From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Some regular-expression performance hacking
Date: Tue, 23 Feb 2021 10:05:35 -0800
Message-ID: <20210223180535.ebgpwkyem33xtt7d@alap3.anarazel.de> (raw)
In-Reply-To: <3875615.1614102748@sss.pgh.pa.us>
References: <1340281.1613018383@sss.pgh.pa.us>
<20210223173437.b3ywijygsy6q42gq@alap3.anarazel.de>
<3874948.1614101949@sss.pgh.pa.us>
<3875615.1614102748@sss.pgh.pa.us>
Hi,
On 2021-02-23 12:52:28 -0500, Tom Lane wrote:
> I wrote:
> > Hmph. There's an "assert(depth >= 0)" immediately in front of that,
> > so I'm not looking too kindly on the compiler thinking it's smarter
> > than I am. Do you have a suggestion on how to shut it up?
gcc can't see the assert though, in an non-cassert optimized build... If
I force assertions to be used, the warning vanishes.
> On reflection, maybe the thing to do is convert the assert into
> an always-on check, "if (depth < 0) return false". The assertion
> is essentially saying that there's no arc leading directly from
> the pre state to the post state. Which there had better not be,
> or a lot of other stuff is going to go wrong; but I suppose there's
> no way to explain that to gcc. It is annoying to have to expend
> an always-on check for a can't-happen case, though.
Wouldn't quite work like that because of the restrictions of what pg
infrastructure we want to expose the regex engine to, but a
if (depth < 0)
pg_unreachable();
would avoid the runtime overhead and does fix the warning.
I have been wondering about making Asserts do something along those
lines - but it'd need to be opt-in, since we clearly have a lot of
assertions that would cost too much.
Greetings,
Andres Freund
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To: pgsql-hackers@postgresql.org
Cc: andres@anarazel.de, tgl@sss.pgh.pa.us, pgsql-hackers@lists.postgresql.org
Subject: Re: Some regular-expression performance hacking
In-Reply-To: <20210223180535.ebgpwkyem33xtt7d@alap3.anarazel.de>
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