user: 请从给定的汇编指令推导出C源代码：

```asm
<cftfsub>:
  endbr64
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  mov    %rsi,%rbx
  sub    $0x8,%rsp
  cmp    $0x8,%edi
  jg     L45
  mov    $0x2,%ebp
  je     L6
L62:
  movslq %ebp,%rax
  xor    %ecx,%ecx
  lea    (%rbx,%rax,8),%rdx
  lea    0x8(%rbx),%rax
  xchg   %ax,%ax
L57:
  movsd  -0x8(%rax),%xmm0
  movsd  (%rdx),%xmm2
  add    $0x2,%ecx
  add    $0x10,%rdx
  movsd  (%rax),%xmm1
  add    $0x10,%rax
  movapd %xmm0,%xmm4
  subsd  %xmm2,%xmm0
  addsd  %xmm2,%xmm4
  movapd %xmm1,%xmm3
  subsd  -0x8(%rdx),%xmm3
  movsd  %xmm4,-0x18(%rax)
  addsd  -0x8(%rdx),%xmm1
  movsd  %xmm1,-0x10(%rax)
  movsd  %xmm0,-0x10(%rdx)
  movsd  %xmm3,-0x8(%rdx)
  cmp    %ebp,%ecx
  jl     L57
L1:
  add    $0x8,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  ret
  nopl   (%rax)
L45:
  mov    %edi,%r12d
  mov    %rdx,%r13
  call   <cft1st@plt>
  cmp    $0x20,%r12d
  jle    L82
  mov    $0x8,%esi
  mov    $0x20,%ebp
  jmp    L107
  nopl   0x0(%rax)
L108:
  mov    %eax,%ebp
L107:
  mov    %r13,%rcx
  mov    %rbx,%rdx
  mov    %r12d,%edi
  call   <cftmdl@plt>
  lea    0x0(,%rbp,4),%eax
  mov    %ebp,%esi
  cmp    %r12d,%eax
  jl     L108
  jne    L121
  test   %ebp,%ebp
  jle    L1
L6:
  movslq %ebp,%rdx
  xor    %eax,%eax
  shl    $0x3,%rdx
  lea    (%rbx,%rdx,1),%rsi
  lea    (%rsi,%rdx,1),%rcx
  add    %rcx,%rdx
  nopl   0x0(%rax)
L16:
  movsd  (%rsi),%xmm5
  movsd  (%rbx,%rax,8),%xmm2
  add    $0x10,%rsi
  add    $0x10,%rcx
  movsd  0x8(%rbx,%rax,8),%xmm0
  movsd  -0x8(%rsi),%xmm1
  add    $0x10,%rdx
  movapd %xmm2,%xmm4
  subsd  %xmm5,%xmm2
  movsd  -0x10(%rdx),%xmm8
  movsd  -0x8(%rdx),%xmm7
  movapd %xmm0,%xmm3
  subsd  %xmm1,%xmm0
  addsd  %xmm5,%xmm4
  addsd  %xmm1,%xmm3
  movsd  -0x10(%rcx),%xmm1
  movapd %xmm2,%xmm9
  movsd  -0x8(%rcx),%xmm2
  movapd %xmm1,%xmm6
  subsd  %xmm8,%xmm1
  addsd  %xmm8,%xmm6
  movapd %xmm2,%xmm5
  subsd  %xmm7,%xmm2
  addsd  %xmm7,%xmm5
  movapd %xmm4,%xmm7
  addsd  %xmm6,%xmm7
  subsd  %xmm6,%xmm4
  movsd  %xmm7,(%rbx,%rax,8)
  movapd %xmm3,%xmm7
  subsd  %xmm5,%xmm3
  addsd  %xmm5,%xmm7
  movsd  %xmm7,0x8(%rbx,%rax,8)
  add    $0x2,%rax
  movsd  %xmm3,-0x8(%rcx)
  movapd %xmm9,%xmm3
  subsd  %xmm2,%xmm3
  movsd  %xmm4,-0x10(%rcx)
  addsd  %xmm9,%xmm2
  movsd  %xmm3,-0x10(%rsi)
  movapd %xmm0,%xmm3
  subsd  %xmm1,%xmm0
  addsd  %xmm1,%xmm3
  movsd  %xmm3,-0x8(%rsi)
  movsd  %xmm2,-0x10(%rdx)
  movsd  %xmm0,-0x8(%rdx)
  cmp    %eax,%ebp
  jg     L16
  add    $0x8,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  ret
  xchg   %ax,%ax
L121:
  test   %ebp,%ebp
  jg     L62
  jmp    L1
L82:
  mov    $0x8,%ebp
  jne    L62
  jmp    L6
```

assistant: ```c
void cftfsub(int n, double *a, double *w) {
  int j, j1, j2, j3, l;
  double x0r, x0i, x1r, x1i, x2r, x2i, x3r, x3i;
  l = 2;
  if (n > 8) {
    cft1st(n, a, w);
    l = 8;
    while ((l << 2) < n) {
      cftmdl(n, l, a, w);
      l <<= 2;
    }
  }
  if ((l << 2) == n) {
    for (j = 0; j < l; j += 2) {
      j1 = j + l;
      j2 = j1 + l;
      j3 = j2 + l;
      x0r = a[j] + a[j1];
      x0i = a[j + 1] + a[j1 + 1];
      x1r = a[j] - a[j1];
      x1i = a[j + 1] - a[j1 + 1];
      x2r = a[j2] + a[j3];
      x2i = a[j2 + 1] + a[j3 + 1];
      x3r = a[j2] - a[j3];
      x3i = a[j2 + 1] - a[j3 + 1];
      a[j] = x0r + x2r;
      a[j + 1] = x0i + x2i;
      a[j2] = x0r - x2r;
      a[j2 + 1] = x0i - x2i;
      a[j1] = x1r - x3i;
      a[j1 + 1] = x1i + x3r;
      a[j3] = x1r + x3i;
      a[j3 + 1] = x1i - x3r;
    }
  } else {
    for (j = 0; j < l; j += 2) {
      j1 = j + l;
      x0r = a[j] - a[j1];
      x0i = a[j + 1] - a[j1 + 1];
      a[j] += a[j1];
      a[j + 1] += a[j1 + 1];
      a[j1] = x0r;
      a[j1 + 1] = x0i;
    }
  }
}
```