user: 这段汇编代码用C语言表示是什么样的？

```asm
<matrix_mult>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  movslq %edi,%rbx
  mov    %rbx,%rax
  imul   %rbx,%rax
  mov    %rcx,-0x30(%rsp)
  imul   %rbx,%rax
  add    %rax,%rax
  mov    %rax,-0x8(%rsp)
  test   %edi,%edi
  jle    L43
  lea    -0x1(%rdi),%eax
  mov    $0xfffffffffffffff8,%rbp
  mov    %rdx,%r8
  mov    %rdx,%r9
  mov    %rax,%rcx
  shl    $0x3,%rax
  mov    %rdx,%r11
  xor    %r12d,%r12d
  shl    $0x3,%rbx
  lea    0x8(%rdx,%rax,1),%r10
  sub    %rax,%rbp
  nopw   0x0(%rax,%rax,1)
L27:
  lea    0x0(%rbp,%r10,1),%rax
  mov    %r11,%rdx
  nopl   0x0(%rax,%rax,1)
L20:
  movsd  (%rdx),%xmm1
  movsd  (%rax),%xmm0
  add    $0x8,%rax
  movsd  %xmm1,-0x8(%rax)
  movsd  %xmm0,(%rdx)
  add    %rbx,%rdx
  cmp    %r10,%rax
  jne    L20
  add    $0x1,%r12d
  add    $0x8,%r11
  lea    (%rax,%rbx,1),%r10
  cmp    %r12d,%edi
  jne    L27
  lea    (%r12,%r12,1),%eax
  add    %rbx,%r8
  mov    %rsi,%rdx
  xor    %r14d,%r14d
  cltq
  xor    %r13d,%r13d
  mov    %r12d,%r10d
  mov    %rax,-0x28(%rsp)
  lea    0x0(,%rax,8),%r11
  mov    -0x30(%rsp),%rax
  add    %rbx,%rax
  mov    %rax,-0x18(%rsp)
  mov    %ecx,%eax
  shr    %eax
  add    %rax,%rax
  mov    %rax,-0x20(%rsp)
  lea    0x10(%rsi),%rax
  mov    %rax,-0x10(%rsp)
  nopw   0x0(%rax,%rax,1)
L24:
  mov    -0x20(%rsp),%rax
  mov    -0x10(%rsp),%rsi
  lea    (%rdx,%rbx,1),%r15
  mov    -0x30(%rsp),%rdi
  mov    %r15,%rcx
  add    %r14,%rax
  lea    (%rsi,%rax,8),%rbp
  mov    -0x18(%rsp),%rsi
  nopw   0x0(%rax,%rax,1)
L29:
  xor    %eax,%eax
  nopw   0x0(%rax,%rax,1)
L10:
  movsd  (%r9,%rax,8),%xmm1
  movsd  (%rdi,%rax,8),%xmm2
  movapd %xmm1,%xmm0
  mulsd  %xmm2,%xmm0
  addsd  (%rdx),%xmm0
  movsd  %xmm0,(%rdx)
  movsd  (%r8,%rax,8),%xmm0
  mulsd  %xmm0,%xmm2
  addsd  (%rcx),%xmm2
  movsd  %xmm2,(%rcx)
  movsd  (%rsi,%rax,8),%xmm2
  add    $0x1,%rax
  mulsd  %xmm2,%xmm1
  addsd  0x8(%rdx),%xmm1
  mulsd  %xmm2,%xmm0
  movsd  %xmm1,0x8(%rdx)
  addsd  0x8(%rcx),%xmm0
  movsd  %xmm0,0x8(%rcx)
  cmp    %rax,%r10
  jne    L10
  add    $0x10,%rdx
  add    %r11,%rdi
  add    $0x10,%rcx
  add    %r11,%rsi
  cmp    %rdx,%rbp
  jne    L29
  add    $0x2,%r13d
  add    -0x28(%rsp),%r14
  add    %r11,%r9
  add    %r11,%r8
  lea    (%r15,%rbx,1),%rdx
  cmp    %r12d,%r13d
  jl     L24
L43:
  mov    -0x8(%rsp),%rax
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
```

assistant: ```c
size_t matrix_mult(int len, double *restrict a, double *restrict b, double *restrict c) {
  int i, j, k, l, m;
  size_t fpops;
  double temp;
  fpops = (size_t)2 * len * len * len;
  for (i = 0; i < len; i++) {
    for (j = 0; j < len; j++) {
      temp = b[i * len + j];
      b[i * len + j] = b[j * len + i];
      b[j * len + i] = temp;
    }
  }
  for (i = 0; i < len; i += 2) {
    for (j = 0; j < len; j += 2) {
      for (k = 0; k < len; k++) {
        a[i * len + j] += b[i * len + k] * c[j * len + k];
        a[(i + 1) * len + j] += b[(i + 1) * len + k] * c[j * len + k];
        a[i * len + (j + 1)] += b[i * len + k] * c[(j + 1) * len + k];
        a[(i + 1) * len + (j + 1)] += b[(i + 1) * len + k] * c[(j + 1) * len + k];
      }
    }
  }
  return fpops;
}
```