'Why does GCC allocate more stack memory than needed?

I'm reading "Computer Systems: A Programmer's Perspective, 3/E" (CS:APP3e) and the following code is an example from the book:

long call_proc() {
    long  x1 = 1;
    int   x2 = 2;
    short x3 = 3;
    char  x4 = 4;
    proc(x1, &x1, x2, &x2, x3, &x3, x4, &x4);
    return (x1+x2)*(x3-x4);
}

The book gives the assembly code generated by GCC:

long call_proc()
call_proc:
    ; Set up arguments to proc
    subq    $32, %rsp           ; Allocate 32-byte stack frame
    movq    $1, 24(%rsp)        ; Store 1 in &x1
    movl    $2, 20(%rsp)        ; Store 2 in &x2
    movw    $3, 18(%rsp)        ; Store 3 in &x3
    movb    $4, 17(%rsp)        ; Store 4 in &x4
    leaq    17(%rsp), %rax      ; Create &x4
    movq    %rax, 8(%rsp)       ; Store &x4 as argument 8
    movl    $4, (%rsp)          ; Store 4 as argument 7
    leaq    18(%rsp), %r9       ; Pass &x3 as argument 6
    movl    $3, %r8d            ; Pass 3 as argument 5
    leaq    20(%rsp), %rcx      ; Pass &x2 as argument 4
    movl    $2, %edx            ; Pass 2 as argument 3
    leaq    24(%rsp), %rsi      ; Pass &x1 as argument 2
    movl    $1, %edi            ; Pass 1 as argument 1
    ; Call proc
    call    proc
    ; Retrieve changes to memory
    movslq  20(%rsp), %rdx      ; Get x2 and convert to long
    addq    24(%rsp), %rdx      ; Compute x1+x2
    movswl  18(%rsp), %eax      ; Get x3 and convert to int
    movsbl  17(%rsp), %ecx      ; Get x4 and convert to int
    subl    %ecx, %eax          ; Compute x3-x4
    cltq                        ; Convert to long
    imulq   %rdx, %rax          ; Compute (x1+x2) * (x3-x4)
    addq    $32, %rsp           ; Deallocate stack frame
    ret                         ; Return

I can understand this code: the compiler allocates 32 bytes of space on the stack, of which the first 16 bytes hold the arguments passed to proc and the last 16 bytes hold 4 local variables.

Then I tested this code on GCC 11.2, using the optimization flag -Og, and got this assembly code:

call_proc():
        subq    $24, %rsp
        movq    $1, 8(%rsp)
        movl    $2, 4(%rsp)
        movw    $3, 2(%rsp)
        movb    $4, 1(%rsp)
        leaq    1(%rsp), %rax
        pushq   %rax
        pushq   $4
        leaq    18(%rsp), %r9
        movl    $3, %r8d
        leaq    20(%rsp), %rcx
        movl    $2, %edx
        leaq    24(%rsp), %rsi
        movl    $1, %edi
        call    proc(long, long*, int, int*, short, short*, char, char*)
        movslq  20(%rsp), %rax
        addq    24(%rsp), %rax
        movswl  18(%rsp), %edx
        movsbl  17(%rsp), %ecx
        subl    %ecx, %edx
        movslq  %edx, %rdx
        imulq   %rdx, %rax
        addq    $40, %rsp
        ret

I noticed that gcc first allocated 24 bytes for 4 local variables. Then it uses pushq to add 2 arguments to the stack, so the final code uses addq $40, %rsp to free stack space.

Compared to the code in the book, GCC allocates 8 more bytes of space here, and it doesn't seem to use the extra space. Why does it need the extra space?

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'Why does GCC allocate more stack memory than needed?

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