Use of stack VECs in df-scan.c:df_bb_verify

[Diego Novillo]

This is rather old code for you folks, but I'm not quite sure how to 
deal with this issue.

I've converted vec.[hc] to C++ and this meant some subtle changes to how 
VEC(T,stack) works.  We no longer need all those macro expansions.

This means that the allocation function for vectors can detect when a 
stack vector is initially allocated on the heap.  This happens when the 
vector is implicitly allocated when the first element is pushed (and the 
vector is nil).

This was causing an ICE in df_bb_verify() because of this code:

  collection_rec.def_vec = VEC_alloc (df_ref, stack, 128);
  collection_rec.use_vec = VEC_alloc (df_ref, stack, 32);
  collection_rec.eq_use_vec = VEC_alloc (df_ref, stack, 32);
  collection_rec.mw_vec = VEC_alloc (df_mw_hardreg_ptr, stack, 32);

  gcc_assert (bb_info);

  /* Scan the block, one insn at a time, from beginning to end.  */
  FOR_BB_INSNS_REVERSE (bb, insn)
    {
      if (!INSN_P (insn))
        continue;
      df_insn_refs_verify (&collection_rec, bb, insn, true);
      df_free_collection_rec (&collection_rec);
    }


Note that we initially allocate the various vectors on the stack, but 
then the FOR_BB_INSNS_REVERSE loop explicitly deallocates them.  The 
next time df_insns_ref_verify tries to push something on these VECs, the 
pushing routine will try to do an initial allocation on the heap, which 
causes an ICE.

I *think* what you meant in that loop is to clear COLLECTION_REC?  Or 
should we clear it and reallocate the vectors?  I am currently running 
with the call to df_free_collection_rec() taken out, but I'm not sure if 
that's the right thing here.


Thanks.  Diego.