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Re: [PATCH GCC][12/13]Workaround reduction statements for distribution
On Fri, Jun 16, 2017 at 6:15 PM, Bin.Cheng <email@example.com> wrote:
> On Fri, Jun 16, 2017 at 11:21 AM, Richard Biener
> <firstname.lastname@example.org> wrote:
>> On Mon, Jun 12, 2017 at 7:03 PM, Bin Cheng <Bin.Cheng@arm.com> wrote:
>>> For now, loop distribution handles variables used outside of loop as reduction.
>>> This is inaccurate because all partitions contain statement defining induction
>> But final induction values are usually not used outside of the loop...
> This is in actuality for induction variable which is used outside of the loop.
>> What is missing is loop distribution trying to change partition order. In fact
>> we somehow assume we can move a reduction across a detected builtin
>> (I don't remember if we ever check for validity of that...).
> Hmm, I am not sure when we can't. If there is any dependence between
> builtin/reduction partitions, it should be captured by RDG or PG,
> otherwise the partitions are independent and can be freely ordered as
> long as reduction partition is scheduled last?
>>> Ideally we should factor out scev-propagation as a standalone interface
>>> which can be called when necessary. Before that, this patch simply workarounds
>>> reduction issue by checking if the statement belongs to all partitions. If yes,
>>> the reduction must be computed in the last partition no matter how the loop is
>>> Bootstrap and test on x86_64 and AArch64. Is it OK?
>> stmt_in_all_partitions is not kept up-to-date during partition merging and if
>> merging makes the reduction partition(s) pass the stmt_in_all_partitions
>> test your simple workaround doesn't work ...
> I think it doesn't matter because:
> A) it's really workaround for induction variables. In general,
> induction variables are included by all partition.
> B) After classify partition, we immediately fuses all reduction
> partitions. More stmt_in_all_partitions means we are fusing
> non-reduction partition with reduction partition, so the newly
> generated (stmt_in_all_partitions) are actually not reduction
> statements. The workaround won't work anyway even the bitmap is
>> As written it's a valid optimization but can you please note it's limitation in
>> some comment please?
> Yeah, I will add comment explaining it.
Comment added in new version patch. It also computes bitmap outside
now, is it OK?
2017-06-07 Bin Cheng <email@example.com>
* tree-loop-distribution.c (classify_partition): New parameter and
better handle reduction statement.
(rdg_build_partitions): Revise comment.
(distribute_loop): Compute statements in all partitions and pass it
From b3502d713309da08d93cd53e5fe8fbfdccf3557b Mon Sep 17 00:00:00 2001
From: Bin Cheng <firstname.lastname@example.org>
Date: Fri, 9 Jun 2017 13:21:07 +0100
Subject: [PATCH 11/13] reduction-workaround-20170607.txt
gcc/tree-loop-distribution.c | 43 ++++++++++++++++++++++++++++++++-----------
1 file changed, 32 insertions(+), 11 deletions(-)
diff --git a/gcc/tree-loop-distribution.c b/gcc/tree-loop-distribution.c
index d741e9e..1b4e238 100644
@@ -1226,17 +1226,18 @@ build_rdg_partition_for_vertex (struct graph *rdg, int v)
/* Classifies the builtin kind we can generate for PARTITION of RDG and LOOP.
- For the moment we detect only the memset zero pattern. */
+ For the moment we detect memset, memcpy and memmove patterns. Bitmap
+ STMT_IN_ALL_PARTITIONS contains statements belonging to all partitions. */
-classify_partition (loop_p loop, struct graph *rdg, partition *partition)
+classify_partition (loop_p loop, struct graph *rdg, partition *partition,
+ bitmap stmt_in_all_partitions)
data_reference_p single_load, single_store;
- bool volatiles_p = false;
- bool plus_one = false;
+ bool volatiles_p = false, plus_one = false, has_reduction = false;
partition->kind = PKIND_NORMAL;
partition->main_dr = NULL;
@@ -1251,16 +1252,31 @@ classify_partition (loop_p loop, struct graph *rdg, partition *partition)
if (gimple_has_volatile_ops (stmt))
volatiles_p = true;
- /* If the stmt has uses outside of the loop mark it as reduction. */
+ /* If the stmt is not included by all partitions and there is uses
+ outside of the loop, then mark the partition as reduction. */
if (stmt_has_scalar_dependences_outside_loop (loop, stmt))
- partition->reduction_p = true;
+ /* Due to limitation in the transform phase we have to fuse all
+ reduction partitions. As a result, this could cancel valid
+ loop distribution especially for loop that induction variable
+ is used outside of loop. To workaround this issue, we skip
+ marking partition as reudction if the reduction stmt belongs
+ to all partitions. In such case, reduction will be computed
+ correctly no matter how partitions are fused/distributed. */
+ if (!bitmap_bit_p (stmt_in_all_partitions, i))
+ partition->reduction_p = true;
+ has_reduction = true;
/* Perform general partition disqualification for builtins. */
+ /* Simple workaround to prevent classifying the partition as builtin
+ if it contains any use outside of loop. */
+ || has_reduction
@@ -1435,9 +1451,9 @@ share_memory_accesses (struct graph *rdg,
-/* Aggregate several components into a useful partition that is
- registered in the PARTITIONS vector. Partitions will be
- distributed in different loops. */
+/* For each seed statement in STARTING_STMTS, this function builds
+ partition for it by adding depended statements according to RDG.
+ All partitions are recorded in PARTITIONS. */
rdg_build_partitions (struct graph *rdg,
@@ -1705,10 +1721,15 @@ distribute_loop (struct loop *loop, vec<gimple *> stmts,
auto_vec<struct partition *, 3> partitions;
rdg_build_partitions (rdg, stmts, &partitions);
+ auto_bitmap stmt_in_all_partitions;
+ bitmap_copy (stmt_in_all_partitions, partitions->stmts);
+ for (i = 1; partitions.iterate (i, &partition); ++i)
+ bitmap_and_into (stmt_in_all_partitions, partitions[i]->stmts);
any_builtin = false;
FOR_EACH_VEC_ELT (partitions, i, partition)
- classify_partition (loop, rdg, partition);
+ classify_partition (loop, rdg, partition, stmt_in_all_partitions);
any_builtin |= partition_builtin_p (partition);