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Subject: Re: jsonapi: scary new warnings with LTO enabled
From: Daniel Gustafsson <daniel@yesql.se>
In-Reply-To: <2078511.1744841564@sss.pgh.pa.us>
Date: Thu, 17 Apr 2025 00:18:23 +0200
Cc: pgsql-hackers@lists.postgresql.org
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References: <2074634.1744839761@sss.pgh.pa.us>
 <0A3CF5C4-9947-42E3-89A9-34FCF7BA95BA@yesql.se>
 <2078511.1744841564@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
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> On 17 Apr 2025, at 00:12, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>=20
> Daniel Gustafsson <daniel@yesql.se> writes:
>>> On 16 Apr 2025, at 23:42, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> I'm not sure
>>> how other than giving up on stack allocation of JsonLexContexts,
>>> though, especially if we consider the jsonapi API frozen.  But =
seeing
>>> that there are only three such call sites and none of them seem in =
the
>>> least performance-critical, maybe we should just do that?
>=20
>> I can't see any other option really, and there is no performance =
angle really
>> so that should be safe.  Since I committed at least one of these, let =
me know
>> if you want me to tackle it.
>=20
> The only alternative I can see that might stop the warning is if we
> can find a way to make it clearer to the optimizer that the FREE()
> isn't reached.  But I'm not sure about a trustworthy way to make that
> happen.  Maybe it'd work to change the signature of freeJsonLexContext
> (or perhaps better, add a separate entry point) so that the caller is
> passing a bool constant that controls whether to free the struct.
> We could have an Assert that compares that to the state of the
> JSONLEX_FREE_STRUCT flag to catch mistakes.  This seems kind of messy
> though.

Yeah, that seems messy enough that someone down the line will go "why on =
earth"
and we'll have to revisit this discussion.  It can probably be made to =
work but
I doubt it will be worth it compared to allocating on the heap.

--
Daniel Gustafsson