Re: Computer architects leaving Intel...

Bernd Linsel wrote:

On 05.09.24 19:04, Terje Mathisen wrote:

One of my alternatives are
>
 Â  unsigned u = start; // Cannot be less than zero
 Â  if (u) {
 Â Â Â  u++;
 Â Â Â  do {
 Â Â Â Â Â  u--;
 Â Â Â Â Â  data[u]...
 Â Â Â  while (u);
 Â  }
>
This typically results in effectively the same asm code as the signed version, except for a bottom JGE (Jump (signed) Greater or Equal instead of JA (Jump Above or Equal, but my version is far more verbose.
>
Alternatively, if you don't need all N bits of the unsigned type, then you can subtract and check if the top bit is set in the result:
>
 Â  for (unsigned u = start; (u & TOPBIT) == 0; u--)
>
Terje
>

 What about:
 for (unsigned u = start; u != ~0u; --u)

I like that one!

    ...
 or even
 for (unsigned u = start; (int)u >= 0; --u)

That is the one that I've actually been using, i.e. casting to the corresponding signed type.

    ...
 ?
 I've compared all variants for x86_64 with -O3 -fexpensive-optimizations on godbolt.org:
- 32 bit version: https://godbolt.org/z/TMhhx3nch
- 64 bit version: https://godbolt.org/z/8oxzTf5Gf

Thanks!
Terje
--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"