[SOLVED] Why does gcc pass char type in 8 byte format to function assembly

Issue

To learn assembly I am viewing the assembly generated by GCC using the -S command for some simple c programs. I have an add function which accepts some ints and some char and adds them together. I am just wondering why the char parameters are pushed onto the stack as 8 bytes (pushq)? Why not just push a single byte?

    .file   "test.c"
    .text
    .globl  add
    .type   add, @function
add:
.LFB0:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    movl    %edi, -4(%rbp)
    movl    %esi, -8(%rbp)
    movl    %edx, -12(%rbp)
    movl    %ecx, -16(%rbp)
    movl    %r8d, -20(%rbp)
    movl    %r9d, -24(%rbp)
    movl    16(%rbp), %ecx
    movl    24(%rbp), %edx
    movl    32(%rbp), %eax
    movb    %cl, -28(%rbp)
    movb    %dl, -32(%rbp)
    movb    %al, -36(%rbp)
    movl    -4(%rbp), %edx
    movl    -8(%rbp), %eax
    addl    %eax, %edx
    movl    -12(%rbp), %eax
    addl    %eax, %edx
    movl    -16(%rbp), %eax
    addl    %eax, %edx
    movl    -20(%rbp), %eax
    addl    %eax, %edx
    movl    -24(%rbp), %eax
    addl    %eax, %edx
    movsbl  -28(%rbp), %eax
    addl    %eax, %edx
    movsbl  -32(%rbp), %eax
    addl    %eax, %edx
    movsbl  -36(%rbp), %eax
    addl    %edx, %eax
    popq    %rbp
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc
.LFE0:
    .size   add, .-add
    .globl  main
    .type   main, @function
main:
.LFB1:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    pushq   $9
    pushq   $8
    pushq   $7
    movl    $6, %r9d
    movl    $5, %r8d
    movl    $4, %ecx
    movl    $3, %edx
    movl    $2, %esi
    movl    $1, %edi
    call    add
    addq    $24, %rsp
    leave
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc
.LFE1:
    .size   main, .-main
    .ident  "GCC: (Ubuntu 4.9.2-10ubuntu13) 4.9.2"
    .section    .note.GNU-stack,"",@progbits

#include <stdio.h>

int add(int a, int b, int c, int d, int e, int f, char g, char h, char i)
{
    return a + b + c + d + e + f + g + h + i;
}

int main()
{
    return add(1, 2, 3, 4, 5, 6, 7, 8, 9);
}

Solution

When pushing values onto the stack, the push must always be based on the word size of the system. If you're an old timer like me, that's 16 bits (though I do have some 12 bit word size systems!), but it really is system dependent.

Since you're talking about X86_64, you will be talking about 64 bit words. My understanding is that the word size is typically connected to the minimum number of bytes required to address any value on the RAM of the system. Since you have a 64 bit memory space, a 64 bit (or 8 bytes, a "quad word" based on the original 16 bit word size) is required.

Answered By - David Hoelzer

Answer Checked By - Senaida (WPSolving Volunteer)