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From: "stefan at bytereef dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/98753] -Wfree-nonheap-object on unreachable code
 with -O0
Date: Tue, 02 Jan 2024 16:08:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98753

--- Comment #16 from Stefan Krah <stefan at bytereef dot org> ---
I have encountered the same issue (gcc emits a false positive warning when
free() is called conditionally) in the mpdecimal project when compiled with
-flto.

Worse, mpdecimal itself as well as a large test suite compile without warni=
ngs,
so distributions will think everything is fine.

Until a user uses the static libmpdec.a for a trivial program:


wget
https://www.bytereef.org/software/mpdecimal/releases/mpdecimal-2.5.1.tar.gz
tar xvf mpdecimal-2.5.1.tar.gz
cd mpdecimal-2.5.1
./configure CFLAGS=3D"-flto=3Dauto" CXXFLAGS=3D"-flto=3Dauto" LDFLAGS=3D"-f=
lto=3Dauto"
LDXXFLAGS=3D"-flto=3Dauto"
make

# The trivial program:
$ cd libmpdec
$ make bench
gcc -Wall -Wextra -Wno-unknown-pragmas -std=3Dc99 -pedantic -DNDEBUG -O2
-flto=3Dauto -o bench bench.c libmpdec.a -lm
In function =E2=80=98mpd_del=E2=80=99,
    inlined from =E2=80=98_mpd_qaddsub.constprop=E2=80=99 at mpdecimal.c:34=
71:5:
mpdecimal.c:470:9: warning: attempt to free a non-heap object =E2=80=98big_=
aligned=E2=80=99
[-Wfree-nonheap-object]
         mpd_free(dec);


Here, the user will get a static library to which he may not even have the
source code readily available and gets a false positive warning for code in
that library.

Like others, I think this warning should be under a category -Wmaybe and be
less decisive in its message.

For the next mpdecimal release the Makefile will filter out -flto for the
static library build.=