user: 请从给定的汇编指令推导出C源代码：

```asm
<blockmix_pwxform>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  sub    $0x40,%rsp
  mov    %rdi,-0x28(%rbp)
  mov    %rsi,-0x30(%rbp)
  mov    %rdx,-0x38(%rbp)
  mov    %rcx,-0x40(%rbp)
  mov    -0x40(%rbp),%rax
  shl    $0x7,%rax
  shr    $0x6,%rax
  mov    %rax,-0x10(%rbp)
  mov    -0x10(%rbp),%rax
  sub    $0x1,%rax
  shl    $0x5,%rax
  mov    %rax,%rdx
  mov    -0x28(%rbp),%rax
  lea    (%rdx,%rax,1),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x8,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <blkcpy@plt>
  mov    -0x28(%rbp),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x8,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <blkxor@plt>
  mov    -0x38(%rbp),%rdx
  mov    -0x30(%rbp),%rax
  mov    %rdx,%rsi
  mov    %rax,%rdi
  call   <block_pwxform@plt>
  movq   $0x1,-0x18(%rbp)
  jmp    L12
L1:
  mov    -0x18(%rbp),%rax
  sub    $0x1,%rax
  shl    $0x5,%rax
  mov    %rax,%rdx
  mov    -0x30(%rbp),%rax
  lea    (%rdx,%rax,1),%rcx
  mov    -0x18(%rbp),%rax
  shl    $0x5,%rax
  mov    %rax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  mov    $0x8,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <blkcpy@plt>
  mov    -0x18(%rbp),%rax
  shl    $0x5,%rax
  mov    %rax,%rdx
  mov    -0x28(%rbp),%rax
  lea    (%rdx,%rax,1),%rcx
  mov    -0x18(%rbp),%rax
  shl    $0x5,%rax
  mov    %rax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  mov    $0x8,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <blkxor@plt>
  mov    -0x18(%rbp),%rax
  shl    $0x5,%rax
  mov    %rax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rax,%rdx
  mov    -0x38(%rbp),%rax
  mov    %rax,%rsi
  mov    %rdx,%rdi
  call   <block_pwxform@plt>
  addq   $0x1,-0x18(%rbp)
L12:
  mov    -0x18(%rbp),%rax
  cmp    -0x10(%rbp),%rax
  jb     L1
  mov    -0x18(%rbp),%rax
  lea    0x0(,%rax,8),%rdx
  mov    -0x40(%rbp),%rax
  shl    $0x4,%rax
  cmp    %rax,%rdx
  jae    L14
  mov    -0x40(%rbp),%rax
  add    %rax,%rax
  sub    -0x18(%rbp),%rax
  lea    0x0(,%rax,8),%rdx
  mov    -0x18(%rbp),%rax
  shl    $0x5,%rax
  mov    %rax,%rcx
  mov    -0x28(%rbp),%rax
  add    %rax,%rcx
  mov    -0x18(%rbp),%rax
  shl    $0x5,%rax
  mov    %rax,%rsi
  mov    -0x30(%rbp),%rax
  add    %rsi,%rax
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <blkcpy@plt>
L14:
  mov    -0x10(%rbp),%rax
  lea    -0x1(%rax),%rdx
  movabs $0x1fffffffffffffff,%rax
  and    %rdx,%rax
  mov    %rax,-0x18(%rbp)
  mov    -0x40(%rbp),%rax
  add    %rax,%rax
  mov    %rax,-0x8(%rbp)
  mov    -0x18(%rbp),%rax
  shl    $0x5,%rax
  mov    %rax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  mov    %rax,%rdi
  call   <salsa20_8@plt>
  addq   $0x1,-0x18(%rbp)
  jmp    L29
L5:
  mov    -0x18(%rbp),%rax
  sub    $0x1,%rax
  shl    $0x5,%rax
  mov    %rax,%rdx
  mov    -0x30(%rbp),%rax
  lea    (%rdx,%rax,1),%rcx
  mov    -0x18(%rbp),%rax
  shl    $0x5,%rax
  mov    %rax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  mov    $0x8,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <blkxor@plt>
  mov    -0x18(%rbp),%rax
  shl    $0x5,%rax
  mov    %rax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  mov    %rax,%rdi
  call   <salsa20_8@plt>
  addq   $0x1,-0x18(%rbp)
L29:
  mov    -0x18(%rbp),%rax
  cmp    -0x8(%rbp),%rax
  jb     L5
  nop
  nop
  leave
  ret
```

assistant: ```c
void blockmix_pwxform(const uint64_t *Bin, uint64_t *Bout, uint64_t *S, size_t r) {
  size_t r1, r2, i;
  r1 = r * 128 / ((4 * 2) * 8);
  blkcpy(Bout, &Bin[(r1 - 1) * (4 * 2)], (4 * 2));
  blkxor(Bout, Bin, (4 * 2));
  block_pwxform(Bout, S);
  for (i = 1; i < r1; i++) {
    blkcpy(&Bout[i * (4 * 2)], &Bout[(i - 1) * (4 * 2)], (4 * 2));
    blkxor(&Bout[i * (4 * 2)], &Bin[i * (4 * 2)], (4 * 2));
    block_pwxform(&Bout[i * (4 * 2)], S);
  }
  if (i * (4 * 2) < r * 16) {
    blkcpy(&Bout[i * (4 * 2)], &Bin[i * (4 * 2)], r * 16 - i * (4 * 2));
  }
  i = (r1 - 1) * (4 * 2) / 8;
  r2 = r * 2;
  salsa20_8(&Bout[i * 8]);
  i++;
  for (; i < r2; i++) {
    blkxor(&Bout[i * 8], &Bout[(i - 1) * 8], 8);
    salsa20_8(&Bout[i * 8]);
  }
}
```