On Mon, 17 Mar 2014, Cong Hou wrote: > On Mon, Mar 17, 2014 at 6:44 AM, Richard Biener <rguent...@suse.de> wrote: > > On Fri, 14 Mar 2014, Cong Hou wrote: > > > >> On Fri, Mar 14, 2014 at 12:58 AM, Richard Biener <rguent...@suse.de> wrote: > >> > On Fri, 14 Mar 2014, Jakub Jelinek wrote: > >> > > >> >> On Fri, Mar 14, 2014 at 08:52:07AM +0100, Richard Biener wrote: > >> >> > > Consider this fact and if there are alias checks, we can safely > >> >> > > remove > >> >> > > the epilogue if the maximum trip count of the loop is less than or > >> >> > > equal to the calculated threshold. > >> >> > > >> >> > You have to consider n % vf != 0, so an argument on only maximum > >> >> > trip count or threshold cannot work. > >> >> > >> >> Well, if you only check if maximum trip count is <= vf and you know > >> >> that for n < vf the vectorized loop + it's epilogue path will not be > >> >> taken, > >> >> then perhaps you could, but it is a very special case. > >> >> Now, the question is when we are guaranteed we enter the scalar > >> >> versioned > >> >> loop instead for n < vf, is that in case of versioning for alias or > >> >> versioning for alignment? > >> > > >> > I think neither - I have plans to do the cost model check together > >> > with the versioning condition but didn't get around to implement that. > >> > That would allow stronger max bounds for the epilogue loop. > >> > >> In vect_transform_loop(), check_profitability will be set to true if > >> th >= VF-1 and the number of iteration is unknown (we only consider > >> unknown trip count here), where th is calculated based on the > >> parameter PARAM_MIN_VECT_LOOP_BOUND and cost model, with the minimum > >> value VF-1. If the loop needs to be versioned, then > >> check_profitability with true value will be passed to > >> vect_loop_versioning(), in which an enhanced loop bound check > >> (considering cost) will be built. So I think if the loop is versioned > >> and n < VF, then we must enter the scalar version, and in this case > >> removing epilogue should be safe when the maximum trip count <= th+1. > > > > You mean exactly in the case where the profitability check ensures > > that n % vf == 0? Thus effectively if n == maximum trip count? > > That's quite a special case, no? > > > Yes, it is a special case. But it is in this special case that those > warnings are thrown out. Also, I think declaring an array with VF*N as > length is not unusual.
Ok, but then for the patch compute the cost model threshold once in vect_analyze_loop_2 and store it in a new LOOP_VINFO_COST_MODEL_THRESHOLD. Also you have to check the return value from max_stmt_executions_int as that may return -1 if the number cannot be computed (or isn't representable in a HOST_WIDE_INT). You also should check for LOOP_REQUIRES_VERSIONING_FOR_ALIGNMENT which should have the same effect on the cost model check. The existing condition is already complicated enough - adding new stuff warrants comments before the (sub-)checks. Richard.
