In order to know how exactly the gcc do the optimization, I have written two program compiling with -O2, but there is some difference of the assembly code. In my programs, I want to output "hello" in the loop, and add some delay between each output. These two programs are only for illustrating my question, and I know I can using volatile or asm in program 1 to achieve my purpose.
Program 1
#include <stdio.h>
int main(int argc, char **argv)
{
unsigned long i = 0;
while (1) {
if (++i > 0x1fffffffUL) {
printf("hello\n");
i = 0;
}
}
}
Compile with -O2, the assembly code is:
Disassembly of section .text.startup:
00000000 <_main>:
#include <stdio.h>
int main(int argc, char **argv)
{
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 83 e4 f0 and $0xfffffff0,%esp
6: 83 ec 10 sub $0x10,%esp
9: e8 00 00 00 00 call e <_main+0xe>
e: 66 90 xchg %ax,%ax
10: c7 04 24 00 00 00 00 movl $0x0,(%esp)
17: e8 00 00 00 00 call 1c <_main+0x1c>
1c: eb f2 jmp 10 <_main+0x10>
1e: 90 nop
1f: 90 nop
Program 2
int main(int argc, char **argv)
{
unsigned long i = 0;
while (1) {
if (i > 0x1fffffffUL) {
printf("hello\n");
i = 0;
}
i++;
}
}
Compile with -O2, the assembly code is:
Disassembly of section .text.startup:
00000000 <_main>:
#include <stdio.h>
int main(int argc, char **argv)
{
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 83 e4 f0 and $0xfffffff0,%esp
6: 83 ec 10 sub $0x10,%esp
9: e8 00 00 00 00 call e <_main+0xe>
e: 31 c0 xor %eax,%eax
10: 83 c0 01 add $0x1,%eax
13: 3d ff ff ff 1f cmp $0x1fffffff,%eax
18: 76 f6 jbe 10 <_main+0x10>
1a: c7 04 24 00 00 00 00 movl $0x0,(%esp)
while (1) {
if (i > 0x1fffffffUL) {
printf("hello\n");
i = 0;
}
i++;
21: e8 00 00 00 00 call 26 <_main+0x26>
int main(int argc, char **argv)
{
unsigned long i = 0;
while (1) {
if (i > 0x1fffffffUL) {
26: 31 c0 xor %eax,%eax
28: eb e6 jmp 10 <_main+0x10>
printf("hello\n");
2a: 90 nop
2b: 90 nop
2c: 90 nop
2d: 90 nop
2e: 90 nop
2f: 90 nop
In program 1, the increase of i is optimized out, but it's not in program 2. Why this happens? What rules is gcc using when optimizing with -O2 for these two programs?
