[Bug target/81580] New: [ARM] C++ zero-sized array generates undefined instruction.

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81580

            Bug ID: 81580
           Summary: [ARM] C++ zero-sized array generates undefined
                    instruction.
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: belagod at gcc dot gnu.org
  Target Milestone: ---

This piece of code:

#include <array>

std::array<int,0> arr;

int foo (int i)
{
  return arr[i];
}

When compiled with:

$ arm-none-eabi-g++ -march=armv7-a -marm za.cc -S -O2 -o -

        .arm
        .fpu softvfp
        .type   _Z3fooi, %function
_Z3fooi:
        .fnstart
.LFB1017:
        @ Volatile: function does not return.
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        mov     r3, #0
        ldr     r3, [r3]
        .inst   0xe7f000f0
        .cantunwind
        .fnend
        .size   _Z3fooi, .-_Z3fooi
        .global arr
        .bss
        .align  2
        .type   arr, %object
        .size   arr, 1
arr:
        .space  1
        .ident  "GCC: (GNU) 8.0.0 20170725 (experimental)"

The C++ Std '23.3.2.8 Zero sized arrays' does say that data() returned from
zero-sized arrays is unspecified, but I don't think the compiler should be
generating UNDEFINED instructions?

$ arm-none-eabi-g++ -v                                       
Using built-in specs.
COLLECT_GCC=/work/embedded/dev-builds/trunk/install/bin/arm-none-eabi-g++
COLLECT_LTO_WRAPPER=/data/embedded/dev-builds/trunk/install/libexec/gcc/arm-none-eabi/8.0.0/lto-wrapper
Target: arm-none-eabi
Configured with: /data/embedded/dev-builds/trunk/src/configure
--prefix=/data/embedded/dev-builds/trunk/install --target=arm-none-eabi
--enable-languages=c,c++ --with-cpu=cortex-m3 --with-mode=thumb --with-newlib
--enable-newlib-io-long-long --enable-newlib-register-fini
--enable-newlib-retargetable-locking --disable-newlib-supplied-syscalls
--disable-multilib --with-libexpat --with-system-zlib --disable-gdbtk
--enable-plugins --disable-threads --disable-tls --disable-libgomp
--disable-libmudflap --disable-libquadmath --disable-libssp
--disable-libstdcxx-pch --disable-nls --disable-rda --disable-sid --disable-tui
--disable-utils --disable-werror --disable-fixed-point
Thread model: single
gcc version 8.0.0 20170725 (experimental) (GCC)