[Bug c++/70075] incorrect initialization of multidimensional VLAs

["egallager at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70075

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org

--- Comment #2 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #0)
> Unlike GCC in C mode, G++ accepts explicitly initialized variable length
> arrays (as specified in WG21 N3639,
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3639.html). 
> However, when initializing VLAs with more elements than explicitly specified
> in the initializer, G++ fails to zero-initialize the remaining elements as
> would be the case for ordinary arrays.
> 
> The following test case demonstrates the problem:
> 
>  cat z.cpp && /build/gcc-trunk/gcc/xgcc -B /build/gcc-trunk/gcc -Wall
> -Wextra -Wpedantic z.cpp && ./a.out
> void __attribute__ ((noclone, noinline)) bar (int n)
> {
>   const int m = 2;
>   char a [m][n] = { { 0, 1, 2 } };
> 
>   for (int i = 0; i < m; ++i)
>     for (int j = 0; j < n; ++j)
>       __builtin_printf ("%hhi ", a [i][j]);
>   __builtin_printf ("\n");
> 
>   if (a [0][n - 1] != 0)
>     __builtin_abort ();
> }
> 
> int main ()
> {
>     bar (4);
> }
> z.cpp: In function ‘void bar(int)’:
> z.cpp:4:15: warning: ISO C++ forbids variable length array ‘a’ [-Wvla]
>    char a [m][n] = { { 0, 1, 2 } };
>                ^
> 0 1 2 12 -1 127 0 0 
> Aborted (core dumped)

I get this output on i386-apple-darwin9.8.0 with gcc8:

$ /usr/local/bin/g++ -Wall -Wextra -Wpedantic -o 70075.exe 70075.cc
70075.cc: In function ‘void bar(int)’:
70075.cc:4:13: warning: ISO C++ forbids variable length array ‘a’ [-Wvla]
  char a[m][n] = { { 0, 1, 2 } };
             ^
$ ./70075.exe
0 1 2 0 0 0 -32 -113 
$

(i.e. no errors)