On Thu, Jun 3, 2021 at 3:08 PM Heikki Linnakangas <hlinn...@iki.fi> wrote: > > On 03/06/2021 17:33, Greg Stark wrote: > >> 3. It's probably cheaper perform the HAS_ZERO check just once on (half1 > > | half2). We have to compute (half1 | half2) anyway. > > > > Wouldn't you have to check (half1 & half2) ? > > Ah, you're right of course. But & is not quite right either, it will > give false positives. That's ok from a correctness point of view here, > because we then fall back to checking byte by byte, but I don't think > it's a good tradeoff.
Ah, of course. > /* > * Check if there are any zero bytes in this chunk. > * > * First, add 0x7f to each byte. This sets the high bit in each byte, > * unless it was a zero. We already checked that none of the bytes had > * the high bit set previously, so the max value each byte can have > * after the addition is 0x7f + 0x7f = 0xfe, and we don't need to > * worry about carrying over to the next byte. > */ > x1 = half1 + UINT64CONST(0x7f7f7f7f7f7f7f7f); > x2 = half2 + UINT64CONST(0x7f7f7f7f7f7f7f7f); > > /* then check that the high bit is set in each byte. */ > x = (x1 | x2); > x &= UINT64CONST(0x8080808080808080); > if (x != UINT64CONST(0x8080808080808080)) > return 0; That seems right, I'll try that and update the patch. (Forgot to attach earlier anyway) -- John Naylor EDB: http://www.enterprisedb.com
