Subject: Re: Yet another fast GiST build
To: Pavel Borisov <pashkin.elfe@gmail.com>
Cc: "Andrey M. Borodin" <x4mmm@yandex-team.ru>, Tom Lane <tgl@sss.pgh.pa.us>,
 Justin Pryzby <pryzby@telsasoft.com>,
 =?UTF-8?Q?Darafei_Kom=d1=8fpa_Praliaskouski?= <me@komzpa.net>,
 Thomas Munro <thomas.munro@gmail.com>, Michael Paquier
 <michael@paquier.xyz>, Alexander Korotkov <a.korotkov@postgrespro.ru>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
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From: Heikki Linnakangas <hlinnaka@iki.fi>
Message-ID: <867aacfd-d1af-3eed-5e40-dfc334668d65@iki.fi>
Date: Tue, 6 Oct 2020 16:19:31 +0300
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On 06/10/2020 14:05, Pavel Borisov wrote:
> I've been making tests with memory sanitizer

Thanks!

> and got one another error 
> in regression test create_index:
> CREATE INDEX gpointind ON point_tbl USING gist (f1);
> server closed the connection unexpectedly
> 
> with logfile:
> gistproc.c:1714:28: runtime error: 1e+300 is outside the range of 
> representable values of type 'float'
> SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior gistproc.c:1714:28
> 
> gistproc.c:
> 1714     z = point_zorder_internal(p->x, p->y);
> 
> Consider this a minor issue but unrelated to the other issues discussed. 
> It is reproduced on the last master 
> commit 0a3c864c32751fd29d021929cf70af421fd27370 after all changes into 
> Gist committed.
> 
> cflags="-DUSE_VALGRIND -Og -O0 -fsanitize=address -fsanitize=undefined 
> -fno-sanitize-recover=all -fno-sanitize=alignment -fstack-protector"

You get the same error with:

select (float8 '1e+300')::float4;

float.c:1204:11: runtime error: 1e+300 is outside the range of 
representable values of type 'float'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior float.c:1204:11 in

It boils down to casting a C double to float, when the value doesn't fit 
in float. I'm surprised that's undefined behavior, but I'm no C99 
lawyer. The code in dtof() expects it to yield Inf.

I'm inclined to shrug this off and say that the sanitizer is being 
over-zealous. Is there some compiler flag we should be setting, to tell 
it that we require specific behavior? Any other ideas?

- Heikki