Watch as the OS rewrites my buggy program. [Windows, SetErrorMode, SEM_NOALIGNMENTFAULTEXCEPT]

movoops.cpp

/*
Watch as the OS rewrites my buggy program.

Compile with:
cl //Ox movoops.cpp

Output:
0f 28 01
xxxxxxxxxxxxxxx1
0f 10 01
*/

#include <immintrin.h>
#include <stdio.h>
#include <windows.h>

__declspec(noinline) __m128 f(char* a) {
  return _mm_load_ps((float*)a);
}

int main()
{
  // Dear OS, please clean up my mess.
  // "SEM_NOALIGNMENTFAULTEXCEPT: The system automatically fixes memory alignment 
  //  faults and makes them invisible to the application."
  // https://msdn.microsoft.com/en-us/library/windows/desktop/ms680621(v=vs.85).aspx
  SetErrorMode(SEM_NOALIGNMENTFAULTEXCEPT);

  // Scratch space for unaligned operation
  __m128 m[2] = { 0 };
  
  // Print the first three bytes of f(): 
  // movaps x, m
  char* b = (char*)&f;
  printf("%02x %02x %02x\n", b[0], b[1], b[2]);
  
  // Unalign that address
  char* up = (char*)m+1;
  
  // Print the pointer value - observe its unalignedness
  printf("%p\n", up);
  
  // Call the function with the unaligned pointer
  m[1] = f(up);
  
  // Print the first three bytes of f() again. Observe how they have changed:
  // movups x, m
  printf("%02x %02x %02x\n", b[0], b[1], b[2]);
}