user: 提供的汇编代码用C语言表示会是什么样子:

```asm
<v4lconvert_spca508_to_yuv420>:
  endbr64
  test   %ecx,%ecx
  jle    L152
  mov    %rdi,%rax
  mov    %edx,%edi
  mov    %ecx,%edx
  push   %r15
  imul   %edi,%edx
  push   %r14
  mov    %ecx,%r14d
  mov    %edi,%r10d
  push   %r13
  movslq %edi,%r15
  mov    %r8d,%r13d
  push   %r12
  mov    %rsi,%r12
  lea    (%rdx,%rdx,4),%ecx
  push   %rbp
  test   %ecx,%ecx
  push   %rbx
  movslq %edx,%rbx
  lea    0x3(%rcx),%edx
  cmovns %ecx,%edx
  shr    $0x1f,%r10d
  xor    %ebp,%ebp
  movabs $0x8080808080808080,%rcx
  add    %edi,%r10d
  sar    $0x2,%edx
  mov    %rbx,-0x10(%rsp)
  sar    %r10d
  movslq %edx,%rbx
  mov    %rbx,-0x18(%rsp)
  lea    (%rdi,%rdi,1),%ebx
  mov    %ebx,-0x4(%rsp)
  xor    %ebx,%ebx
  nopl   0x0(%rax)
L80:
  test   %edi,%edi
  jle    L28
  xor    %edx,%edx
  cs nopw 0x0(%rax,%rax,1)
L62:
  mov    (%rax),%r8
  add    $0x8,%rax
  xor    %rcx,%r8
  mov    %r8,(%rsi,%rdx,1)
  add    $0x8,%rdx
  cmp    %edx,%edi
  jg     L62
L28:
  test   %ebx,%ebx
  lea    0x3(%rbx),%r8d
  cmovns %ebx,%r8d
  sar    $0x2,%r8d
  movslq %r8d,%r8
  test   %r13d,%r13d
  je     L92
  mov    -0x18(%rsp),%rdx
  lea    (%rdx,%r8,1),%r11
  add    %r12,%r11
  cmp    $0x1,%edi
  jle    L134
L68:
  xor    %edx,%edx
  nopl   0x0(%rax,%rax,1)
L107:
  mov    (%rax),%r9
  add    $0x8,%rax
  xor    %rcx,%r9
  mov    %r9,(%r11,%rdx,1)
  add    $0x8,%rdx
  cmp    %edx,%r10d
  jg     L107
  test   %r13d,%r13d
  je     L145
L134:
  add    -0x10(%rsp),%r8
  xor    %edx,%edx
  add    %r12,%r8
  cmp    $0x1,%edi
  jle    L118
  nopl   0x0(%rax,%rax,1)
L32:
  mov    (%rax),%r9
  add    $0x8,%rax
  xor    %rcx,%r9
  mov    %r9,(%r8,%rdx,1)
  add    $0x8,%rdx
  cmp    %edx,%r10d
  jg     L32
  add    %r15,%rsi
L87:
  xor    %edx,%edx
  nopl   0x0(%rax)
L128:
  mov    (%rax),%r8
  add    $0x8,%rax
  xor    %rcx,%r8
  mov    %r8,(%rsi,%rdx,1)
  add    $0x8,%rdx
  cmp    %edx,%edi
  jg     L128
  add    $0x2,%ebp
  add    -0x4(%rsp),%ebx
  add    %r15,%rsi
  cmp    %ebp,%r14d
  jg     L80
L124:
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  nopl   0x0(%rax)
L92:
  mov    -0x10(%rsp),%rdx
  lea    (%rdx,%r8,1),%r11
  add    %r12,%r11
  cmp    $0x1,%edi
  jg     L68
  nopl   (%rax)
L145:
  add    -0x18(%rsp),%r8
  xor    %edx,%edx
  add    %r12,%r8
  cmp    $0x1,%edi
  jg     L32
L118:
  add    %r15,%rsi
  cmp    $0x1,%edi
  je     L87
  add    $0x2,%ebp
  add    -0x4(%rsp),%ebx
  add    %r15,%rsi
  cmp    %ebp,%r14d
  jg     L80
  jmp    L124
L152:
  ret
```

assistant: ```c
void v4lconvert_spca508_to_yuv420(const unsigned char *src, unsigned char *dst, int width, int height, int yvu) {
  int i, j;
  unsigned long *lsrc = (unsigned long *)src;
  for (i = 0; i < height; i += 2) {
    unsigned long *ldst = (unsigned long *)(dst + i * width);
    for (j = 0; j < width; j += sizeof(long)) {
      *ldst = *lsrc++;
      *ldst++ ^= 0x8080808080808080ULL;
    }
    if (yvu)
      ldst = (unsigned long *)(dst + (width * height * 5) / 4 + i * width / 4);
    else
      ldst = (unsigned long *)(dst + width * height + i * width / 4);
    for (j = 0; j < width / 2; j += sizeof(long)) {
      *ldst = *lsrc++;
      *ldst++ ^= 0x8080808080808080ULL;
    }
    if (yvu)
      ldst = (unsigned long *)(dst + width * height + i * width / 4);
    else
      ldst = (unsigned long *)(dst + (width * height * 5) / 4 + i * width / 4);
    for (j = 0; j < width / 2; j += sizeof(long)) {
      *ldst = *lsrc++;
      *ldst++ ^= 0x8080808080808080ULL;
    }
    ldst = (unsigned long *)(dst + i * width + width);
    for (j = 0; j < width; j += sizeof(long)) {
      *ldst = *lsrc++;
      *ldst++ ^= 0x8080808080808080ULL;
    }
  }
}
```