user: 下面的汇编指令对应的C源代码是什么:

```asm
<libc_ldtos>:
  endbr64
  push   %r15
  mov    $0x20,%edi
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x18,%rsp
  call   <malloc@plt>
  fldt   0x50(%rsp)
  fldz
  mov    %rax,%r12
  fcomip %st(1),%st
  fstp   %st(0)
  ja     L82
  movb   $0x0,(%rax)
  mov    %rax,%rdx
  mov    $0x12,%edi
  mov    $0x11,%ebx
  mov    $0x1,%r13d
  mov    $0x1,%esi
  xor    %r8d,%r8d
  xor    %ebp,%ebp
  mov    $0x2,%r15d
L30:
  fnstcw 0xe(%rsp)
  fldt   0x50(%rsp)
  movzwl 0xe(%rsp),%eax
  or     $0xc,%ah
  mov    %ax,0xc(%rsp)
  fldcw  0xc(%rsp)
  fistpll (%rsp)
  fldcw  0xe(%rsp)
  mov    (%rsp),%rax
  test   %rax,%rax
  jne    L102
  movb   $0x30,(%rdx)
L80:
  fildll (%rsp)
  movb   $0x2e,(%r12,%r13,1)
  xor    %esi,%esi
  lea    (%r12,%r15,1),%rdi
  movabs $0x6666666666666667,%r8
  fldt   0x50(%rsp)
  fsubp  %st,%st(1)
  flds   0xe17(%rip)        # 2000 <_fini+0xc9c>
  jmp    L47
  nopl   0x0(%rax,%rax,1)
L2:
  fxch   %st(1)
L47:
  fmul   %st,%st(1)
  fxch   %st(1)
  fld    %st(0)
  fldcw  0xc(%rsp)
  fistpll (%rsp)
  fldcw  0xe(%rsp)
  mov    (%rsp),%rcx
  mov    %rcx,%rax
  imul   %r8
  mov    %rcx,%rax
  sar    $0x3f,%rax
  sar    $0x2,%rdx
  sub    %rax,%rdx
  lea    (%rdx,%rdx,4),%rax
  add    %rax,%rax
  sub    %rax,%rcx
  add    $0x30,%ecx
  mov    %cl,(%rdi,%rsi,1)
  add    $0x1,%rsi
  cmp    %rsi,%rbx
  jg     L2
  fstp   %st(0)
  fstp   %st(0)
  lea    -0x1(%rbx),%rax
  xor    %edx,%edx
  test   %rbx,%rbx
  cmovle %rdx,%rax
  movb   $0x0,(%rdi,%rax,1)
  add    $0x18,%rsp
  mov    %r12,%rax
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  nopl   0x0(%rax,%rax,1)
L102:
  jle    L4
  movabs $0xcccccccccccccccd,%r9
  mov    %rax,%rdi
  mov    %rbp,%rsi
  cs nopw 0x0(%rax,%rax,1)
L68:
  mov    %rdi,%rax
  mov    %rsi,%r14
  lea    0x1(%rsi),%rsi
  mul    %r9
  mov    %rsi,%r13
  shr    $0x3,%rdx
  lea    (%rdx,%rdx,4),%r8
  mov    %rdx,%rax
  mov    %rdi,%rdx
  add    %r8,%r8
  sub    %r8,%rdx
  mov    %rdi,%r8
  mov    %rax,%rdi
  add    $0x30,%edx
  mov    %dl,-0x1(%r12,%rsi,1)
  cmp    $0x9,%r8
  jg     L68
  lea    0x2(%r14),%r15
  mov    $0x13,%ebx
  mov    $0x7,%eax
  sub    %r15,%rbx
  cmp    %rax,%rbx
  cmovl  %rax,%rbx
  cmp    %r14,%rbp
  jge    L80
  nopl   0x0(%rax)
L89:
  movsbl (%r12,%r14,1),%esi
  movsbl (%r12,%rbp,1),%edi
  sub    $0x1,%r14
  add    $0x1,%rbp
  call   <swap@plt>
  cmp    %r14,%rbp
  jl     L89
  fnstcw 0xe(%rsp)
  movzwl 0xe(%rsp),%eax
  or     $0xc,%ah
  mov    %ax,0xc(%rsp)
  jmp    L80
  nopw   0x0(%rax,%rax,1)
L82:
  fldt   0x50(%rsp)
  movb   $0x2d,(%rax)
  lea    0x1(%rax),%rdx
  mov    $0x11,%edi
  mov    $0x10,%ebx
  mov    $0x2,%r13d
  mov    $0x2,%esi
  mov    $0x3,%r15d
  mov    $0x1,%r8d
  mov    $0x1,%ebp
  fchs
  fstpt  0x50(%rsp)
  jmp    L30
L4:
  mov    %r8,%r13
  mov    %rdi,%rbx
  mov    %rsi,%r15
  jmp    L80
```

assistant: ```c
char *libc_ldtos(long double n) {
  char *buff = malloc(32);
  *buff = +0;
  char *a2 = buff;
  long long i = 0, j = 0, k = 0;
  long long x = 0, y = 0, t = 0, z = 0, u = 0;
  if (n < 0) {
    a2[j++] = '-';
    n = -n;
  }
  y = x = n;
  if (x == 0) {
    a2[j++] = '0';
  }
  k = j;
  while (x > 0) {
    t = x % 10;
    x /= 10;
    a2[k++] = t + '0';
  }
  i = k--;
  while (k > j) {
    swap(a2[j], a2[k]);
    j++;
    k--;
  }
  j = i;
  n = n - y;
  a2[j++] = '.';
  u = (19 - j);
  if (u < 7) {
    u = 7;
  }
  z = 0;
  while (z < (u)) {
    n = n * 10;
    t = (long long)n % 10;
    a2[j++] = t + '0';
    z++;
  }
  a2[j - 1] = '\0';
  return a2;
}
```