On Wed, 2015-12-09 at 11:24 +0100, Richard Biener wrote:
> > This second case (without the preference for the original IV)
> > generates better code on MIPS because the final assembly
> > has the increment instructions between the loads and the tests
> > of the values being loaded and so there is no delay (or less delay)
> > between the load and use. It seems like this could easily be
> > the case for other platforms too so I was wondering what people
> > thought of this patch:
>
> You don't comment on the comment you remove ... debugging
> programs is also important!
>
> So if then the cost of both cases should be distinguished
> somewhere else, like granting a bonus for increment before
> exit test or so.
>
> Richard.
Here is new patch that tries to do that. It accomplishes the same thing
as my original patch but by checking different features. Basically, for
machines with no autoinc/autodec it has a preference for IVs that don't
change during loop (i.e. var_before == var_after).
What do you think about this approach?
Steve Ellcey
sell...@imgtec.com
2015-12-11 Steve Ellcey <sell...@imgtec.com>
* tree-ssa-loop-ivopts.c (determine_iv_cost): Add cost to ivs that
need to be updated during loop.
diff --git a/gcc/tree-ssa-loop-ivopts.c b/gcc/tree-ssa-loop-ivopts.c
index 98dc451..ecf9737 100644
--- a/gcc/tree-ssa-loop-ivopts.c
+++ b/gcc/tree-ssa-loop-ivopts.c
@@ -5826,6 +5826,14 @@ determine_iv_cost (struct ivopts_data *data, struct
iv_cand *cand)
|| DECL_ARTIFICIAL (SSA_NAME_VAR (cand->var_before)))
cost++;
+ /* If we are not using autoincrement or autodecrement, prefer ivs that
+ do not have to be incremented/decremented during the loop. This can
+ move loads ahead of the instructions that update the address. */
+ if (cand->pos != IP_BEFORE_USE
+ && cand->pos != IP_AFTER_USE
+ && cand->var_before != cand->var_after)
+ cost++;
+
/* Prefer not to insert statements into latch unless there are some
already (so that we do not create unnecessary jumps). */
if (cand->pos == IP_END
