[Mingw-users] Unexpected inaccuracy in double to int conversion

[Martin W. <mai...@ma...>]

After updating to the latest version of the MinGW GCC (4.7.0), one
of my regression tests started failing. I've isolated the problem
to the following test case:

#include <stdio.h>
#include <math.h>

volatile double scale = 3.0;

int main(int argc, char *argv[])
{
   printf("%20.20f\n", 123000.0 / pow(10.0, scale));
   printf("%d\n", (int)(123000.0 / pow(10.0, scale)));
   printf("%d\n", (int)(123000.0 / pow(10.0, 3.0)));
}

The output from this is:

123.00000000000000000000
122
123


whereas I would have expected the middle number to also be 123.

Is this allowed behaviour, or is it a bug? I realise double to int
conversion is not necessarily exact, but it appears that the double
value is an exact representation of the desired integer value, so I
would have expected it to survive conversion. As shown by the third
line of output, it does when the expression is evaluated at compile
time.