Re: Computer architects leaving Intel...

Terje Mathisen <terje.mathisen@tmsw.no> writes:

Bernd Linsel wrote:
>

On 05.09.24 19:04, Terje Mathisen wrote:
>

One of my alternatives are
>
  unsigned u = start; // Cannot be less than zero
  if (u) {
    u++;
    do {
      u--;
      data[u]...
    while (u);
  }
>
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:
>
  for (unsigned u = start; (u & TOPBIT) == 0; u--)

>
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 don't like either of these because they need a redundant
specification of the index variable's type (and similarly the
definition of TOPBIT depends on knowing that type).  Needing to
redundantly know the type is dangerous because the two type
specifications might get out of sync.  Instead, either

    for (unsigned u = start; u != -1; --u)

or

    for (unsigned u = start; u+1 != 0; --u)

avoids the danger of having types be out of sync (and also can be
used with signed types, not that I would advocate doing that).