guard page

[Shelwien]

http://rextester.com/GSMTMH44081

winapi has this feature which allows to mostly remove bound checks from compression algorithms,
so I made a class which supposedly makes it easier to use.

  guard_buf< (1<<20) > buf;

  buf.init();

  for( j=0; j<3; j++ ) {

    GUARD_BEGIN(buf);

    for( i=0,p=buf;; i++ ) {

      *p++ = byte((i+j)>>0);

      *p++ = byte((i+j)>>8);

      *p++ = byte((i+j)>>16);

      *p++ = byte((i+j)>>24);

      if( buf.flag ) break;

    }

    GUARD_END(buf);

    uint x = (uint&)buf[buf.bufsize];

    printf( "x=%X i=%i/%i\n", x, i,buf.bufsize/4 );

  }

The idea is that instead of explicit checks in the code, like if( p>=buf+bufsize) on each memory write,
we can use hardware support (it has almost zero overhead when bound check is never triggered).

"Guard page" is like normal memory, but triggers the SEH handler on first access, where buf.flag is set.