[Bug c++/109609] New: Invalid strncpy/strncat optimization in GCC 12

["gburca-gnu at ebixio dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109609

            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 using
the commented out strncpy() call, the output is as expected. See also how the
generated assembly changes when the printf() inside the function is commented
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 = 23;
char dst[N + 1];

void invert(const char *id) {
  constexpr int MAX_LEN = 13;
  char buf[MAX_LEN];
  char *ptr = buf + sizeof(buf);  // start from the end of buf
  *(--ptr) = '\0';                // terminate string
  while (*id && ptr > buf) {
    *(--ptr) = *(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=/home/gb/.fighome/runtime/gcc/12.2.0-1/bin/g++
COLLECT_LTO_WRAPPER=/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/staging
--build=x86_64-linux-gnu --disable-multilib --disable-multiarch
--enable-clocale=gnu --enable-languages=c,c++,fortran --enable-ld=yes
--enable-gold=yes --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--enable-linker-build-id --enable-lto --enable-plugins --enable-threads=posix
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-pkgversion=''\''internal_build'\''' --with-system-zlib --disable-werror
--with-libelf=/data/work/chhq-sudbld10-001-CENTOS7/15df1254b8bb2e5c/scratch/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='-v' '-save-temps' '-O2' '-shared-libgcc' '-mtune=generic'
'-march=x86-64'

/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=generic -march=x86-64 -O2 -fpch-preprocess