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 B12BC385842B; Mon, 24 Apr 2023 19:34:02 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org B12BC385842B
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1682364842;
	bh=2HSEkb3MrlXPYLGoT8pBtPvMMrBmGw9wPYKZi2gy4Ms=;
	h=From:To:Subject:Date:From;
	b=n48XriIDVUnMFtJuJGuvR8xvche7vO5yHFXtg8LO5gEJ/nsfz2hjX/CHM5+84mDW9
	 PcMWH4+9Pfedu5eLLw3wGEhx/qwqgjKrG7DQDpJDCqaWw62x4bY495ds+L+teKGhj3
	 llbbI3dsCDQpOKuIBeRayhLbfDZr5T8Hfxk2DB+c=
From: "gburca-gnu at ebixio dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109609] New: Invalid strncpy/strncat optimization in GCC 12
Date: Mon, 24 Apr 2023 19:34:01 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: gburca-gnu at ebixio dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-109609-4@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=3D109609

            Bug ID: 109609
           Summary: Invalid strncpy/strncat optimization in GCC 12
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gburca-gnu at ebixio dot com
  Target Milestone: ---

The following code prints uninitialized garbage when compiled with -O2 or -=
O3
in all versions of GCC 12 (including trunk). It works correctly in GCC 11.
Replacing strncpy() with strncat() gives the same results, however when usi=
ng
the commented out strncpy() call, the output is as expected. See also how t=
he
generated assembly changes when the printf() inside the function is comment=
ed
out.

Expected output:
Src: edcba
Dst: edcba

Actual output:
Src: edcba
Dst: <random uninitialized data>

https://godbolt.org/z/5E51Gdh9s

Compile args:
g++ -O2 file.cpp

8<----------- file.cpp --------------
#include <cstring>
#include <stdio.h>
#include <algorithm>

static constexpr unsigned N =3D 23;
char dst[N + 1];

void invert(const char *id) {
  constexpr int MAX_LEN =3D 13;
  char buf[MAX_LEN];
  char *ptr =3D buf + sizeof(buf);  // start from the end of buf
  *(--ptr) =3D '\0';                // terminate string
  while (*id && ptr > buf) {
    *(--ptr) =3D *(id++);           // copy id backwards
  }
  printf("Src: %s\n", ptr);
  strncpy(dst, ptr, N);           // copy ptr/buf to dst
  //strncpy(dst, ptr, std::min<size_t>(N, strlen(ptr)));
}


int main() {
  invert("abcde");
  printf("Dst: %s\n", dst);
}
8<----------- file.cpp --------------

g++ -v -save-temps -O2 file.cpp
Using built-in specs.
COLLECT_GCC=3D/home/gb/.fighome/runtime/gcc/12.2.0-1/bin/g++
COLLECT_LTO_WRAPPER=3D/home/gb/.fighome/runtime/gcc/12.2.0-1/bin/../libexec=
/gcc/x86_64-linux-gnu/12.2.0/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../gcc-12.2.0/configure --prefix
/data/work/chhq-sudbld10-001-CENTOS7/15df1254b8bb2e5c/scratch/gcc/12.2.0/st=
aging
--build=3Dx86_64-linux-gnu --disable-multilib --disable-multiarch
--enable-clocale=3Dgnu --enable-languages=3Dc,c++,fortran --enable-ld=3Dyes
--enable-gold=3Dyes --enable-libstdcxx-debug --enable-libstdcxx-time=3Dyes
--enable-linker-build-id --enable-lto --enable-plugins --enable-threads=3Dp=
osix
--host=3Dx86_64-linux-gnu --target=3Dx86_64-linux-gnu
--with-pkgversion=3D''\''internal_build'\''' --with-system-zlib --disable-w=
error
--with-libelf=3D/data/work/chhq-sudbld10-001-CENTOS7/15df1254b8bb2e5c/scrat=
ch/gcc/12.2.0/build/libelf-0.8.13
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.2.0 ('internal_build')
COLLECT_GCC_OPTIONS=3D'-v' '-save-temps' '-O2' '-shared-libgcc' '-mtune=3Dg=
eneric'
'-march=3Dx86-64'

/home/gb/.fighome/runtime/gcc/12.2.0-1/bin/../libexec/gcc/x86_64-linux-gnu/=
12.2.0/cc1plus
-E -quiet -v -iprefix
/home/gb/.fighome/runtime/gcc/12.2.0-1/bin/../lib/gcc/x86_64-linux-gnu/12.2=
.0/
-D_GNU_SOURCE file.cpp -mtune=3Dgeneric -march=3Dx86-64 -O2 -fpch-preproces=
s=