Is a stack frame required for all functions in C on x86-64?

Question

I made a function to calculate the length of a C string (I'm trying to beat the clang optimizer with -O3). I am running macOS.

_string_length1:
    push rbp
    mov rbp, rsp
    xor rax, rax
.body:
    cmp byte [rdi], 0
    je .exit
    inc rdi
    inc rax
    jmp .body
.exit:
    pop rbp
    ret

This is the C function I'm trying to defeat:

size_t string_length2(const char *str) {
  size_t ret = 0;
  while (str[ret]) {
    ret++;
  }
  return ret;
}

And he figured it out:

string_length2:
    push rbp
    mov rbp, rsp
    mov rax, -1
LBB0_1:
    cmp byte ptr [rdi + rax + 1], 0
    lea rax, [rax + 1]
    jne LBB0_1
    pop rbp
    ret

Each C function sets the stack frame with push rbpand mov rbp, rspand splits it with pop rbp. But I do not use the stack in any way, I only use processor registers. It worked without using the stack frame (when I tested on x86-64), but is this necessary?

Answer 1

, , , . . , inline ( ) tail call ( ).

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(, gcc -flto -O2), .

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BTW, long fact(int n) GCC 7.1 ( Debian/Sid/x86-64) -O3 (.. gcc -fverbose-asm -S -O3 fact.c). fact.s call.

Answer 2

C , ...

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