From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id 0E1923858426; Fri, 12 May 2023 09:00:48 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 0E1923858426
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1683882048;
	bh=0mSjUgwE950RCjukCTQwmcFtfL3JZds2ZIN3ToydaaQ=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=VKoH1AErKA+b2SAksJvjmWGy4+dYYL5cDXp0ZQmAgIQ9/hV5PPlycm2qgHPf6G6tM
	 4abmfj7xrbRaoZeZWM2cuF4cTx7zlpAFfvR6sv12/QwhvpN3uNTVYRXXVjzBqBbojm
	 JpV2jsanlxfyybZA02JNcFw7x1JlA5kvfk2+gyNE=
From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug analyzer/109789] analyzer-use-of-uninitialized-value false
 positive inside function when array passed to the function is pre-initialized
Date: Fri, 12 May 2023 09:00:47 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: analyzer
X-Bugzilla-Version: 13.1.1
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: redi at gcc dot gnu.org
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: dmalcolm at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: 
Message-ID: <bug-109789-4-GDGJg1ckeA@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-109789-4@http.gcc.gnu.org/bugzilla/>
References: <bug-109789-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109789

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #5)
> float dsp_abs_max(float *buf, size_t size) {
>   for (size_t i =3D 0; i < size; i++)
>     if (fabsf(buf[i]) > 1e-20f)
>       dsp_abs_max_ret =3D fabsf(buf[i]);
>   return dsp_abs_max_ret;
> }
> void export_audio(int nframes, float init, int count) {
>   do {
>     float tmp_l[nframes];
>     for (int i =3D 0; i < nframes; i++)
>       tmp_l[i] =3D init;
>     float max_amp =3D dsp_abs_max(tmp_l, nframes);

I think the problem is that frames is signed int and is converted to size_t
when calling this function. The analyzer is complaining that if nframes is
negative, then you'll get a very large size_t and the loop inside dsp_abs_m=
ax
will read more variables than were init'd.

Of course if nframes is negative, the program has undefined behaviour anywa=
y,
C17 6.7.6.2 says "each time it is evaluated
it shall have a value greater than zero".

So I think the analyzer should assume the size is greater than zero, or warn
about *that* possibility, at the point of the array declaration.

If you add this to export_audio() then there's no analyzer warning:

  if (nframes < 1)
    return;=