user: 请从给定的汇编指令推导出C源代码：

```asm
<fib>:
  endbr64
  push   %r15
  mov    %rdi,%rax
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x68,%rsp
  movq   $0x0,(%rsp)
  cmp    $0x1,%rdi
  jbe    L171
  lea    -0x2(%rdi),%r11
  mov    %r11,%r10
L186:
  movq   $0x0,0x8(%rsp)
  lea    -0x2(%r10),%r11
  cmp    $0x1,%r10
  jbe    L300
  mov    %r10,0x18(%rsp)
L42:
  movq   $0x0,0x10(%rsp)
  lea    -0x2(%r11),%rsi
  cmp    $0x1,%r11
  jbe    L117
L196:
  lea    -0x2(%rsi),%r8
  xor    %r10d,%r10d
  cmp    $0x1,%rsi
  jbe    L66
L121:
  lea    -0x2(%r8),%r12
  xor    %ecx,%ecx
  cmp    $0x1,%r8
  jbe    L81
L49:
  lea    -0x2(%r12),%rbx
  xor    %ebp,%ebp
  cmp    $0x1,%r12
  jbe    L6
L157:
  lea    -0x4(%rbx),%r15
  xor    %r14d,%r14d
  cmp    $0x1,%rbx
  jbe    L20
L32:
  lea    0x2(%r15),%rax
  mov    %r15,%r13
  xor    %edx,%edx
  cmp    $0xfffffffffffffffe,%r15
  jae    L78
L205:
  lea    -0x2(%r13),%rdi
  xor    %r9d,%r9d
  cmp    $0x1,%r13
  jbe    L283
  nopl   0x0(%rax)
L195:
  mov    %r11,0x58(%rsp)
  mov    %rdx,0x50(%rsp)
  mov    %rcx,0x48(%rsp)
  mov    %r10,0x40(%rsp)
  mov    %r9,0x38(%rsp)
  mov    %r8,0x30(%rsp)
  mov    %rsi,0x28(%rsp)
  mov    %rdi,0x20(%rsp)
  call   <fib>
  mov    0x20(%rsp),%rdi
  mov    0x38(%rsp),%r9
  mov    0x28(%rsp),%rsi
  mov    0x30(%rsp),%r8
  sub    $0x1,%rdi
  add    %rax,%r9
  mov    0x40(%rsp),%r10
  mov    0x48(%rsp),%rcx
  cmp    $0xffffffffffffffff,%rdi
  mov    0x50(%rsp),%rdx
  mov    0x58(%rsp),%r11
  jne    L195
  sub    $0x1,%r13
  lea    0x1(%r9,%rdx,1),%rdx
  xor    %r9d,%r9d
  lea    -0x2(%r13),%rdi
  cmp    $0x1,%r13
  ja     L195
L283:
  add    %r13,%rdx
  test   %r13,%r13
  jne    L35
  sub    $0x1,%r15
  lea    0x1(%r14,%rdx,1),%r14
  xor    %edx,%edx
  lea    0x2(%r15),%rax
  mov    %r15,%r13
  cmp    $0xfffffffffffffffe,%r15
  jb     L205
L78:
  add    %rax,%r14
  cmp    $0xfffffffffffffffe,%r15
  jne    L141
  sub    $0x1,%rbx
  lea    0x1(%rbp,%r14,1),%rbp
  xor    %r14d,%r14d
  lea    -0x4(%rbx),%r15
  cmp    $0x1,%rbx
  ja     L32
L20:
  add    %rbx,%rbp
  test   %rbx,%rbx
  jne    L17
  sub    $0x1,%r12
  lea    0x1(%rcx,%rbp,1),%rcx
  xor    %ebp,%ebp
  lea    -0x2(%r12),%rbx
  cmp    $0x1,%r12
  ja     L157
L6:
  add    %r12,%rcx
  test   %r12,%r12
  jne    L104
  sub    $0x1,%r8
  lea    0x1(%r10,%rcx,1),%r10
  xor    %ecx,%ecx
  lea    -0x2(%r8),%r12
  cmp    $0x1,%r8
  ja     L49
L81:
  add    %r8,%r10
  test   %r8,%r8
  jne    L167
  mov    0x10(%rsp),%rax
  sub    $0x1,%rsi
  lea    -0x2(%rsi),%r8
  lea    0x1(%rax,%r10,1),%rax
  xor    %r10d,%r10d
  mov    %rax,0x10(%rsp)
  cmp    $0x1,%rsi
  ja     L121
L66:
  add    %rsi,0x10(%rsp)
  test   %rsi,%rsi
  jne    L9
  mov    0x8(%rsp),%rax
  mov    0x10(%rsp),%rdi
  sub    $0x1,%r11
  lea    0x1(%rax,%rdi,1),%rax
  mov    %rax,0x8(%rsp)
  jmp    L42
L117:
  add    %r11,0x8(%rsp)
  test   %r11,%r11
  jne    L241
  mov    (%rsp),%rax
  mov    0x8(%rsp),%rdi
  mov    0x18(%rsp),%r10
  lea    0x1(%rax,%rdi,1),%rax
  mov    %rax,(%rsp)
  sub    $0x1,%r10
  jmp    L186
L300:
  add    %r10,(%rsp)
  test   %r10,%r10
  jne    L165
  mov    (%rsp),%rax
  add    $0x1,%rax
L171:
  add    $0x68,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
L17:
  xor    %ebx,%ebx
  jmp    L157
L165:
  xor    %r10d,%r10d
  jmp    L186
L241:
  xor    %r11d,%r11d
  jmp    L42
L9:
  xor    %esi,%esi
  jmp    L196
L167:
  xor    %r8d,%r8d
  jmp    L121
L104:
  xor    %r12d,%r12d
  jmp    L49
L35:
  xor    %r13d,%r13d
  jmp    L205
L141:
  mov    $0xfffffffffffffffe,%r15
  jmp    L32
```

assistant: ```c
size_t fib(size_t n) {
  if (n < 2)
    return n;
  return fib(n - 2) + fib(n - 1);
}
```