user: Demonstrate the C code that generates the assembly output below:

```asm
<dstsub>:
  endbr64
  push   %r15
  mov    %edx,%eax
  mov    %rsi,%r8
  mov    %edx,%esi
  push   %r14
  cltd
  idiv   %edi
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  mov    %edi,%ebx
  sar    %ebx
  mov    %ebx,-0x4c(%rsp)
  cmp    $0x3,%edi
  jle    L65
  movslq %eax,%r11
  movslq %esi,%rdx
  mov    %eax,%ebp
  xor    %r14d,%r14d
  sub    %r11,%rdx
  neg    %ebp
  mov    %r11,-0x48(%rsp)
  lea    -0x1(%rbx),%r11d
  lea    0x0(,%rdx,8),%r15
  movslq %ebp,%rdx
  lea    -0x1(%r11),%r10
  mov    %r11d,-0x18(%rsp)
  lea    0x0(,%rdx,8),%rbp
  movslq %edi,%r12
  mov    %rbp,-0x10(%rsp)
  imul   %r10,%rbp
  test   %eax,%eax
  mov    %rbp,%rbx
  mov    %r14,%rbp
  cmovle %rbx,%rbp
  neg    %r11
  shl    $0x3,%r12
  shl    $0x3,%r11
  mov    %rbp,%rdx
  lea    (%r11,%r12,1),%r13
  add    %r15,%rdx
  test   %eax,%eax
  lea    (%r8,%r13,1),%r9
  lea    (%rcx,%rdx,1),%rbp
  cmovle %r14,%rbx
  lea    0x8(%r8),%rdx
  mov    %r9,-0x40(%rsp)
  mov    %rbp,-0x38(%rsp)
  mov    %r8,%rbp
  lea    -0x8(%r8,%r12,1),%r9
  sub    %r11,%rbp
  mov    -0x48(%rsp),%r11
  add    %r15,%rbx
  mov    %r9,-0x28(%rsp)
  add    %rcx,%rbx
  mov    %rbp,-0x20(%rsp)
  mov    %r14,%rbp
  shl    $0x3,%r11
  imul   %r11,%r10
  test   %eax,%eax
  cmovns %r10,%rbp
  add    %r11,%rbp
  add    %rcx,%rbp
  test   %eax,%eax
  cmovns %r14,%r10
  lea    (%r8,%r12,1),%r14
  mov    %r14,-0x30(%rsp)
  mov    -0x4c(%rsp),%r14d
  add    %r11,%r10
  shl    $0x3,%r14
  add    %rcx,%r10
  cmp    %r13,%r14
  setle  %r13b
  cmp    -0x30(%rsp),%rdx
  setae  %r14b
  or     %r13d,%r14d
  cmpl   $0x2,-0x4c(%rsp)
  setne  %r13b
  and    %r13d,%r14d
  cmp    %r9,%rbx
  mov    -0x40(%rsp),%r9
  seta   %r13b
  cmp    -0x38(%rsp),%r9
  seta   -0x11(%rsp)
  movzbl -0x11(%rsp),%r9d
  or     %r9d,%r13d
  mov    -0x20(%rsp),%r9
  and    %r14d,%r13d
  cmp    %rdx,-0x38(%rsp)
  setb   %r14b
  cmp    %r9,%rbx
  seta   %bl
  or     %ebx,%r14d
  and    %r13d,%r14d
  cmp    %rbp,-0x40(%rsp)
  seta   %bl
  cmp    %r10,-0x28(%rsp)
  setb   %r13b
  or     %r13d,%ebx
  test   %bl,%r14b
  je     L29
  cmp    %rbp,%rdx
  seta   %bl
  cmp    %r10,%r9
  setb   %r10b
  or     %bl,%r10b
  je     L29
  mov    -0x18(%rsp),%r14d
  lea    (%rcx,%r15,1),%r10
  mov    %r11,%r13
  lea    (%rcx,%r11,1),%rbx
  lea    -0x10(%r8,%r12,1),%r12
  mov    -0x48(%rsp),%r9
  neg    %r13
  shr    %r14d
  lea    -0x10(%r12),%rbp
  lea    -0x1(%r14),%r15d
  mov    %rbp,%r14
  shl    $0x4,%r9
  shl    $0x4,%r15
  sub    %r15,%r14
  jmp    L16
  nopl   0x0(%rax,%rax,1)
L13:
  sub    $0x10,%rbp
L16:
  movsd  (%rbx),%xmm0
  movsd  (%r10),%xmm1
  add    $0x10,%rdx
  movupd -0x10(%rdx),%xmm4
  movupd (%r12),%xmm2
  movhpd (%rbx,%r11,1),%xmm0
  movhpd (%r10,%r13,1),%xmm1
  add    %r9,%rbx
  sub    %r9,%r10
  movapd %xmm0,%xmm3
  addpd  %xmm1,%xmm0
  shufpd $0x1,%xmm2,%xmm2
  subpd  %xmm1,%xmm3
  movapd %xmm0,%xmm5
  mulpd  %xmm4,%xmm5
  movapd %xmm3,%xmm1
  mulpd  %xmm2,%xmm1
  mulpd  %xmm2,%xmm0
  mulpd  %xmm4,%xmm3
  addpd  %xmm5,%xmm1
  subpd  %xmm3,%xmm0
  shufpd $0x1,%xmm1,%xmm1
  movups %xmm1,(%r12)
  mov    %rbp,%r12
  movups %xmm0,-0x10(%rdx)
  cmp    %r14,%rbp
  jne    L13
  mov    -0x18(%rsp),%ebx
  mov    %ebx,%edx
  or     $0x1,%edx
  and    $0x1,%ebx
  je     L65
  imul   %edx,%eax
  sub    %edx,%edi
  movslq %edx,%rdx
  movslq %edi,%rdi
  movslq %eax,%r9
  sub    %eax,%esi
  lea    (%r8,%rdx,8),%rax
  movsd  (%rcx,%r9,8),%xmm0
  movslq %esi,%rsi
  movsd  (%rax),%xmm3
  movsd  (%rcx,%rsi,8),%xmm2
  lea    (%r8,%rdi,8),%rsi
  movapd %xmm0,%xmm1
  movsd  (%rsi),%xmm4
  addsd  %xmm2,%xmm0
  subsd  %xmm2,%xmm1
  movapd %xmm0,%xmm5
  movapd %xmm1,%xmm2
  mulsd  %xmm4,%xmm2
  mulsd  %xmm3,%xmm5
  mulsd  %xmm4,%xmm0
  mulsd  %xmm3,%xmm1
  addsd  %xmm5,%xmm2
  subsd  %xmm1,%xmm0
  movsd  %xmm2,(%rsi)
  movsd  %xmm0,(%rax)
L65:
  movslq -0x4c(%rsp),%rax
  lea    (%r8,%rax,8),%rax
  movsd  (%rax),%xmm0
  mulsd  (%rcx),%xmm0
  movsd  %xmm0,(%rax)
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  nopl   (%rax)
L29:
  mov    -0x4c(%rsp),%ebx
  mov    -0x30(%rsp),%rax
  lea    (%rcx,%r11,1),%rdi
  lea    (%rcx,%r15,1),%rsi
  mov    -0x10(%rsp),%r10
  lea    -0x2(%rbx),%r9d
  lea    0x10(%r8,%r9,8),%r9
  nop
L5:
  movsd  (%rdi),%xmm0
  movsd  (%rsi),%xmm2
  add    $0x8,%rdx
  add    %r11,%rdi
  movsd  -0x8(%rax),%xmm4
  movsd  -0x8(%rdx),%xmm3
  add    %r10,%rsi
  sub    $0x8,%rax
  movapd %xmm0,%xmm1
  addsd  %xmm2,%xmm0
  subsd  %xmm2,%xmm1
  movapd %xmm0,%xmm5
  mulsd  %xmm3,%xmm5
  movapd %xmm1,%xmm2
  mulsd  %xmm4,%xmm2
  mulsd  %xmm4,%xmm0
  mulsd  %xmm3,%xmm1
  addsd  %xmm5,%xmm2
  subsd  %xmm1,%xmm0
  movsd  %xmm2,(%rax)
  movsd  %xmm0,-0x8(%rdx)
  cmp    %rdx,%r9
  jne    L5
  jmp    L65
```

assistant: ```c
void dstsub(int n, double *a, int nc, double *c) {
  int j, k, kk, ks, m;
  double wkr, wki, xr;
  m = n >> 1;
  ks = nc / n;
  kk = 0;
  for (j = 1; j < m; j++) {
    k = n - j;
    kk += ks;
    wkr = c[kk] - c[nc - kk];
    wki = c[kk] + c[nc - kk];
    xr = wki * a[k] - wkr * a[j];
    a[k] = wkr * a[k] + wki * a[j];
    a[j] = xr;
  }
  a[m] *= c[0];
}
```