The following code:
----------
#include <stdio.h>
#include <inttypes.h>
// Rounds in current rounding mode, which is "round to closest
integer" by default on Pentium & Xeon
// Valid for |fval| <= 2^22 - 1, that is, -4194303 <= fval <= 4914303
// based on code from www.lomont.org
inline int32_t fast_round(float x) {
x+=12582912; // 2^23+2^22
int32_t i=*(int32_t*)&x;
i&=8388607; // 2^23-1;
i-=4194304; // 2^22
return i;
}
int main(void) {
printf("%f rounds to %i\n", 1.4f, fast_round(1.4f));
return 0;
}
----------
produces the correct output when compiled using g++ -O:
1.400000 rounds to 1
However, when it is compiled with g++ -O2, it prints:
1.400000 rounds to -4194304
I don't know anything about assembly, so I can't diagnose the reason
for this problem. I am using g++ Ubuntu 4.3.3 on AMD Opteron. Any
ideas?
Thanks!
Ed
________________________________
F. Edward Boas
3375 Alma St # 166
Palo Alto, CA 94306
650-787-1688
http://www.stanford.edu/~boas
