How does GCC align struct in x64?

TL;DR

When aligning the stack to multiple of 16 bytes, why are the paddings at the higher address for struct while at the lower address for primitive types?

More details

First of all, my machine follows System V AMD64 ABI, and I understand that the stack is aligned to 16 bytes. But when I try to understand how structs are stored on the stack, I found some interesting behavior of GCC that I don't understand.

I have the following program:

struct S {
    long i;
};

void take_struct_addr(struct S *sp);
void take_long_addr(long *l);

void access_member() {
    struct S s;
    take_struct_addr(&s);
}

void access_local() {
    long l;
    take_long_addr(&l);
}

I compile it using the following command:

$ gcc -ansi -pedantic -S -Og access-member.c

And I get the following assembly code:

    .file   "access-member.c"
    .text
    .globl  access_member
    .type   access_member, @function
access_member:
.LFB0:
    .cfi_startproc
    subq    $24, %rsp
    .cfi_def_cfa_offset 32
    movq    %rsp, %rdi # <---------- start address of the struct
    call    take_struct_addr
    addq    $24, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
.LFE0:
    .size   access_member, .-access_member
    .globl  access_local
    .type   access_local, @function
access_local:
.LFB1:
    .cfi_startproc
    subq    $24, %rsp
    .cfi_def_cfa_offset 32
    leaq    8(%rsp), %rdi # <---------- start address of the long
    call    take_long_addr
    addq    $24, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
.LFE1:
    .size   access_local, .-access_local
    .ident  "GCC: (GNU) 4.8.5 20150623 (Red Hat 4.8.5-39)"
    .section    .note.GNU-stack,"",@progbits

So, my question is why are the padding locations different in these two cases? Thanks in advance!