Ed Boas wrote:
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
$ gcc -Wall -O2 ed.c
ed.c: In function 'int32_t fast_round(float)':
ed.c:9:25: warning: dereferencing type-punned pointer will break
strict-aliasing
rules
TCPRINCE@tcprince-MOBL5 /cygdrive/c/Documents and Settings/tcprince/My
Documents
/tim/tim/src/net
$ ./a
1.400000 rounds to 1
gcc 4.5 doesn't show the problem of gcc 4.3.2 on this broken source
code. You invoke Undefined Behavior with a past release of gcc, you get
what you paid for. Who's to say what is "correct output?"
Set -no-strict-aliasing, the warning goes away, and even gcc 4.3 gives
your "correct output."
