On 12/05/14 05:15, Bin Cheng wrote:
Hi,
Though PR62178 is hidden by recent cost change in aarch64 backend, the ivopt
issue still exists.
Current candidate selecting algorithm tends to select fewer candidates given
below reasons:
1) to better handle loops with many induction uses but the best choice is
one generic basic induction variable;
2) to keep compilation time low.
One fundamental weakness of the strategy is the opposite situation can't be
handled properly sometimes. For these cases the best choice is each
induction variable has its own candidate.
This patch fixes the problem by shuffling candidate set after fix-point is
reached by current implementation. The reason why this strategy works is it
replaces candidate set by selecting local optimal candidate for some
induction uses, and the new candidate set (has lower cost) is exact what we
want in the mentioned case. Instrumentation data shows this can find better
candidates set for ~6% loops in spec2006 on x86_64, and ~4% on aarch64.
This patch actually is extension to the first version patch posted at
https://gcc.gnu.org/ml/gcc-patches/2014-09/msg02620.html, that only adds
another selecting pass with special seed set (more or less like the shuffled
set in this patch). Data also confirms this patch can find optimal sets for
most loops found by the first one, as well as optimal sets for many new
loops.
Bootstrap and test on x86_64, no regression on benchmarks. Bootstrap and
test on aarch64.
Since this patch only selects candidate set with lower cost, any regressions
revealed are latent bugs of other components in GCC.
I also collected GCC bootstrap time on x86_64, no regression either.
Is this OK?
2014-12-03 Bin chengbin.ch...@arm.com
PR tree-optimization/62178
* tree-ssa-loop-ivopts.c (iv_ca_replace): New function.
(try_improve_iv_set): Shuffle candidates set in order to handle
case in which candidate wrto each iv use should be selected.
gcc/testsuite/ChangeLog
2014-12-03 Bin chengbin.ch...@arm.com
PR tree-optimization/62178
* gcc.target/aarch64/pr62178.c: New test.
pr62178-20141202.txt
Index: gcc/tree-ssa-loop-ivopts.c
===================================================================
--- gcc/tree-ssa-loop-ivopts.c (revision 217828)
+++ gcc/tree-ssa-loop-ivopts.c (working copy)
@@ -5718,6 +5718,85 @@ iv_ca_extend (struct ivopts_data *data, struct iv_
return cost;
}
+/* Try replacing candidates in IVS which are recorded by list ACT_DELTA to
+ lower cost candidates. CAND is the one won't be replaced. Replacement
+ of candidate is recorded in list DELTA. */
Is this better written as:
Try replacing candidates in IVS with IVs related to CAND (which is not
changed) if doing so lowers the IV cost. ACT_DELTA is the recorded
list of candidates. Replacement of candidate is recorded in list DELTA.
?
+ if (data->consider_all_candidates)
+ {
+ for (j = 0; j < n_iv_cands (data); j++)
+ {
+ if (j == old_cp->cand->id)
+ continue;
+
+ cnd = iv_cand (data, j);
+ cp = get_use_iv_cost (data, use, cnd);
+ if (!cp)
+ continue;
+
+ if (best_cp == NULL || cheaper_cost_pair (cp, best_cp))
+ best_cp = cp;
+ }
+ }
+ else
+ {
+ EXECUTE_IF_SET_IN_BITMAP (use->related_cands, 0, j, bi)
+ {
+ if (j == old_cp->cand->id)
+ continue;
+
+ cnd = iv_cand (data, j);
+ cp = get_use_iv_cost (data, use, cnd);
+ if (!cp)
+ continue;
+
+ if (best_cp == NULL || cheaper_cost_pair (cp, best_cp))
+ best_cp = cp;
+ }
+ }
The loop bodies here are duplicated. Can you factor them into a
function so that this looks something like
if (data->consider_all_candidates)
{
for (...)
refactored_code (some arguments)
}
else
{
EXECUTE_IF_SET_IN_BITMAP (...)
refactored_code (some arguments)
@@ -6042,8 +6121,50 @@ try_improve_iv_set (struct ivopts_data *data, stru
/* Try removing the candidates from the set instead. */
best_cost = iv_ca_prune (data, ivs, NULL, &best_delta);
- /* Nothing more we can do. */
if (!best_delta)
+ {
+ /* So far candidate selecting algorithm tends to choose fewer IVs
+ so that it can handle cases in which loops have many variables
+ but the best choice is often to use only one general biv. One
+ weakness is it can't handle opposite cases, in which different
+ candidates should be chosen with respect to each use. To solve
+ the problem, we replace candidate of some uses with lower cost
+ one, thus general algorithm can have a chance to find optimal
+ set for these cases. */
So, in essence we've computed a best cost with "minimal" IVs and you're
using that result as an initial state for expanding the IV set. You
expand on a candidate by candidate basis if and only if the estimated
cost lowers. Right?
At each expansion step you keep a single candidate fixed and you try to
derive other IVs from that fixed IV if doing so lowers the cost? Right?
That's what it looks like to me when reading the code/comments. Just
want to make sure it's working the way I think it is.
FWIW, if I read the slides correctly, GCC's IV code was called out as
being one of the reasons GCC is generating better code for AArch64 than
LLVM at a recent LLVM conference. Glad to see that we're called out in
that way and that we continue to try to improve in that space as well.
Jeff
