floating-WTF

[Caligo]

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

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?