GCC problem with raw double type comparisons

Question

I have the following bit of code, however when compiling it with GCC 4.4 with various optimization flags I get some unexpected results when its run.

#include         


        

Answer 1

The fact that the result depends on the optimization settings suggests it might be the x87 extended precision messing with things (as Michael Burr says).

Here's some code I use (with gcc on x86 processors) to switch the extended precision off:

static const unsigned int PRECISION_BIT_MASK = 0x300;
///< bitmask to mask out all non-precision bits in the fpu control word \cite{INTEL}.
static const unsigned int EXTENDED_PRECISION_BITS = 0x300;
///< add to the fpu control word (after zeroing precision bits) to turn on extended precision \cite{INTEL}.
static const unsigned int STANDARD_PRECISION_BITS = 0x200;
///< add to the fpu control word (after zeroing precision bits) to turn off extended precision \cite{INTEL}.

void set_fpu_control_word(unsigned int mode)
{
  asm ("fldcw %0" : : "m" (*&mode));
}

unsigned int get_fpu_control_word()
{
  volatile unsigned int mode = 0;
  asm ("fstcw %0" : "=m" (*&mode));
  return mode;
}

bool fpu_set_extended_precision_is_on(bool state)
{
  unsigned int old_cw = get_fpu_control_word();
  unsigned int masked = old_cw & ~PRECISION_BIT_MASK;
  unsigned int new_cw;
  if(state)
    new_cw = masked + EXTENDED_PRECISION_BITS;
  else
    new_cw = masked + STANDARD_PRECISION_BITS;
  set_fpu_control_word(new_cw);
  return true;
}

bool fpu_get_extended_precision_is_on()
{
  unsigned int old_cw = get_fpu_control_word();
  return  ((old_cw & PRECISION_BIT_MASK) == 0x300);
}

Or you can just run your code with valgrind, which doesn't simulate the 80-bit registers, and is probably easier for a short program like this!