[PATCH GCC 6/9]Simplify control flow graph for vectorized loop

[Jeff Law]

On 09/14/2016 07:21 AM, Richard Biener wrote:
> On Tue, Sep 6, 2016 at 8:52 PM, Bin Cheng <Bin.Cheng@arm.com> wrote:
>> 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.
>
> I've skimmed over the patch and it looks reasonable to me.
THanks.  I was maybe 15% of the way through the main patch.  Nothing 
that gave me cause for concern, but I wasn't ready to ACK it myself yet.

jeff