user: 请从给定的汇编指令推导出C源代码：

```asm
<str_decode>:
  endbr64
  push   %r15
  mov    %rsi,%r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x138,%rsp
  mov    %rdi,0x8(%rsp)
  mov    %rsi,%rdi
  mov    %fs:0x28,%rax
  mov    %rax,0x128(%rsp)
  xor    %eax,%eax
  call   <ignorestr@plt>
  test   %rax,%rax
  jne    L321
  lea    0x20(%rsp),%rdi
  mov    $0x100,%edx
  mov    %r15,%rsi
  xor    %eax,%eax
  mov    %rdi,0x10(%rsp)
  mov    %r15,%rbx
  call   <memcpy@plt>
  movzbl (%r15),%r13d
  mov    0x8(%rsp),%r12
  mov    %r13d,%eax
  test   %r13d,%r13d
  je     L13
  movl   $0x0,0x18(%rsp)
  xor    %r14d,%r14d
  movl   $0x0,0x4(%rsp)
  jmp    L442
  nopw   0x0(%rax,%rax,1)
L502:
  cmp    $0xa,%r13d
  je     L20
  test   %r14d,%r14d
  je     L249
  cmp    $0x3d,%r13d
  jne    L249
  lea    0x1(%r15),%rax
  movzbl 0x1(%r15),%edi
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jg     L474
  lea    0x2(%r15),%r13
  movzbl 0x2(%r15),%r15d
  add    $0x1,%r12
  call   <qp_code@plt>
  mov    %eax,%ebp
  mov    %r15d,%edi
  call   <qp_code@plt>
  shl    $0x4,%ebp
  or     %eax,%ebp
  mov    %bpl,-0x1(%r12)
L117:
  lea    0x1(%r13),%r15
  movzbl 0x1(%r13),%r13d
  mov    %r13d,%eax
  test   %r13d,%r13d
  je     L13
  mov    %r15,%rdx
  sub    %rbx,%rdx
  cmp    $0xfe,%rdx
  jg     L474
L442:
  cmp    $0x3f,%r13d
  jne    L502
  cmpb   $0x3d,0x1(%r15)
  je     L358
  mov    0x4(%rsp),%ecx
  test   %ecx,%ecx
  je     L238
  mov    0x18(%rsp),%edx
  test   %edx,%edx
  je     L521
L238:
  mov    %al,(%r12)
  mov    %r15,%r13
  add    $0x1,%r12
  jmp    L117
  nopl   (%rax)
L16:
  mov    %r15,%rax
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jg     L474
  movzbl 0x1(%r15),%r13d
  add    $0x1,%r15
L521:
  mov    %r13d,%edi
  call   <isspace@plt>
  test   %rax,%rax
  jne    L16
  test   %r13d,%r13d
  jne    L168
  jmp    L13
  cs nopw 0x0(%rax,%rax,1)
L233:
  movzbl 0x1(%r15),%ebp
  add    $0x1,%r15
  mov    %ebp,%edi
  call   <isspace@plt>
  test   %rax,%rax
  je     L299
L168:
  mov    %r15,%rax
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jle    L233
L474:
  mov    0x10(%rsp),%rsi
  mov    0x8(%rsp),%rdi
  mov    $0x100,%edx
  xor    %eax,%eax
  call   <memcpy@plt>
L196:
  mov    0x128(%rsp),%rax
  sub    %fs:0x28,%rax
  jne    L153
  add    $0x138,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  cs nopw 0x0(%rax,%rax,1)
L20:
  movl   $0x0,0x18(%rsp)
  lea    0x1(%r15),%r13
  xor    %r14d,%r14d
  movl   $0x0,0x4(%rsp)
  jmp    L117
  nopl   0x0(%rax)
L249:
  mov    0x4(%rsp),%edi
  test   %edi,%edi
  je     L341
  mov    0x18(%rsp),%esi
  test   %esi,%esi
  je     L521
L341:
  cmp    $0x3d,%r13d
  jne    L238
  cmpb   $0x3f,0x1(%r15)
  jne    L238
  mov    %r15,%rax
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jg     L474
  xor    %ecx,%ecx
  xor    %edx,%edx
  jmp    L235
  nopl   0x0(%rax)
L199:
  or     $0x20,%eax
  cmp    $0x2,%edx
  movzbl %al,%eax
  cmove  %eax,%ecx
L455:
  mov    %r15,%rax
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jg     L474
L235:
  movzbl 0x1(%r15),%eax
  add    $0x1,%r15
  cmp    $0x3f,%al
  jne    L199
  add    $0x1,%edx
  cmp    $0x2,%edx
  jle    L455
  mov    %r15,%r13
  cmp    $0x71,%ecx
  je     L71
  cmp    $0x62,%ecx
  mov    $0x1,%eax
  cmovne 0x4(%rsp),%eax
  mov    %eax,0x4(%rsp)
  jmp    L117
  nopl   (%rax)
L321:
  mov    0x128(%rsp),%rax
  sub    %fs:0x28,%rax
  jne    L153
  mov    0x8(%rsp),%rdi
  add    $0x138,%rsp
  mov    %r15,%rsi
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  jmp    10c0 <strcpy@plt>
  nopl   (%rax)
L358:
  lea    0x1(%r15),%r13
  jmp    L117
  nopl   0x0(%rax)
L299:
  test   %ebp,%ebp
  jne    L178
  jmp    L13
  xchg   %ax,%ax
L291:
  movzbl 0x1(%r15),%r9d
  add    $0x1,%r15
  mov    %r9d,%edi
  mov    %r9d,0x18(%rsp)
  call   <isspace@plt>
  mov    0x18(%rsp),%r9d
  test   %rax,%rax
  je     L516
L178:
  mov    %r15,%rax
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jle    L291
  jmp    L474
  nopl   0x0(%rax,%rax,1)
L13:
  movb   $0x0,(%r12)
  jmp    L196
  nopw   0x0(%rax,%rax,1)
L516:
  test   %r9d,%r9d
  jne    L174
  jmp    L13
  nop
L87:
  movzbl 0x1(%r15),%r10d
  add    $0x1,%r15
  mov    %r10d,%edi
  mov    %r10d,0x1c(%rsp)
  call   <isspace@plt>
  mov    0x1c(%rsp),%r10d
  mov    0x18(%rsp),%r9d
  test   %rax,%rax
  je     L132
L174:
  mov    %r15,%rax
  mov    %r9d,0x18(%rsp)
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jle    L87
  jmp    L474
L132:
  test   %r10d,%r10d
  mov    %r9d,0x1c(%rsp)
  mov    %r10d,0x18(%rsp)
  je     L13
  cmp    $0x3d,%r13d
  je     L312
  cmp    $0x3d,%ebp
  je     L312
  mov    %ebp,%edi
  call   <base64_code@plt>
  mov    %r13d,%edi
  mov    %eax,%ebp
  call   <base64_code@plt>
  mov    0x1c(%rsp),%r9d
  mov    0x18(%rsp),%r10d
  mov    %eax,%r8d
  mov    %ebp,%eax
  sar    $0x4,%eax
  lea    0x0(,%r8,4),%edx
  and    $0x3,%eax
  or     %edx,%eax
  cmp    $0x3d,%r9d
  mov    %al,(%r12)
  jne    L43
  movl   $0x1,0x18(%rsp)
  add    $0x1,%r12
  mov    %r15,%r13
  jmp    L117
L312:
  movl   $0x1,0x18(%rsp)
  mov    %r15,%r13
  jmp    L117
L43:
  mov    %r9d,%edi
  mov    %r10d,0x18(%rsp)
  shl    $0x4,%ebp
  call   <base64_code@plt>
  mov    0x18(%rsp),%r10d
  mov    %eax,%r13d
  sar    $0x2,%eax
  and    $0xf,%eax
  or     %ebp,%eax
  mov    %al,0x1(%r12)
  cmp    $0x3d,%r10d
  jne    L311
  movl   $0x1,0x18(%rsp)
  add    $0x2,%r12
  mov    %r15,%r13
  jmp    L117
L71:
  mov    $0x1,%r14d
  jmp    L117
L311:
  mov    %r10d,%edi
  add    $0x3,%r12
  call   <base64_code@plt>
  mov    %r13d,%edx
  movl   $0x0,0x18(%rsp)
  mov    %r15,%r13
  shl    $0x6,%edx
  or     %eax,%edx
  mov    %dl,-0x1(%r12)
  jmp    L117
L153:
  call   <__stack_chk_fail@plt>
```

assistant: ```c
void str_decode(unsigned char *dst, unsigned char *src) {
  register int is_qp, is_base64, is_done;
  register int c1, c2, c3, c4;
  char *dsttmp;
  char *srctmp;
  char srcbuf[256];
  long pos;
  if (ignorestr((char *)src)) {
    strcpy((char *)dst, (char *)src);
    return;
  }
  memcpy(srcbuf, src, 256);
  dsttmp = (char *)dst;
  srctmp = (char *)src;
  pos = (long)src - (long)srctmp;
  for (is_done = is_qp = is_base64 = 0; (c1 = *src) != 0; src++) {
    pos = (long)src - (long)srctmp;
    if (pos > 254) {
      memcpy(dsttmp, srcbuf, 256);
      return;
    }
    if (c1 == '?' && src[1] == '=') {
      src++;
      continue;
    } else if (c1 == '\n') {
      src++;
      is_done = is_qp = is_base64 = 0;
      continue;
    } else if (is_qp && c1 == '=') {
      pos = (long)src - (long)srctmp;
      if (pos > 254) {
        memcpy(dsttmp, srcbuf, 256);
        return;
      }
      c1 = *++src;
      pos = (long)src - (long)srctmp;
      if (pos > 254) {
        memcpy(dsttmp, srcbuf, 256);
        return;
      }
      c2 = *++src;
      *dst++ = (qp_code(c1) << 4) | qp_code(c2);
    } else if (is_base64 && !is_done) {
      while (isspace(c1)) {
        pos = (long)src - (long)srctmp;
        if (pos > 254) {
          memcpy(dsttmp, srcbuf, 256);
          return;
        }
        c1 = *++src;
      }
      if (!c1)
        break;
      do {
        pos = (long)src - (long)srctmp;
        if (pos > 254) {
          memcpy(dsttmp, srcbuf, 256);
          return;
        }
        c2 = *++src;
      } while (isspace(c2));
      if (!c2)
        break;
      do {
        pos = (long)src - (long)srctmp;
        if (pos > 254) {
          memcpy(dsttmp, srcbuf, 256);
          return;
        }
        c3 = *++src;
      } while (isspace(c3));
      if (!c3)
        break;
      do {
        pos = (long)src - (long)srctmp;
        if (pos > 254) {
          memcpy(dsttmp, srcbuf, 256);
          return;
        }
        c4 = *++src;
      } while (isspace(c4));
      if (!c4)
        break;
      if (c1 == '=' || c2 == '=') {
        is_done = 1;
        continue;
      }
      c2 = base64_code(c2);
      *dst++ = (base64_code(c1) << 2) | ((c2 & 0x30) >> 4);
      if (c3 == '=')
        is_done = 1;
      else {
        c3 = base64_code(c3);
        *dst++ = ((c2 & 0xF) << 4) | ((c3 & 0x3c) >> 2);
        if (c4 == '=')
          is_done = 1;
        else {
          *dst++ = ((c3 & 0x03) << 6) | base64_code(c4);
        }
      }
    } else if ((c1 == '=') && (src[1] == '?')) {
      c2 = c3 = 0;
      for (;;) {
        pos = (long)src - (long)srctmp;
        if (pos > 254) {
          memcpy(dsttmp, srcbuf, 256);
          return;
        }
        c1 = *++src;
        if (c1 != '?') {
          if (c2 == 2)
            c3 = c1 | 0x20;
        } else {
          if (++c2 >= 3)
            break;
        }
      }
      if (c3 == 'q')
        is_qp = 1;
      else if (c3 == 'b')
        is_base64 = 1;
    } else
      *dst++ = c1;
  }
  *dst = '\0';
}
```