Question

I searched for x++ vs ++x and I found a great answer here, So I decide to see the assembly output of gcc to see how x++ and ++x implemented:

main() { int s = 0; ++s; return 0; }

Compiling the example:

gcc mul.c -masm=intel -o mul.asm

Output of ++s:

    .file   "mul.c"
    .intel_syntax
    .text
    .p2align 4,,15
.globl main
    .type   main, @function
main:
    lea %ecx, [%esp+4]
    and %esp, -16
    push    DWORD PTR [%ecx-4]
    push    %ebp
    mov %ebp, %esp
    push    %ecx
    sub %esp, 16
    mov DWORD PTR [%ebp-8], 0
    add DWORD PTR [%ebp-8], 1
    mov %eax, 0
    add %esp, 16
    pop %ecx
    pop %ebp
    lea %esp, [%ecx-4]
    ret
    .size   main, .-main
    .ident  "GCC: (GNU) 4.2.1 20070719  [FreeBSD]"      

Output for x++:

    .file   "mul.c"
    .intel_syntax
    .text
    .p2align 4,,15
.globl main
    .type   main, @function
main:
    lea %ecx, [%esp+4]
    and %esp, -16
    push    DWORD PTR [%ecx-4]
    push    %ebp
    mov %ebp, %esp
    push    %ecx
    sub %esp, 16
    mov DWORD PTR [%ebp-8], 0
    add DWORD PTR [%ebp-8], 1
    mov %eax, 0
    add %esp, 16
    pop %ecx
    pop %ebp
    lea %esp, [%ecx-4]
    ret
    .size   main, .-main
    .ident  "GCC: (GNU) 4.2.1 20070719  [FreeBSD]"

So, I am asking if x++ and ++x have different meaning, why GCC outputs the some assembly for them, shouldn’t have different output?

Answer 1

This is an example of a not-well written test case. The actual result of the expression value++ or ++value is never stored, as such the compiler can handle both equivalent for basic types.

Use this examples instead:

main() { int s = 0, x; x = ++s; return 0; }

main() { int s = 0, x; x = s++; return 0; }

Post-increment:

(gdb) disas /m main
Dump of assembler code for function main():
1       int main(){
   0x0040138c :     push   %ebp
   0x0040138d :     mov    %esp,%ebp
   0x0040138f :     and    $0xfffffff0,%esp
   0x00401392 :     sub    $0x10,%esp
   0x00401395 :     call   0x4018d4 

2               int s = 0;
   0x0040139a :    movl   $0x0,0xc(%esp)

3               int x;
4               x = s++;
   0x004013a2 :    mov    0xc(%esp),%eax
   0x004013a6 :    mov    %eax,0x8(%esp)
   0x004013aa :    incl   0xc(%esp)

5               return 0;
   0x004013ae :    mov    $0x0,%eax

6       }   0x004013b3 :   leave
   0x004013b4 :    ret

End of assembler dump.
(gdb)

Pre-increment:

(gdb) disas /m main
Dump of assembler code for function main():
1       int main(){
   0x0040138c :     push   %ebp
   0x0040138d :     mov    %esp,%ebp
   0x0040138f :     and    $0xfffffff0,%esp
   0x00401392 :     sub    $0x10,%esp
   0x00401395 :     call   0x4018d4 

2               int s = 0;
   0x0040139a :    movl   $0x0,0xc(%esp)

3               int x;
4               x = ++s;
   0x004013a2 :    incl   0xc(%esp)
   0x004013a6 :    mov    0xc(%esp),%eax
   0x004013aa :    mov    %eax,0x8(%esp)

5               return 0;
   0x004013ae :    mov    $0x0,%eax

6       }   0x004013b3 :   leave
   0x004013b4 :    ret

End of assembler dump.
(gdb)