user: 请从给定的汇编指令推导出C源代码:

```asm
<cvt_420p_to_rgb>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  mov    %edi,-0x18(%rsp)
  mov    %esi,-0x14(%rsp)
  mov    %rdx,%r9
  mov    %edi,%eax
  imul   %esi,%eax
  lea    (%rax,%rax,2),%edx
  movslq %edx,%rdx
  lea    (%rcx,%rdx,1),%r13
  test   %esi,%esi
  jle    L44
  lea    0x3(%rax),%ebx
  test   %eax,%eax
  cmovns %eax,%ebx
  sar    $0x2,%ebx
  movslq %ebx,%rbx
  lea    -0x1(%rdi),%ebp
  lea    0x1(%rbp),%rdx
  lea    0x0(,%rdx,4),%rcx
  sub    %rcx,%rdx
  mov    %rdx,-0x10(%rsp)
  mov    $0x0,%r14d
  mov    $0x0,%r12d
  cltq
  mov    %rax,-0x8(%rsp)
  jmp    L54
L21:
  movslq %r14d,%r11
  mov    %r12d,%eax
  shr    $0x1f,%eax
  add    %r12d,%eax
  sar    %eax
  lea    (%rax,%rax,4),%r10d
  shl    $0x5,%r10d
  movslq %r10d,%r10
  add    -0x8(%rsp),%r10
  mov    %r13,%r8
  mov    $0x0,%edi
  add    %r9,%r11
L0:
  movzbl (%r11,%rdi,1),%eax
  mov    %edi,%edx
  shr    $0x1f,%edx
  add    %edi,%edx
  sar    %edx
  movslq %edx,%rdx
  add    %r10,%rdx
  movzbl (%r9,%rdx,1),%esi
  add    %r9,%rdx
  movzbl (%rdx,%rbx,1),%ecx
  add    $0xffffff80,%esi
  add    $0xffffff80,%ecx
  imul   $0xc6,%esi,%edx
  sar    $0x8,%edx
  lea    (%rax,%rsi,1),%r15d
  add    %r15d,%edx
  mov    $0xff,%r15d
  cmp    %r15d,%edx
  cmovg  %r15d,%edx
  test   %edx,%edx
  mov    $0x0,%r15d
  cmovs  %r15d,%edx
  mov    %dl,(%r8)
  imul   $0x67,%ecx,%edx
  sar    $0x8,%edx
  lea    (%rax,%rcx,1),%r15d
  add    %r15d,%edx
  mov    $0xff,%r15d
  cmp    %r15d,%edx
  cmovg  %r15d,%edx
  test   %edx,%edx
  mov    $0x0,%r15d
  cmovs  %r15d,%edx
  mov    %dl,-0x1(%r8)
  imul   $0x58,%esi,%esi
  sar    $0x8,%esi
  sub    %esi,%eax
  imul   $0xb7,%ecx,%ecx
  sar    $0x8,%ecx
  sub    %ecx,%eax
  mov    $0xff,%edx
  cmp    %edx,%eax
  cmovg  %edx,%eax
  test   %eax,%eax
  mov    $0x0,%edx
  cmovs  %edx,%eax
  mov    %al,-0x2(%r8)
  sub    $0x3,%r8
  mov    %rdi,%rax
  add    $0x1,%rdi
  cmp    %rbp,%rax
  jne    L0
  add    -0x10(%rsp),%r13
L31:
  add    $0x1,%r12d
  add    $0x140,%r14d
  cmp    %r12d,-0x14(%rsp)
  je     L44
L54:
  cmpl   $0x0,-0x18(%rsp)
  jg     L21
  jmp    L31
L44:
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
```

assistant: ```c
void cvt_420p_to_rgb(int width, int height, const unsigned char *src, unsigned char *dst) {
  int r, g, b;
  int rdif, gdif, bdif, y;
  int yy, uu, vv;
  int xoff, yoff;
  int numpix = width * height;
  unsigned char *pout = dst + width * height * 3;
  for (yoff = 0; yoff < height; yoff++) {
    for (xoff = 0; xoff < width; xoff++) {
      yy = *(src + yoff * 320 + xoff);
      uu = *(src + (yoff / 2) * 320 / 2 + xoff / 2 + numpix);
      vv = *(src + (yoff / 2) * 320 / 2 + xoff / 2 + numpix + numpix / 4);
      uu -= 128;
      vv -= 128;
      r = yy + vv + ((vv * 103) >> 8);
      g = yy - ((uu * 88) >> 8) - ((vv * 183) >> 8);
      b = yy + uu + ((uu * 198) >> 8);
      if (r > 255) {
        r = 255;
      }
      if (g > 255) {
        g = 255;
      }
      if (b > 255) {
        b = 255;
      }
      if (r < 0) {
        r = 0;
      }
      if (g < 0) {
        g = 0;
      }
      if (b < 0) {
        b = 0;
      }
      *pout = (unsigned char)b;
      pout--;
      *pout = (unsigned char)r;
      pout--;
      *pout = (unsigned char)g;
      pout--;
    }
  }
}
```