Thread

1. [PATCH] Add pg_lfind8_nonzero()

   cca5507 <cca5507@qq.com> — 2025-12-14T13:33:00Z

   Hi hackers,
   
   I'd like to add pg_lfind8_nonzero() to optimize some code like this:
   
   ```
   	for (i = 0; i < numberOfAttributes; i++)
   	{
   		if (isnull[i])
   		{
   			hasnull = true;
   			break;
   		}
   	}
   ```
   
   With pg_lfind8_nonzero(), we can write the code like this:
   
   ```
   	if (likely(numberOfAttributes > 0))
   		hasnull = pg_lfind8_nonzero((uint8 *) isnull, numberOfAttributes);
   ```
   
   The pg_lfind8_nonzero() is faster because we can handle 8 bool values at a time.
   
   v1-0001 add the pg_lfind8_nonzero().
   
   v1-0002 use the pg_lfind8_nonzero() in some places.
   
   v1-0003 add a extension "test_patch" only for testing, the test like this:
   
   ```
       for (i = 0; i < 1000; i++)
       {
           for (int j = 0; j < natts; j++)
           {
               if (isnull[j])
               {
                   hasnull = true;
                   break;
               }
           }
       }
   =======================
       for (i = 0; i < 1000; i++)
       {
           if (likely(natts > 0))
               hasnull = pg_lfind8_nonzero((uint8 *) isnull, natts);
       }
   ```
   
   create extension test_patch;
   # 4 is the natts
   select test_head(4);
   select test_patch(4);
   
   Test result:
   natts: 4   head: 1984ns  patch: 2094ns
   natts: 8   head: 3196ns  patch: 641ns
   natts: 16  head: 4589ns  patch: 752ns
   natts: 32  head: 8036ns  patch: 1152ns
   natts: 64  head: 19367ns patch: 2455ns
   natts: 128 head: 33445ns patch: 4018ns
   
   Looking forward to your comments!
   
   --
   Regards,
   ChangAo Chen
   

2. Re: [PATCH] Add pg_lfind8_nonzero()

   Andres Freund <andres@anarazel.de> — 2025-12-14T15:27:06Z

   Hi,
   
   On 2025-12-14 21:33:00 +0800, cca5507 wrote:
   > Hi hackers,
   > 
   > I'd like to add pg_lfind8_nonzero() to optimize some code like this:
   > 
   > ```
   > 	for (i = 0; i < numberOfAttributes; i++)
   > 	{
   > 		if (isnull[i])
   > 		{
   > 			hasnull = true;
   > 			break;
   > 		}
   > 	}
   > ```
   
   Is code like this ever actually relevant for performance? In cases where the
   compiler can't also optimize the code? Unless it is a legitimate bottleneck,
   adding more complicated code to optimize the case doesn't make sense.
   
   
   > create extension test_patch;
   > # 4 is the natts
   > select test_head(4);
   > select test_patch(4);
   > 
   > Test result:
   > natts: 4   head: 1984ns  patch: 2094ns
   > natts: 8   head: 3196ns  patch: 641ns
   > natts: 16  head: 4589ns  patch: 752ns
   > natts: 32  head: 8036ns  patch: 1152ns
   > natts: 64  head: 19367ns patch: 2455ns
   > natts: 128 head: 33445ns patch: 4018ns
   > 
   > Looking forward to your comments!
   > 
   
   The benchmark really should show performance benefits in at least somewhat
   realistic cases (i.e. real users of the functions, even if the workload is a
   bit more extreme than it'd be most of the time).
   
   Greetings,
   
   Andres
   
   
   
   

3. Re: [PATCH] Add pg_lfind8_nonzero()

   cca5507 <cca5507@qq.com> — 2025-12-16T02:33:15Z

   Hi,
   
   Thank you for your reply!
   
   > Is code like this ever actually relevant for performance? In cases where the
   > compiler can't also optimize the code? Unless it is a legitimate bottleneck,
   > adding more complicated code to optimize the case doesn't make sense.
   
   I just think the patch isn't that complicated.
   
   > The benchmark really should show performance benefits in at least somewhat
   > realistic cases (i.e. real users of the functions, even if the workload is a
   > bit more extreme than it'd be most of the time).
   
   Yeah, this makes sense to me. Maybe I need to think more on it.
   
   --
   Regards,
   ChangAo Chen
   

4. Re: [PATCH] Add pg_lfind8_nonzero()

   Peter Eisentraut <peter@eisentraut.org> — 2025-12-17T15:09:42Z

   On 14.12.25 14:33, cca5507 wrote:
   > With pg_lfind8_nonzero(), we can write the code like this:
   > 
   > ```
   > 	if (likely(numberOfAttributes > 0))
   > 		hasnull = pg_lfind8_nonzero((uint8 *) isnull, numberOfAttributes);
   > ```
   > 
   > The pg_lfind8_nonzero() is faster because we can handle 8 bool values at a time.
   
   Maybe you could try popcount for this?
   
        hasnull = (pg_popcount((char *) isnull, numberOfAttributes) > 0);
   
   
   
   
   

5. Re: [PATCH] Add pg_lfind8_nonzero()

   cca5507 <cca5507@qq.com> — 2025-12-18T02:29:00Z

   Hi,
   
   > Maybe you could try popcount for this?
   > 
   >      hasnull = (pg_popcount((char *) isnull, numberOfAttributes) > 0);
   
   It seems that pg_popcount() cannot early return when we find a nonzero value, so I don't
   think it's a good choice for this case.
   
   --
   Regards,
   ChangAo Chen