Vectorization: Loop peeling with misaligned support.

["Bingfeng Mei" <bmei at broadcom dot com>]

Hi,
In loop vectorization, I found that vectorizer insists on loop peeling even our target supports misaligned memory access. This results in much bigger code size for a very simple loop. I defined TARGET_VECTORIZE_SUPPORT_VECTOR_MISALGINMENT and also TARGET_VECTORIZE_BUILTIN_VECTORIZATION_COST to make misaligned accesses almost as cheap as an aligned one. But the vectorizer still does peeling anyway.

In vect_enhance_data_refs_alignment function, it seems that result of vect_supportable_dr_alignment is not used in decision of whether to do peeling. 

      supportable_dr_alignment = vect_supportable_dr_alignment (dr, true);
      do_peeling = vector_alignment_reachable_p (dr);

Later on, there is code to compare load/store costs. But it only decides whether to do peeling for load or store, not whether to do peeling.

Currently I have a workaround. For the following simple loop, the size is 80bytes vs. 352 bytes without patch (-O2 -ftree-vectorize gcc 4.8.3 20131114)
int A[100];
int B[100];
void foo2() {
  int i;
  for (i = 0; i < 100; ++i)
    A[i] = B[i] + 100;
}

What is the best way to tell vectorizer not to do peeling in such situation? 


Thanks,
Bingfeng Mei
Broadcom UK