[Bug c/66661] New: incorrect memory access in optimization with flexible array member

["P at draigBrady dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66661

            Bug ID: 66661
           Summary: incorrect memory access in optimization with flexible
                    array member
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: P at draigBrady dot com
  Target Milestone: ---

On a heap allocated structure, direct access to flexible array members with
optimization at -O2 can result in reads to memory beyond the heap object.
I.E. gcc assumes alignment/padding is allocated when accessing flexible array
members. The attached file is a summary of the code involved though does _not_
reproduce the issue.

To reproduce one can:

  git clone --depth=1 git://git.sv.gnu.org/coreutils.git
  cd coreutils/
  git checkout 53883af0
  export LSAN_OPTIONS=exitcode=0
  ./bootstrap && ./configure --quiet && \
  make -j8 AM_CFLAGS='-fsanitize=address -fsanitize=undefined'
  src/chmod a+rx ..

Also attached is the disassembly of the problematic code,
and for comparison good code achieved by using a (char*) cast
on the flexi array to force byte at a time access.