http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49321
Summary: Incorrect Optimization O2 and O3 on Shift Operation
for 64 bit Integers
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: [email protected]
ReportedBy: [email protected]
// Shift operation yielded different results on different
// level of optimization for the same source. O1 is working but
// not O2 and O3
// Tested environments: Fedora 10. GCC 4.3.2.20081105
// and Windows XP, MingGW GCC 4.5.2
// Compiler output and warning: nothing
// source file: one attached (no header file)
#include "stdio.h"
/*
* Incorrect -O2 and -O3 optimization for 64 bit integer shift operator.
* -O1 is working correclty
g++ -O1 main.cpp && ./a.out
i[0]=0, i[1]=8
g++ -O2 main.cpp && ./a.out
i[0]=1, i[1]=0
g++ -O3 main.cpp && ./a.out
i[0]=1, i[1]=0
*/
int main(int argc, char* argv[]) {
int i[2];
i[0] = 1;
i[1] = 0;
*((unsigned long long *) (&i)) <<= 35;
/* correct result: linux test i[0]=0, i[1]=8 */
printf ("i[0]=%d, i[1]=%d\n", i[0],i[1]);
return 0;
}
