Re: Reduce build times of pg_trgm GIN indexes

On 12/04/2026 21:05, Tom Lane wrote:
> Heikki Linnakangas <hlinnaka@iki.fi> writes:
>> Pushed 0001 as commit 6f5ad00ab7.
> 
> This commit has caused Coverity to start complaining that
> most of ginExtractEntries() is unreachable:
> 
> *** CID 1691468:         Control flow issues  (DEADCODE)
> /srv/coverity/git/pgsql-git/postgresql/src/backend/access/gin/ginutil.c: 495             in ginExtractEntries()
> 489         /*
> 490          * Scan the items for any NULLs.  All NULLs are considered equal, so we
> 491          * just need to check and remember if there are any.  We remove them from
> 492          * the array here, and after deduplication, put back one NULL entry to
> 493          * represent them all.
> 494          */
>>>>      CID 1691468:         Control flow issues  (DEADCODE)
>>>>      Execution cannot reach this statement: "hasNull = false;".
> 495         hasNull = false;
> 496         if (nullFlags)
> 497         {
> 498             int32        numNonNulls = 0;
> 499
> 500             for (int32 i = 0; i < nentries; i++)
> 
> Evidently, it does not realize that the extractValueFn() can change
> nentries from its initial value of zero.  I wouldn't be too surprised
> if that's related to our casting of the pointer to uintptr_t --- that
> may cause it to not see the passed pointer as a potential reference
> mechanism.
> 
> I would just write that off as Coverity not being smart enough, except
> that I'm worried that some compiler might make a similar deduction and
> break the function completely.  Was the switch to a local variable
> for nentries really a useful win performance-wise?

I didn't do it for performance, but because I find the function easier 
to read that way. We could change it back.

It's a pretty scary thought that a compiler might misoptimize that 
though. In the same function we have 'nullFlags', too, as a local 
variable, even before this commit. Not sure why Coverity doesn't 
complain about that.

> /*
>  * PointerGetDatum
>  *        Returns datum representation for a pointer.
>  */
> static inline Datum
> PointerGetDatum(const void *X)
> {
>     return (Datum) (uintptr_t) X;
> }

Hmm, is that 'const' incorrect? This function doesn't modify *X, but the 
resulting address will be used to modify it. Maybe changing it to 
non-const "void *X" would give Coverity a hint.

- Heikki