user: 请将这段汇编代码片段转换成等效的C代码:

```asm
<square_step>:
  endbr64
  push   %r15
  mov    %rdi,%r15
  movslq %esi,%rdi
  movslq %edx,%rsi
  mov    %esi,%eax
  push   %r14
  lea    (%r15,%rsi,8),%r9
  lea    (%r15,%rdi,8),%r8
  push   %r13
  sub    %ecx,%eax
  push   %r12
  cltq
  mov    %rsi,%r12
  push   %rbp
  lea    (%r15,%rax,8),%rdx
  mov    %ecx,%ebp
  mov    %edi,%ecx
  push   %rbx
  sub    %ebp,%ecx
  mov    %ebp,%r13d
  mov    %rdi,%rbx
  movslq %ecx,%rcx
  sub    $0x48,%rsp
  mov    (%rdx),%r10
  mov    %rcx,0x38(%rsp)
  mov    (%r10,%rdi,4),%eax
  add    (%r10,%rcx,4),%eax
  mov    %rdx,0x28(%rsp)
  mov    (%r9),%r10
  mov    %rdi,0x20(%rsp)
  mov    %r9,0x10(%rsp)
  add    (%r10,%rcx,4),%eax
  mov    (%r8),%r10
  mov    %r8,0x8(%rsp)
  mov    %rsi,(%rsp)
  add    (%r10,%rsi,4),%eax
  lea    0x3(%rax),%r10d
  cmovs  %r10d,%eax
  shr    $0x1f,%r13d
  add    %ebp,%r13d
  sar    $0x2,%eax
  sar    %r13d
  neg    %r13d
  mov    %eax,%r14d
  xor    %eax,%eax
  call   <salt@plt>
  lea    0x0(%r13,%r12,1),%r10d
  mov    0x28(%rsp),%rdx
  lea    0x0(%r13,%rbx,1),%r11d
  movslq %r10d,%r10
  add    %eax,%r14d
  movslq %r11d,%r11
  mov    0x20(%rsp),%rdi
  lea    (%r15,%r10,8),%r10
  mov    0x10(%rsp),%r9
  mov    0x8(%rsp),%r8
  mov    %r11,0x30(%rsp)
  mov    (%r10),%rax
  mov    (%rsp),%rsi
  mov    %r10,0x18(%rsp)
  mov    %rdi,0x28(%rsp)
  mov    %r14d,(%rax,%r11,4)
  mov    (%rdx),%r14
  lea    0x0(%rbp,%rbx,1),%edx
  movslq %edx,%rdx
  mov    (%r14,%rdx,4),%eax
  add    (%r14,%rdi,4),%eax
  mov    %rdx,0x20(%rsp)
  mov    (%r9),%r14
  add    (%r14,%rdx,4),%eax
  mov    (%r8),%r14
  add    (%r14,%rsi,4),%eax
  lea    0x3(%rax),%r14d
  cmovns %eax,%r14d
  xor    %eax,%eax
  sub    %r13d,%ebx
  add    %r12d,%ebp
  call   <salt@plt>
  mov    0x18(%rsp),%r10
  movslq %ebx,%rbx
  movslq %ebp,%rbp
  sar    $0x2,%r14d
  mov    0x10(%rsp),%r9
  mov    0x38(%rsp),%rcx
  mov    (%r10),%r10
  add    %eax,%r14d
  mov    0x8(%rsp),%r8
  mov    0x28(%rsp),%rdi
  mov    (%rsp),%rsi
  mov    %r9,0x18(%rsp)
  mov    %r14d,(%r10,%rbx,4)
  lea    (%r15,%rbp,8),%r14
  mov    (%r9),%rbp
  mov    (%r14),%r10
  mov    %rdi,0x10(%rsp)
  mov    (%r10,%rcx,4),%eax
  add    0x0(%rbp,%rcx,4),%eax
  mov    (%r8),%rcx
  add    (%r10,%rdi,4),%eax
  add    (%rcx,%rsi,4),%eax
  lea    0x3(%rax),%ebp
  cmovns %eax,%ebp
  sub    %r13d,%r12d
  xor    %eax,%eax
  call   <salt@plt>
  movslq %r12d,%r12
  mov    0x30(%rsp),%r11
  mov    0x18(%rsp),%r9
  lea    (%r15,%r12,8),%r12
  sar    $0x2,%ebp
  mov    0x20(%rsp),%rdx
  mov    (%rsp),%rsi
  mov    (%r12),%rcx
  add    %eax,%ebp
  mov    0x8(%rsp),%r8
  mov    0x10(%rsp),%rdi
  mov    (%r9),%r9
  mov    %ebp,(%rcx,%r11,4)
  mov    (%r14),%rcx
  mov    (%rcx,%rdi,4),%eax
  add    (%r9,%rdx,4),%eax
  add    (%rcx,%rdx,4),%eax
  mov    (%r8),%rdx
  add    (%rdx,%rsi,4),%eax
  lea    0x3(%rax),%ebp
  cmovns %eax,%ebp
  xor    %eax,%eax
  call   <salt@plt>
  mov    %eax,%r8d
  sar    $0x2,%ebp
  mov    (%r12),%rax
  add    %r8d,%ebp
  mov    %ebp,(%rax,%rbx,4)
  add    $0x48,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
```

assistant: ```c
void square_step(int **map, int x, int y, int length) {
  map[y - length / 2][x - length / 2] = (map[y - length][x - length] + map[y - length][x] + map[y][x - length] + map[x][y]) / 4 + salt();
  map[y - length / 2][x + length / 2] = (map[y - length][x] + map[y - length][x + length] + map[y][x + length] + map[x][y]) / 4 + salt();
  map[y + length / 2][x - length / 2] = (map[y][x - length] + map[y + length][x - length] + map[y + length][x] + map[x][y]) / 4 + salt();
  map[y + length / 2][x + length / 2] = (map[y][x + length] + map[y + length][x] + map[y + length][x + length] + map[x][y]) / 4 + salt();
}
```