To find out exactly how gcc does the optimization, I wrote two compilations of the program with -O2, but there is some difference in the build code. In my programs, I want to output "hello" in a loop and add some delay between each output. These two programs are intended only to illustrate my question, and I know that I can use volatile or asm in program 1 to achieve my goal.
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, build code:
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, build code:
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 in i optimized, but this is not in program 2. Why is this happening? What rules do gcc use when optimizing with -O2 for these two programs?