user: 请从给定的汇编指令推导出C源代码：

```asm
<kernel_dtrmv_u_n_2_lib4>:
  endbr64
  test   %edi,%edi
  jle    L18
  push   %rbx
  mov    %rsi,%rax
  mov    %rcx,%r10
  prefetcht0 (%rsi)
  lea    0x40(%rsi),%r9
  prefetcht0 (%r9)
  prefetcht0 0x80(%rsi)
  prefetcht0 0xc0(%rsi)
  movsd  0x8(%rdx),%xmm6
  movsd  (%rsi),%xmm0
  mulsd  (%rdx),%xmm0
  pxor   %xmm1,%xmm1
  addsd  %xmm1,%xmm0
  movapd %xmm6,%xmm3
  mulsd  0x20(%rsi),%xmm3
  addsd  %xmm0,%xmm3
  mulsd  0x28(%rsi),%xmm6
  addsd  %xmm1,%xmm6
  lea    0x10(%rdx),%r11
  cmp    $0x5,%edi
  jle    L63
  lea    -0x6(%rdi),%ebx
  shr    $0x2,%ebx
  mov    %ebx,%esi
  mov    %rsi,%rdx
  shl    $0x7,%rdx
  lea    0xc0(%rax,%rdx,1),%rcx
  mov    %r11,%rdx
  mov    %r9,%rax
L72:
  sub    $0xffffffffffffff80,%rax
  prefetcht0 (%rax)
  prefetcht0 0x40(%rax)
  movsd  (%rdx),%xmm2
  movsd  0x8(%rdx),%xmm1
  movsd  0x10(%rdx),%xmm0
  movsd  0x18(%rdx),%xmm4
  movapd %xmm2,%xmm5
  mulsd  -0x80(%rax),%xmm5
  addsd  %xmm3,%xmm5
  mulsd  -0x78(%rax),%xmm2
  addsd  %xmm6,%xmm2
  movapd %xmm1,%xmm3
  mulsd  -0x60(%rax),%xmm3
  addsd  %xmm5,%xmm3
  mulsd  -0x58(%rax),%xmm1
  addsd  %xmm2,%xmm1
  movapd %xmm0,%xmm2
  mulsd  -0x40(%rax),%xmm2
  addsd  %xmm3,%xmm2
  mulsd  -0x38(%rax),%xmm0
  addsd  %xmm1,%xmm0
  movapd %xmm4,%xmm3
  mulsd  -0x20(%rax),%xmm3
  addsd  %xmm2,%xmm3
  mulsd  -0x18(%rax),%xmm4
  movapd %xmm4,%xmm6
  addsd  %xmm0,%xmm6
  add    $0x20,%rdx
  cmp    %rcx,%rax
  jne    L72
  lea    0x1(%rsi),%rcx
  mov    %rcx,%rax
  shl    $0x7,%rax
  add    %rax,%r9
  shl    $0x5,%rcx
  add    %rcx,%r11
  lea    0x6(,%rbx,4),%edx
L58:
  cmp    %edx,%edi
  jle    L48
  lea    -0x1(%rdi),%eax
  sub    %edx,%eax
  lea    0x8(,%rax,8),%rdx
  mov    $0x0,%eax
L64:
  movsd  (%r11,%rax,1),%xmm0
  movapd %xmm0,%xmm1
  mulsd  (%r9,%rax,4),%xmm1
  addsd  %xmm1,%xmm3
  mulsd  0x8(%r9,%rax,4),%xmm0
  addsd  %xmm0,%xmm6
  add    $0x8,%rax
  cmp    %rdx,%rax
  jne    L64
L48:
  test   %r8d,%r8d
  jne    L10
  movsd  %xmm3,(%r10)
  movsd  %xmm6,0x8(%r10)
L73:
  pop    %rbx
  ret
L63:
  mov    $0x2,%edx
  jmp    L58
L10:
  cmp    $0x1,%r8d
  je     L42
  movsd  (%r10),%xmm0
  subsd  %xmm3,%xmm0
  movsd  %xmm0,(%r10)
  movsd  0x8(%r10),%xmm0
  subsd  %xmm6,%xmm0
  movsd  %xmm0,0x8(%r10)
  jmp    L73
L42:
  addsd  (%r10),%xmm3
  movsd  %xmm3,(%r10)
  addsd  0x8(%r10),%xmm6
  movsd  %xmm6,0x8(%r10)
  jmp    L73
L18:
  ret
```

assistant: ```c
void kernel_dtrmv_u_n_2_lib4(int kmax, double *A, double *x, double *y, int alg) {
  if (kmax <= 0)
    return;
  __builtin_prefetch(A);
  __builtin_prefetch(A + 8);
  const int lda = 4;
  int k;
  double x_0, x_1, x_2, x_3, y_0 = 0, y_1 = 0;
  __builtin_prefetch(A + 4 * lda);
  __builtin_prefetch(A + 4 * lda + 8);
  x_0 = x[0];
  x_1 = x[1];
  y_0 += A[0 + lda * 0] * x_0;
  y_0 += A[0 + lda * 1] * x_1;
  y_1 += A[1 + lda * 1] * x_1;
  A += 2 * lda;
  x += 2;
  k = 2;
  for (; k < kmax - 3; k += 4) {
    __builtin_prefetch(A + 4 * lda);
    __builtin_prefetch(A + 4 * lda + 8);
    x_0 = x[0];
    x_1 = x[1];
    x_2 = x[2];
    x_3 = x[3];
    y_0 += A[0 + lda * 0] * x_0;
    y_1 += A[1 + lda * 0] * x_0;
    y_0 += A[0 + lda * 1] * x_1;
    y_1 += A[1 + lda * 1] * x_1;
    y_0 += A[0 + lda * 2] * x_2;
    y_1 += A[1 + lda * 2] * x_2;
    y_0 += A[0 + lda * 3] * x_3;
    y_1 += A[1 + lda * 3] * x_3;
    A += 4 * lda;
    x += 4;
  }
  for (; k < kmax; k++) {
    x_0 = x[0];
    y_0 += A[0 + lda * 0] * x_0;
    y_1 += A[1 + lda * 0] * x_0;
    A += 1 * lda;
    x += 1;
  }
  if (alg == 0) {
    y[0] = y_0;
    y[1] = y_1;
  } else if (alg == 1) {
    y[0] += y_0;
    y[1] += y_1;
  } else {
    y[0] -= y_0;
    y[1] -= y_1;
  }
}
```