[Bug c++/101831] New: Spurious maybe-uninitialized warning on std::array::size

[Bug c++/101831] New: Spurious maybe-uninitialized warning on std::array::size

[m101010a at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

            Bug ID: 101831
           Summary: Spurious maybe-uninitialized warning on
                    std::array::size
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m101010a at gmail dot com
  Target Milestone: ---

When calling std::array::size on an uninitialized array, gcc emits a warning:

$ cat x.cpp
#include <array>
int f() {
        std::array<char, 1> a;
        return a.size();
}
$ g++ -v -S -o /dev/null -Wmaybe-uninitialized x.cpp
Using built-in specs.
COLLECT_GCC=g++
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --with-isl
--with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit
--enable-cet=auto --enable-checking=release --enable-clocale=gnu
--enable-default-pie --enable-default-ssp --enable-gnu-indirect-function
--enable-gnu-unique-object --enable-install-libiberty --enable-linker-build-id
--enable-lto --enable-multilib --enable-plugin --enable-shared
--enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-libunwind-exceptions --disable-werror
gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.1.0 (GCC) 
COLLECT_GCC_OPTIONS='-v' '-S' '-o' '/dev/null' '-Wmaybe-uninitialized'
'-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' '/dev/'
 /usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/cc1plus -quiet -v -D_GNU_SOURCE x.cpp
-quiet -dumpdir /dev/ -dumpbase x.cpp -dumpbase-ext .cpp -mtune=generic
-march=x86-64 -Wmaybe-uninitialized -version -o /dev/null
GNU C++17 (GCC) version 11.1.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 11.1.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP

warning: MPFR header version 4.1.0 differs from library version 4.1.0-p13.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../include/c++/11.1.0

/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../include/c++/11.1.0/x86_64-pc-linux-gnu

/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../include/c++/11.1.0/backward
 /usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/include
 /usr/local/include
 /usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/include-fixed
 /usr/include
End of search list.
GNU C++17 (GCC) version 11.1.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 11.1.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP

warning: MPFR header version 4.1.0 differs from library version 4.1.0-p13.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 45eb9a71cc15bfdc579557bea4b77e51
x.cpp: In function ‘int f()’:
x.cpp:4:22: warning: ‘a’ may be used uninitialized [-Wmaybe-uninitialized]
    4 |         return a.size();
      |                ~~~~~~^~
In file included from x.cpp:1:
/usr/include/c++/11.1.0/array:176:7: note: by argument 1 of type ‘const
std::array<char, 1>*’ to ‘constexpr std::array<_Tp, _Nm>::size_type
std::array<_Tp, _Nm>::size() const [with _Tp = char; long unsigned int _Nm =
1]’ declared here
  176 |       size() const noexcept { return _Nm; }
      |       ^~~~
x.cpp:3:29: note: ‘a’ declared here
    3 |         std::array<char, 1> a;
      |                             ^
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-S' '-o' '/dev/null' '-Wmaybe-uninitialized'
'-shared-libgcc' '-mtune=generic' '-march=x86-64' '-dumpdir' '/dev/'

It also happens on the following reduced test case:

struct array {
  int d;
  int e() const { return 1; }
};
int f() {
  array a;
  return a.e();
}

This does not happen when optimization is enabled, and does not happen if
"-fsyntax-only" is used instead of "-S -o /dev/null".  This also happens with
GCC 11.2, but does not happen with GCC 10.

[Bug tree-optimization/101831] Spurious maybe-uninitialized warning on std::array::size

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-08-16

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
-Wmaybe-uninitialized is also issued when an uninitialized object is passed by
reference to a const-qualified argument.  This includes passing the address of
an such object to the implicit this pointer in calls to member functions.  This
form of the warning runs very early on and before any function calls have been
inlined, so it can't tell that the function doesn't actually read from the
uninitialized object.  The same effect can be reproduced in C in a call to a
non-member function (see below).  It's possible to run the early uninitialized
pass later but probably not without introducing some false negatives.  I'm not
sure that the std::array use case is common enough to justify the  potential
for the false negatives (or conversely, that the potential is significant
enough not to avoid the false positives).  So confirmed.  It requires some
thought and testing.

$ cat a.c && gcc -S -Wall -Wextra a.c
inline __attribute__ ((always_inline))
int f (const int *p) { (void)&p; return 0; }

int g (void)
{
  int i;
  return f (&i);
}
a.c: In function ‘g’:
a.c:7:10: warning: ‘i’ may be used uninitialized [-Wmaybe-uninitialized]
    7 |   return f (&i);
      |          ^~~~~~
a.c:2:5: note: by argument 1 of type ‘const int *’ to ‘f’ declared here
    2 | int f (const int *p) { (void)&p; return 0; }
      |     ^
a.c:6:7: note: ‘i’ declared here
    6 |   int i;
      |       ^

[Bug tree-optimization/101831] Spurious maybe-uninitialized warning on std::array::size

[kim.walisch at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

kim.walisch at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kim.walisch at gmail dot com

--- Comment #2 from kim.walisch at gmail dot com ---
> I'm not sure that the std::array use case is common enough to justify the  potential for the false negatives

I just hit the same GCC warning on completely valid code. Both Clang & MSVC
correctly do not issue any warning.

void Foo::func()
{
  std::array<uint64_t, 8> pos;
  assert(pos.size() == static_global_array.size());
  ...
}

In member function ‘void Foo::func()’:
warning: ‘pos’ may be used uninitialized [-Wmaybe-uninitialized]
  312 |   assert(pos.size() == buffers_.size());

[Bug tree-optimization/101831] [11/12 Regression] Spurious maybe-uninitialized warning on std::array::size

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |11.2.0, 12.0
            Summary|Spurious                    |[11/12 Regression] Spurious
                   |maybe-uninitialized warning |maybe-uninitialized warning
                   |on std::array::size         |on std::array::size

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
Declaring std::array::size() with attribute ((const)) avoids the warning so
that might be a solution.

[Bug tree-optimization/101831] [11/12 Regression] Spurious maybe-uninitialized warning on std::array::size

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |msebor at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

[Bug tree-optimization/101831] [11/12 Regression] Spurious maybe-uninitialized warning on std::array::size

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.3

[Bug tree-optimization/101831] [11/12 Regression] Spurious maybe-uninitialized warning on std::array::size

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
libstdc++ patch:
https://gcc.gnu.org/pipermail/gcc-patches/2022-February/589612.html

[Bug tree-optimization/101831] [11/12 Regression] Spurious maybe-uninitialized warning on std::array::size

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Martin Sebor <msebor@gcc.gnu.org>:

https://gcc.gnu.org/g:756eabacfcd767e39eea63257a026f61a4c4e661

commit r12-6992-g756eabacfcd767e39eea63257a026f61a4c4e661
Author: Martin Sebor <msebor@redhat.com>
Date:   Tue Feb 1 17:19:11 2022 -0700

    Declare std::array members with attribute const [PR101831].

    Resolves:
    PR libstdc++/101831 - Spurious maybe-uninitialized warning on
std::array::size

    libstdc++-v3/ChangeLog:

            PR libstdc++/101831
            * include/std/array (begin): Declare const member function
attribute
            const.
            (end, rbegin, rend, size, max_size, empty, data): Same.
            * testsuite/23_containers/array/capacity/empty.cc: Add test cases.
            * testsuite/23_containers/array/capacity/max_size.cc: Same.
            * testsuite/23_containers/array/capacity/size.cc: Same.
            * testsuite/23_containers/array/iterators/begin_end.cc: New test.

[Bug tree-optimization/101831] [11 Regression] Spurious maybe-uninitialized warning on std::array::size

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[11/12 Regression] Spurious |[11 Regression] Spurious
                   |maybe-uninitialized warning |maybe-uninitialized warning
                   |on std::array::size         |on std::array::size

--- Comment #6 from Martin Sebor <msebor at gcc dot gnu.org> ---
Fixed in r12-6992.  Let me backport it after a bit.

[Bug tree-optimization/101831] [11 Regression] Spurious maybe-uninitialized warning on std::array::size

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.3                        |11.4

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 11.3 is being released, retargeting bugs to GCC 11.4.

[Bug tree-optimization/101831] [11 Regression] Spurious maybe-uninitialized warning on std::array::size

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.4                        |11.5

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 11.4 is being released, retargeting bugs to GCC 11.5.