[Bug middle-end/80533] New: Alias analysis of zero length array does not recognize accesses beyond end of array

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80533

            Bug ID: 80533
           Summary: Alias analysis of zero length array does not recognize
                    accesses beyond end of array
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sje at gcc dot gnu.org
  Target Milestone: ---

Created attachment 41274
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41274&action=edit
Test case

GCC should treat zero length arrays like C99 flexible arrays when they are
at the end of a structure.  I.e. recognize that accesses to that array may
go beyond the end of the structure.   GCC has been treating zero length
arrays and flexible arrays differently since at least 5.0.

This was found on aarch64 but the bug is not platform specific.  A test case
is attached, it cannot be run but if compiled for aarch64 with
-O2 -fno-strict-aliasing and either -UFLEX or -DFLEX you can see the
different code.  In the -UFLEX case it generates a load/load/store/store
sequence and in the -DFLEX case it generates load/store/load/store for
the code in the main loop.

See also:

https://gcc.gnu.org/ml/gcc/2017-04/msg00118.html
https://gcc.gnu.org/ml/gcc-patches/2017-04/msg01257.html