Re: [U-Boot] [PATCH 1/3] memcpy: use 32-bit copies if possible

Dear Alessandro Rubini,

In message 20091008074114.GA30203@mail.gnudd.com you wrote:

...

Since int is 32 also on 64bit systems, I used unsigned long.

Note that this is not guaranteed, though. It could be 64 bit as well.

...

   /* do it one word at a time (32 bits or 64 bits) if possible */
   if ( ((count | (int)s) & (sizeof(long) - 1)) == 0) {
      count /= sizeof(long);
      cl = (c & 0xff) | ((c & 0xff) << 8);
      cl |= cl << 16;
      if (sizeof(long) > 4)
         cl |= cl << 32;

How about:

  cl = 0;
  for (i=0; i<sizeof(long); ++i) {
     cl <<= 8;
     cl |= c & 0xff;
  }

GCC optimization will do the rest...

If you want gcc to optimise well, make it easy to do so. Changing the for loop into: for (i=sizeof(long); i; --i) makes it easier for gcc and more likely to result in optimal code.

Similarly for copy ops for(--top,--fromp, i = len/4; i; --i) *++top = *++fromp; /* pre incr can be one op on some archs */

Jocke