user: 请从给定的汇编指令推导出C源代码:

```asm
<memset_c>:
  endbr64
  cmp    $0x3,%edx
  jbe    L213
  shl    $0x18,%esi
  push   %r13
  mov    %esi,%eax
  push   %r12
  shr    $0x8,%eax
  push   %rbp
  mov    %edx,%ebp
  or     %esi,%eax
  push   %rbx
  add    %rdi,%rbp
  mov    %eax,%ebx
  shr    $0x10,%ebx
  or     %eax,%ebx
  sub    $0x8,%rsp
  mov    %edi,%eax
  test   $0x1,%dil
  je     L57
  lea    0x1(%rdi),%rdx
  mov    %bl,(%rdi)
  mov    %edx,%eax
  mov    %rdx,%rdi
L57:
  test   $0x2,%al
  je     L222
  mov    %bx,(%rdi)
  add    $0x2,%rdi
L222:
  mov    %rbp,%rax
  sub    %rdi,%rax
  mov    %eax,%r13d
  cmp    $0x3,%eax
  ja     L193
L152:
  mov    %r13d,%r12d
  sub    $0x8,%r12d
  js     L187
  mov    %r12d,%ebp
  nopl   (%rax)
L165:
  mov    %ebx,%edx
  mov    %ebx,%esi
  call   <WriteUnsigned2@plt>
  mov    %rax,%rdi
  sub    $0x8,%ebp
  jns    L165
L228:
  mov    %r12d,%eax
  sub    $0x9,%r13d
  xor    %edx,%edx
  and    $0xfffffff8,%eax
  neg    %eax
  cmp    $0xffffffff,%r13d
  cmovl  %edx,%eax
  lea    -0x8(%r12,%rax,1),%r12d
L187:
  test   $0x4,%r12b
  je     L82
  mov    %ebx,(%rdi)
  add    $0x4,%rdi
L82:
  test   $0x2,%r12b
  je     L175
  mov    %bx,(%rdi)
  add    $0x2,%rdi
L175:
  xor    %eax,%eax
  and    $0x1,%r12d
  je     L101
  mov    %bl,(%rdi)
L101:
  add    $0x8,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  ret
  nopl   0x0(%rax)
L193:
  test   $0x4,%dil
  jne    L154
L162:
  cmp    $0x1f,%r13d
  jbe    L152
  mov    %edi,%eax
  test   $0x8,%dil
  jne    L81
L63:
  test   $0x10,%al
  jne    L142
L33:
  sub    %rdi,%rbp
  mov    %ebp,%r13d
  sub    $0x20,%r13d
  js     L207
  mov    %r13d,%r12d
  nopl   0x0(%rax,%rax,1)
L23:
  mov    %ebx,%r8d
  mov    %ebx,%ecx
  mov    %ebx,%edx
  mov    %ebx,%esi
  call   <WriteUnsigned4@plt>
  mov    %ebx,%r8d
  mov    %ebx,%ecx
  mov    %ebx,%edx
  mov    %rax,%rdi
  mov    %ebx,%esi
  call   <WriteUnsigned4@plt>
  mov    %rax,%rdi
  sub    $0x20,%r12d
  jns    L23
  and    $0xffffffe0,%r13d
  lea    -0x21(%rbp),%eax
  neg    %r13d
  cmp    $0xffffffff,%eax
  mov    $0x0,%eax
  cmovge %r13d,%eax
  lea    -0x40(%rax,%rbp,1),%r13d
L207:
  test   $0x10,%r13b
  jne    L25
L182:
  and    $0xf,%r13d
  mov    %r13d,%r12d
  sub    $0x8,%r12d
  js     L187
  mov    %ebx,%edx
  mov    %ebx,%esi
  call   <WriteUnsigned2@plt>
  mov    %rax,%rdi
  jmp    L228
  nopl   0x0(%rax,%rax,1)
L213:
  mov    %rdi,%rax
  test   %edx,%edx
  je     L145
  mov    %sil,(%rdi)
  cmp    $0x1,%edx
  je     L70
  mov    %sil,0x1(%rdi)
  cmp    $0x3,%edx
  jne    L70
  mov    %sil,0x2(%rdi)
  ret
  nopl   (%rax)
L145:
  ret
  nopl   0x0(%rax)
L154:
  mov    %ebx,(%rdi)
  lea    -0x4(%rax),%r13d
  add    $0x4,%rdi
  jmp    L162
  nopw   0x0(%rax,%rax,1)
L70:
  mov    %rdi,%rax
  ret
  nopl   0x0(%rax)
L25:
  mov    %ebx,%r8d
  mov    %ebx,%ecx
  mov    %ebx,%edx
  mov    %ebx,%esi
  call   <WriteUnsigned4@plt>
  mov    %rax,%rdi
  jmp    L182
  xchg   %ax,%ax
L142:
  mov    %ebx,%r8d
  mov    %ebx,%ecx
  mov    %ebx,%edx
  mov    %ebx,%esi
  call   <WriteUnsigned4@plt>
  mov    %rax,%rdi
  jmp    L33
  cs nopw 0x0(%rax,%rax,1)
L81:
  mov    %ebx,%edx
  mov    %ebx,%esi
  call   <WriteUnsigned2@plt>
  mov    %rax,%rdi
  jmp    L63
```

assistant: ```c
char *memset_c(char *restrict ptr, unsigned char fillC, unsigned len) {
  if (len <= 3) {
    if (len >= 1)
      ptr[0] = fillC;
    if (len > 1)
      ptr[1] = fillC;
    if (len == 3)
      ptr[2] = fillC;
    return ptr;
  }
  unsigned fill = (unsigned)fillC;
  char *end = ptr + len;
  {
    unsigned shift = fill << 24;
    shift = shift | (shift >> 8);
    fill = shift | (shift >> 16);
  }
  if ((unsigned)ptr & 1)
    *ptr++ = fill;
  if ((unsigned)ptr & 2) {
    *(unsigned short *)ptr = fill;
    ptr += 2;
  }
  unsigned remain = end - ptr;
  unsigned *restrict wordptr = (unsigned *restrict)ptr;
  if (remain >= 4 && ((unsigned)wordptr & 4)) {
    *wordptr++ = fill;
    remain -= 4;
  }
  if (remain >= 32) {
    if ((unsigned)wordptr & 8)
      wordptr = WriteUnsigned2(wordptr, fill, fill);
    if ((unsigned)wordptr & 16)
      wordptr = WriteUnsigned4(wordptr, fill, fill, fill, fill);
    remain = end - (char *)wordptr;
    remain -= 32;
    while ((int)remain >= 0) {
      wordptr = WriteUnsigned4(wordptr, fill, fill, fill, fill);
      wordptr = WriteUnsigned4(wordptr, fill, fill, fill, fill);
      remain -= 32;
    }
    if ((unsigned)remain & 16)
      wordptr = WriteUnsigned4(wordptr, fill, fill, fill, fill);
    remain &= 0xFU;
  }
  remain -= 8;
  while ((int)remain >= 0) {
    wordptr = WriteUnsigned2(wordptr, fill, fill);
    remain -= 8;
  }
  if (remain & 4)
    *wordptr++ = fill;
  ptr = (char *restrict)wordptr;
  if (remain & 2) {
    *(unsigned short *)ptr = fill;
    ptr += 2;
  }
  if (remain & 1)
    *ptr++ = fill;
  return 0;
}
```