Zero initializers faster than memset?

Question

I keep the legacy C code where in many places they have small arrays, such as int a[32];followed by a memset(a, 0, sizeof a);zero initializing it.

I am going to reorganize this into int a[32] = {0};and delete the memset.

The question is: do zero initializers use faster code than calling memset?

Answer 1

TL DR: use an initializer - it is never worse memset().

It depends on your compiler. It should not be slower than a call memset()(since a call memset()is one of the options available to the compiler).

, ; , , - , .

---

, , GCC :

#include <string.h>

int f1()
{
    int a[32] = {0};
    return a[31];
}

int f2()
{
    int a[32];
    memset(a, 0, sizeof a);
    return a[31];
}

gcc -S -std=c11 :

f1:
.LFB0:
    .file 1 "40786375.c"
    .loc 1 4 0
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    subq    $8, %rsp
    .loc 1 5 0
    leaq    -128(%rbp), %rdx
    movl    $0, %eax
    movl    $16, %ecx
    movq    %rdx, %rdi
    rep stosq
    .loc 1 6 0
    movl    -4(%rbp), %eax
    .loc 1 7 0
    leave
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc
f2:
.LFB1:
    .loc 1 10 0
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    addq    $-128, %rsp
    .loc 1 12 0
    leaq    -128(%rbp), %rax
    movl    $128, %edx
    movl    $0, %esi
    movq    %rax, %rdi
    call    memset@PLT
    .loc 1 13 0
    movl    -4(%rbp), %eax
    .loc 1 14 0
    leave
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc

, f1() rep stosq , f2() , , C. , memset() , , , , , .

a volatile, , (gcc -S -std=c11 -O3):

f1:
.LFB4:
    .cfi_startproc
    subq    $16, %rsp
    .cfi_def_cfa_offset 24
    xorl    %eax, %eax
    movl    $16, %ecx
    leaq    -120(%rsp), %rdi
    rep stosq
    movl    4(%rsp), %eax
    addq    $16, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
f2:
.LFB5:
    .cfi_startproc
    subq    $16, %rsp
    .cfi_def_cfa_offset 24
    xorl    %eax, %eax
    movl    $16, %ecx
    leaq    -120(%rsp), %rdx
    movq    %rdx, %rdi
    rep stosq
    movl    4(%rsp), %eax
    addq    $16, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc

, .