Hacker News Reader

Comment by RossBencina | Hacker News Reader

x and y are integers represented as floating point

  > d = trunc(x/y);     // floor works for unsigned
  >
  > // NOTE: if only want 'd' and it's being converted to an
  > // integer then the truncate or floor operation is
  > // free in the float to integer conversion.

Please show me how to portably truncate or floor a floating point value to an int in C for "free".

View on HN

They're typically like a 3-cycle op, right? Obviously that's not zero, but as far as floating point ops get it's a cheap as it gets, like an add, mul, fma, that sorta thing.

I am probably dating myself here, but in C the default rounding mode is round-to-nearest and the cost of switching the x86 FPU to truncate (and back) in order to perform the truncation is extremely expensive. Perhaps other instruction sets have truncate-to-integer instructions that ignore the active rounding mode.

I think you misread?

{within float-to-integer conversion} trunc or floor is free

that is, if you are converting, you already get it by default

I think you are correct. Clearly I am still traumatised by the cost of converting floats to ints in the x86 era.

Perhaps they meant implicit instead of free, since the comment also explains that `d` is an integer and the intent is to truncate, so the trunc() call is not even necessary when the assigning type is an int.

Reply on news.ycombinator.com