[Bug middle-end/78295] New: [7 Regression] Spurious -Wuninitialized warning for vector element assignment

["jsm28 at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78295

            Bug ID: 78295
           Summary: [7 Regression] Spurious -Wuninitialized warning for
                    vector element assignment
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jsm28 at gcc dot gnu.org
  Target Milestone: ---

On x86_64-linux-gnu, with -O2 -Wall, the code

typedef double vectype __attribute__ ((__vector_size__ (16)));

vectype
f (double x)
{
  vectype t;
  for (int i = 0; i < 2; i++)
    t[i] = x;
  return t;
}

produces a spurious warning

t.c: In function 'f':
t.c:8:10: warning: 't' is used uninitialized in this function [-Wuninitialized]
     t[i] = x;
     ~~~~~^~~

which does not appear with GCC 6.  Even if storing to a vector element is
represented internally as loading the whole vector then modifying it, this
should not result in a warning, as *logically* the assignment does not use an
uninitialized value.

This results in errors building the glibc testsuite for x86_64, as it has code
like the above to initialize vectors.