Re: Computer architects leaving Intel...

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--)

Terje

What about:

for (unsigned u = start; u != ~0u; --u)
    ...

or even

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

?

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


--
Bernd Linsel