Is there any performance difference between these two different programs?

#define K 50

void main() {
    int k = K;
}

void main() {
    int k = 50;
}

CodePudding user response：

Macros in C perform token substitution before the actual compilation stage.

This means that after preprocessing, the second piece of code is exactly the same as the first.

CodePudding user response：

In this case: No, it makes 0 difference (not even at -O0!).

This is the assembler gcc 10 on ubuntu 20.04 produced on my machine:

.file   "compare_assign.c"
.text
.globl  main
.type   main, @function

main:
.LFB0:
    .cfi_startproc
    endbr64
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    movl    $50, -4(%rbp)
    nop
    popq    %rbp
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc
.LFE0:
    .size   main, .-main
    .ident  "GCC: (Ubuntu 10.3.0-1ubuntu1~20.04) 10.3.0"
    .section    .note.GNU-stack,"",@progbits
    .section    .note.gnu.property,"a"
    .align 8
    .long    1f - 0f
    .long    4f - 1f
    .long    5
0:
    .string  "GNU"
1:
    .align 8
    .long    0xc0000002
    .long    3f - 2f
2:
    .long    0x3
3:
    .align 8
4:

Only difference is the title of the source file (duh):

diff compare_assign_noopt.asm compare_define_noopt.asm 
1c1
<   .file   "compare_assign.c"
---
>   .file   "compare_define.c"