[Bug tree-optimization/84958] New: int loads not eliminated against larger stores

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84958

            Bug ID: 84958
           Summary: int loads not eliminated against larger stores
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

[ As discussed here: https://gcc.gnu.org/ml/gcc-patches/2018-03/msg00800.html ]

This test-case:
...
int foo()
{
  int a[10];
  for(int i = 0; i < 10; ++i)
    a[i] = i*i;
  int res = 0;
  for(int i = 0; i < 10; ++i)
    res += a[i];
  return res;
}
...

compiled with -O3 results in this gimple at optimized:
...
  MEM[(int *)&a] = { 0, 1 };
  MEM[(int *)&a + 8B] = { 4, 9 };
  MEM[(int *)&a + 16B] = { 16, 25 };
  MEM[(int *)&a + 24B] = { 36, 49 };
  MEM[(int *)&a + 32B] = { 64, 81 };
  _6 = a[0];
  _28 = a[1];
  res_29 = _6 + _28;
  _35 = a[2];
  res_36 = res_29 + _35;
  _42 = a[3];
  res_43 = res_36 + _42;
  _49 = a[4];
  res_50 = res_43 + _49;
  _56 = a[5];
  res_57 = res_50 + _56;
  _63 = a[6];
  res_64 = res_57 + _63;
  _70 = a[7];
  res_71 = res_64 + _70;
  _77 = a[8];
  res_78 = res_71 + _77;
  _2 = a[9];
  res_11 = _2 + res_78;
  a ={v} {CLOBBER};
  return res_11;
...

Loop vectorization has no effect, and the scalar loops are completely unrolled.
Then slp vectorization vectorizes the stores. 

When disabling slp vectorization, we have instead:
...
return 285;
...


[ FWIW, adding an extra fre pass here also results in optimal gimple:
...
diff --git a/gcc/passes.def b/gcc/passes.def
index 3ebcfc30349..6b64f600c4a 100644
--- a/gcc/passes.def
+++ b/gcc/passes.def
@@ -325,6 +325,7 @@ along with GCC; see the file COPYING3.  If not see
       NEXT_PASS (pass_tracer);
       NEXT_PASS (pass_thread_jumps);
       NEXT_PASS (pass_dominator, false /* may_peel_loop_headers_p */);
+      NEXT_PASS (pass_fre);
       NEXT_PASS (pass_strlen);
       NEXT_PASS (pass_thread_jumps);
       NEXT_PASS (pass_vrp, false /* warn_array_bounds_p */);
...
]