[Bug c++/100430] New: False positive for -Warray-bounds and pointers

[jens.maurer at gmx dot net via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100430

            Bug ID: 100430
           Summary: False positive for -Warray-bounds and pointers
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jens.maurer at gmx dot net
  Target Milestone: ---

This code triggers a -Warray-bounds warning in gcc 11.1 (gcc 9.3 was silent;
gcc 10 did show the warning). The test case is reduced from another case (not
shown) that only triggered the warning starting with gcc 11.

char * g();
static char c = 0;
unsigned int x = 0;

void f(char *&p)
{
  if (x > 1)
    p[1] = 0;
  p = g();
}

void h()
{
  char * p = &c;
  f(p);
}


$ g++ -c -O2 -Warray-bounds c.cxx
c.cxx: In function ‘void h()’:
c.cxx:9:10: warning: array subscript 1 is outside array bounds of ‘char [1]’
[-Warray-bounds]
    9 |     p[1] = 0;
      |     ~~~~~^~~
c.cxx:3:13: note: while referencing ‘c’
    3 | static char c = 0;
      |             ^


Note: using -O3 silences the warning.



$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-pc-linux-gnu/11.1.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-11.1.0/configure --enable-languages=c,c++
--with-system-zlib --enable-lto --with-abi=m64 --disable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.1.0 (GCC)