------- Comment #5 from gdr at cs dot tamu dot edu 2006-12-18 10:16 -------
Subject: Re: New: -O2 generates bad code
"dcb314 at hotmail dot com" <[EMAIL PROTECTED]> writes:
| I compiled the following C++ code on a x86_64 machine
| without optimisation.
|
| #include <iostream.h>
|
| int
| main()
| {
| long long n = 1;
|
| cout << sizeof( n) << endl;
| for (int i = 0; i < 100; ++i)
| {
| cout << n << ' ' << (float) n << '\n';
| n *= 2;
| if (n == 0)
| {
| break;
| }
| }
| return 0;
| }
|
| When the compiled code runs, it is fine.
| There are 64 executions of the loop.
|
| However, I added flag -O2, and the code runs differently.
| There are 100 executions of the loop.
|
| Suspect optimiser bug.
Rather, user bug.
Your program contains an undefined behaviour which is provoked by the
overflow of n *= 2; the compiler is therefore technically allowed to
do whatever it likes. In this case, the optimizer assumes that you
can never get from 1 to 0 by successive exponentiation. Therefore the
test n == 0 is never executed.
-- Gaby
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30245
