Message-ID: <195097d6-64cd-4adb-b8a3-1d86ae31c411@iki.fi>
Date: Wed, 15 Apr 2026 14:06:14 +0300
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Subject: Re: Reduce build times of pg_trgm GIN indexes
To: David Geier <geidav.pg@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>,
 Peter Eisentraut <peter@eisentraut.org>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>,
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From: Heikki Linnakangas <hlinnaka@iki.fi>
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On 14/04/2026 10:02, David Geier wrote:
>> 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.

This was briefly discussed when PointerGetDatum() was changed from a 
macro to a static inline function [1]. On that email, Peter pointed out 
that the compiler was doing the same deduction that Coverity did now, 
i.e. that if you pass the Datum returned by PointerGetDatum(&foo) to a 
function, it cannot change *foo. I'm surprised we dismissed that worry 
so quickly. If the compiler optimizes based on that assumption, you can 
get incorrect code.

Three alternative fixes were discussed on that thread. Here's a fourth 
one that I think is better;

#define PointerGetDatum(X) \
	((Datum) (uintptr_t) (true ? (X) : NULL))

I found this trick with the dummy conditional expression at [2]. It 
always evaluates to just (X), but it has the effect that you get a 
compiler error if (X) is not a pointer.


[1] 
https://www.postgresql.org/message-id/812568f2-ff1d-ebd9-aee6-e00d8f2e0fb6%40enterprisedb.com

[2] See "TO_VOID_PTR_EXPR()" at 
https://medium.com/@pauljlucas/generic-in-c-d7ab47e3b5ab

> Ah, that could be it.
> Is there a way for me to run Coverity on a patch to test that out?

Not really I'm afraid. I can commit a fix and we'll see if it helps the 
next time that Coverity runs (= Sunday).

> Which Coverity CI do we actually use? Is it this one here [1]?
> 
> [1] https://scan.coverity.com/projects/209?

Yeah, that's the one, but only the security team has access.

- Heikki