user: 请从给定的汇编指令推导出C源代码：

```asm
<update>:
  endbr64
  lea    -0x1(%rdi),%ecx
  movaps %xmm0,%xmm2
  cmp    $0x2,%ecx
  jle    L110
  cmp    $0x1,%esi
  jne    L40
  lea    -0x4(%rdi),%eax
  cmp    $0x7,%eax
  jbe    L48
  shr    $0x3,%eax
  movaps %xmm0,%xmm4
  mov    %eax,%r9d
  lea    0x4(%rdx),%rax
  shufps $0x0,%xmm4,%xmm4
  lea    -0x1(%r9),%r8d
  shl    $0x5,%r8
  lea    0x24(%rdx,%r8,1),%r8
  cs nopw 0x0(%rax,%rax,1)
L8:
  movups (%rax),%xmm1
  movups 0x10(%rax),%xmm6
  add    $0x20,%rax
  movups -0x18(%rax),%xmm0
  movups -0x8(%rax),%xmm5
  movaps %xmm1,%xmm3
  shufps $0xdd,%xmm6,%xmm1
  shufps $0x88,%xmm5,%xmm0
  shufps $0x88,%xmm6,%xmm3
  addps  %xmm3,%xmm0
  mulps  %xmm4,%xmm0
  addps  %xmm1,%xmm0
  movaps %xmm0,%xmm1
  movss  %xmm0,-0x1c(%rax)
  shufps $0x55,%xmm0,%xmm1
  movss  %xmm1,-0x14(%rax)
  movaps %xmm0,%xmm1
  unpckhps %xmm0,%xmm1
  shufps $0xff,%xmm0,%xmm0
  movss  %xmm0,-0x4(%rax)
  movss  %xmm1,-0xc(%rax)
  cmp    %r8,%rax
  jne    L8
  lea    0x2(,%r9,8),%r8d
L54:
  movslq %r8d,%rax
  shl    $0x2,%rax
  lea    (%rdx,%rax,1),%r9
  movss  0x4(%rdx,%rax,1),%xmm0
  movss  -0x4(%r9),%xmm1
  addss  %xmm0,%xmm1
  mulss  %xmm2,%xmm1
  addss  (%r9),%xmm1
  movss  %xmm1,(%r9)
  lea    0x2(%r8),%r9d
  cmp    %r9d,%ecx
  jle    L84
  movss  0xc(%rdx,%rax,1),%xmm1
  lea    0x8(%rdx,%rax,1),%r9
  addss  %xmm1,%xmm0
  mulss  %xmm2,%xmm0
  addss  (%r9),%xmm0
  movss  %xmm0,(%r9)
  lea    0x4(%r8),%r9d
  cmp    %r9d,%ecx
  jle    L84
  movss  0x14(%rdx,%rax,1),%xmm0
  lea    0x10(%rdx,%rax,1),%r9
  add    $0x6,%r8d
  addss  %xmm0,%xmm1
  mulss  %xmm2,%xmm1
  addss  (%r9),%xmm1
  movss  %xmm1,(%r9)
  cmp    %r8d,%ecx
  jle    L84
  addss  0x1c(%rdx,%rax,1),%xmm0
  lea    0x18(%rdx,%rax,1),%r8
  mulss  %xmm2,%xmm0
  addss  (%r8),%xmm0
  movss  %xmm0,(%r8)
L84:
  mov    $0x1,%r9d
L32:
  imul   %esi,%ecx
  and    $0x1,%edi
  movss  (%rdx),%xmm1
  movss  (%rdx,%r9,4),%xmm0
  movslq %ecx,%rax
  lea    (%rdx,%rax,4),%rax
  jne    L10
  addss  (%rax),%xmm0
  mulss  %xmm2,%xmm0
  addss  %xmm1,%xmm0
  movss  %xmm0,(%rdx)
  ret
  nopl   0x0(%rax)
L10:
  cvtss2sd %xmm2,%xmm2
  addsd  %xmm2,%xmm2
  cvtss2sd %xmm0,%xmm0
  cvtss2sd %xmm1,%xmm1
  sub    %esi,%ecx
  movslq %ecx,%rcx
  mulsd  %xmm2,%xmm0
  addsd  %xmm1,%xmm0
  pxor   %xmm1,%xmm1
  cvtsd2ss %xmm0,%xmm0
  movss  %xmm0,(%rdx)
  pxor   %xmm0,%xmm0
  cvtss2sd (%rdx,%rcx,4),%xmm0
  mulsd  %xmm2,%xmm0
  cvtss2sd (%rax),%xmm1
  addsd  %xmm1,%xmm0
  cvtsd2ss %xmm0,%xmm0
  movss  %xmm0,(%rax)
  ret
  nop
L40:
  lea    (%rsi,%rsi,1),%r10d
  movslq %esi,%r9
  mov    $0x2,%r8d
  movslq %r10d,%r10
  mov    %r9,%r11
  shl    $0x2,%r10
  neg    %r11
  lea    (%rdx,%r10,1),%rax
  shl    $0x2,%r11
  nopw   0x0(%rax,%rax,1)
L46:
  movss  (%rax,%r11,1),%xmm0
  addss  (%rax,%r9,4),%xmm0
  add    $0x2,%r8d
  mulss  %xmm2,%xmm0
  addss  (%rax),%xmm0
  movss  %xmm0,(%rax)
  add    %r10,%rax
  cmp    %ecx,%r8d
  jl     L46
  jmp    L32
  nopl   0x0(%rax)
L110:
  movslq %esi,%r9
  jmp    L32
L48:
  mov    $0x2,%r8d
  jmp    L54
```

assistant: ```c
void update(int n, float a, int str, float *x) {
  for (int i = 2; i < n - 1; i += 2)
    x[i * str] += a * (x[(i - 1) * str] + x[(i + 1) * str]);
  if (0 == n % 2)
    x[0] += a * (x[(n - 1) * str] + x[1 * str]);
  else {
    x[0] += 2. * a * x[1 * str];
    x[(n - 1) * str] += 2. * a * x[(n - 2) * str];
  }
}
```