Is it defined what will happen if you shift a float?

Question

I am following This video tutorial to implement a raycaster. It contains this code:

if(ra > PI) { ry = (((int)py>>6)<<6)-0.0001; rx=(py-ry)*aTan+px; yo=-64; xo=-yo*aTan; }//looking up

I hope I have transcribed this correctly. In particular, my question is about casting py (it's declared as float) to integer, shifting it back and forth, subtracting something, and then assigning it to a ry (also a float) This line of code is entered at time 7:24, where he also explains that he wants to

round the y position to the nearest 64th value

(I'm unsure if that means the nearest multiple of 64 or the nearest (1/64), but I know that the 6 in the source is derived from the number 64, being 2⁶)

For one thing, I think that it would be valid for the compiler to load (say) a 32-bit float into a machine register, and then shift that value down by six spaces, and then shift it back up by six spaces (these two operations could interfere with the mantissa, or the exponent, or maybe something else, or these two operations could be deleted by a peephole optimisation step.)

Also I think it would be valid for the compiler to make demons fly out of your nose when this statement is executed.

So my question is, is (((int)py>>6)<<6) defined in C when py is float?

Answer 1

is (((int)py>>6)<<6) defined in C when py is float?

It is certainly undefined behavior (UB) for many float. The cast to an int is UB for float with a whole number value outside the [INT_MIN ... INT_MAX] range.

So code is UB for about 38% of all typical float - the large valued ones, NaNs and infinities.

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For typical float, a cast to int128_t is defined for nearly all float.

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To get to OP's goal, code could use the below, which I believe to be well defined for all float.

If anything, use the below to assess the correctness of one's crafted code.

// round the y position to the nearest 64th value
float round_to_64th(float x) {
  if (isfinite(x)) {
    float ipart;
    // The modf functions break the argument value into integral and fractional parts
    float frac = modff(x, &ipart);
    x = ipart + roundf(frac*64)/64;
  }
  return x;
}

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"I'm unsure if that means the nearest multiple of 64 or the nearest (1/64)"

On review, OP's code is attempting to truncate to the nearest multiple of 64 or 2⁶.

It is still UB for many float.