chasep255 chasep255 - 1 month ago 9
C Question

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

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



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);
}

Answer Source

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.