[Bug tree-optimization/65363] New: trivial redundant code reordering makes code less optimal

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65363

            Bug ID: 65363
           Summary: trivial redundant code reordering makes code less
                    optimal
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org

Consider this test-case test.c (based on PR65270 comment 27/28):
...
#define N 100000
struct a 
{
  int a[N];
};
typedef struct a misaligned_t __attribute__ ((aligned (8)));
typedef struct a aligned_t __attribute__ ((aligned (32)));

static void
__attribute__ ((noinline))
__attribute__ ((noclone))
__attribute__ ((used))
t (void *a, aligned_t *d)
{
  int v, v2;
  int i;
  for (i=0; i < N; i++)
    {
#if REORDER
      v2 = ((misaligned_t *)a)->a[i];
      v = ((aligned_t *)a)->a[i];
#else
      v = ((aligned_t *)a)->a[i];
      v2 = ((misaligned_t *)a)->a[i];
#endif
      d->a[i] += v + v2;
    }
}

aligned_t aa;
aligned_t d;

int
main (void)
{
  t (&aa, &d);
  return 0;
}
...

Changing the order of loads in the loop body results in different instructions
(and I assume the unaligned one (movdqu) is more expensive than the aligned one
(movdqa)):
...
$ n=0; gcc -O2 -ftree-vectorize test.c -DREORDER=$n -S -o $n
$ n=1; gcc -O2 -ftree-vectorize test.c -DREORDER=$n -S -o $n
$ diff -u 0 1
--- 0    2015-03-09 15:46:41.395919753 +0100
+++ 1    2015-03-09 15:46:43.747919840 +0100
@@ -19,7 +19,7 @@
     .p2align 4,,10
     .p2align 3
 .L4:
-    movdqa    (%rdi,%rax), %xmm0
+    movdqu    (%rdi,%rax), %xmm0
     paddd    %xmm0, %xmm0
     paddd    (%rsi,%rax), %xmm0
     movaps    %xmm0, (%rsi,%rax)
...

The two loads are redundant, and fre is the pass that picks the first one and
eliminates the second one. I'm not sure though whether you want to fix this
particular example in fre. Perhaps you want to propagate alignment before doing
fre.

OTOH, fre does not take the cost of the value producing statements into account
when determining which to choose as representative and which to eliminate.