Hi,
This is the main patch improving control flow graph for vectorized loop.  It generally rewrites loop peeling stuff in vectorizer.  As described in patch, for a typical loop to be vectorized like:

       preheader:
     LOOP:
       header_bb:
	 loop_body
	 if (exit_loop_cond) goto exit_bb
	 else                goto header_bb
       exit_bb:

This patch peels prolog and epilog from the loop, adds guards skipping PROLOG and EPILOG for various conditions.  As a result, the changed CFG would look like:

       guard_bb_1:
	 if (prefer_scalar_loop) goto merge_bb_1
	 else                    goto guard_bb_2

       guard_bb_2:
         if (skip_prolog) goto merge_bb_2
         else             goto prolog_preheader

       prolog_preheader:
     PROLOG:
       prolog_header_bb:
	 prolog_body
	 if (exit_prolog_cond) goto prolog_exit_bb
	 else                  goto prolog_header_bb
       prolog_exit_bb:

       merge_bb_2:

       vector_preheader:
     VECTOR LOOP:
       vector_header_bb:
	 vector_body
	 if (exit_vector_cond) goto vector_exit_bb
	 else                  goto vector_header_bb
       vector_exit_bb:

       guard_bb_3:
	 if (skip_epilog) goto merge_bb_3
	 else             goto epilog_preheader

       merge_bb_1:

       epilog_preheader:
     EPILOG:
       epilog_header_bb:
	 epilog_body
	 if (exit_epilog_cond) goto merge_bb_3
	 else                  goto epilog_header_bb

       merge_bb_3:


Note this patch peels prolog and epilog only if it's necessary, as well as adds different guard_conditions/branches.  Also the first guard/branch could be further improved by merging it with loop versioning.

Before this patch, up to 4 branch instructions need to be executed before the vectorized loop is reached in the worst case, while the number is reduced to 2 with this patch.  The patch also does better in compile time analysis to avoid unnecessary peeling/branching.
From implementation's point of view, vectorizer needs to update induction variables and iteration bounds along with control flow changes.  Unfortunately, it also becomes much harder to follow because slpeel_* functions updates SSA by itself, rather than using update_ssa interface.  This patch tries to factor out SSA/IV/Niter_bound changes from CFG changes.  This should make the implementation easier to read, and I think it maybe a step forward to replace slpeel_* functions with generic GIMPLE loop copy interfaces as Richard suggested.

Thanks,
bin

2016-09-01  Bin Cheng  <bin.cheng@arm.com>

	* tree-vect-loop-manip.c (adjust_vec_debug_stmts): Don't release
	adjust_vec automatically.
	(slpeel_add_loop_guard): Remove param cond_expr_stmt_list.  Rename
	param exit_bb to guard_to.
	(slpeel_checking_verify_cfg_after_peeling):
	(set_prologue_iterations):
	(create_lcssa_for_virtual_phi): New func which is factored out from
	slpeel_tree_peel_loop_to_edge.
	(slpeel_tree_peel_loop_to_edge):
	(iv_phi_p): New func.
	(vect_can_advance_ivs_p): Call iv_phi_p.
	(vect_update_ivs_after_vectorizer): Call iv_phi_p.  Directly insert
	new gimple stmts in basic block.
	(vect_do_peeling_for_loop_bound):
	(vect_do_peeling_for_alignment):
	(vect_gen_niters_for_prolog_loop): Rename to...
	(vect_gen_prolog_loop_niters): ...Rename from.  Change parameters and
	adjust implementation.
	(vect_update_inits_of_drs): Fix code style issue.  Convert niters to
	sizetype if necessary.
	(vect_build_loop_niters): Move to here from tree-vect-loop.c.  Change
	it to external function.
	(vect_gen_scalar_loop_niters, vect_gen_vector_loop_niters): New.
	(vect_gen_vector_loop_niters_mult_vf): New.
	(slpeel_update_phi_nodes_for_loops): New.
	(slpeel_update_phi_nodes_for_guard1): Reimplement.
	(find_guard_arg, slpeel_update_phi_nodes_for_guard2): Reimplement.
	(slpeel_update_phi_nodes_for_lcssa, vect_do_peeling): New.
	* tree-vect-loop.c (vect_build_loop_niters): Move to file
	tree-vect-loop-manip.c
	(vect_generate_tmps_on_preheader): Delete.
	(vect_transform_loop): Rename vectorization_factor to vf.  Call
	vect_do_peeling instead of vect_do_peeling-* functions.
	* tree-vectorizer.h (vect_do_peeling): New decl.
	(vect_build_loop_niters, vect_gen_vector_loop_niters): New decls.
	(vect_do_peeling_for_loop_bound): Delete.
	(vect_do_peeling_for_alignment): Delete.