[Bug tree-optimization/68379] [6 Regression] BB vectorization: definition in block 13 follows the use for SSA_NAME

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68379

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
The PR68614 testcase

int a, b[3], c[3][5];

void
fn1 ()
{
  int e;
  for (a = 2; a >= 0; a--)
    for (e = 0; e < 4; e++)
      c[a][e] = b[a];
}

exposes the same issue in basic-block vectorization.  We fully unroll the
nest leaving a SLP group of size 3 for the b loads where each individual
load is used in a different SLP target (so the load isn't really grouped
which shows an opportunity for a cheap fix improving code-gen as well).