, rt2=1.50 :::
t=0.75, a=-2.000000, b=2.00, s1=-0.50, s2=0.00, rt1=-0.50, rt2=1.50 :::
difference:
%.2f ---> %f
>
> I have no idea why 'a' is zero. 'rt1' and 'rt2' do not have the same
> value, and 'lerp' does not return 1.0. Compiling with 'dmd -inline
> -unittest' does produce the correct result as before, though.
>
> You can play with different compiler options, such as -O, to get the
> same weird behavior.
>
> I presume this is another DMD bug?
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