[SLP] Stop counting cost of gather sequences with multiple uses

When building the SLP tree, we look for reuse among the vectorized tree
entries. However, each gather sequence is represented by a unique tree entry,
even though the sequence may be identical to another one. This means, for
example, that a gather sequence with two uses will be counted twice when
computing the cost of the tree. We should only count the cost of the definition
of a gather sequence rather than its uses. During code generation, the
redundant gather sequences are emitted, but we optimize them away with CSE. So
it looks like this problem just affects the cost model.

Differential Revision: https://reviews.llvm.org/D44742

llvm-svn: 328316

1 files changed, 22 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index b21a9e0250a..af74acb4150 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -2457,7 +2457,28 @@ int BoUpSLP::getTreeCost() {
 
   unsigned BundleWidth = VectorizableTree[0].Scalars.size();
 
-  for (TreeEntry &TE : VectorizableTree) {
+  for (unsigned I = 0, E = VectorizableTree.size(); I < E; ++I) {
+    TreeEntry &TE = VectorizableTree[I];
+
+    // We create duplicate tree entries for gather sequences that have multiple
+    // uses. However, we should not compute the cost of duplicate sequences.
+    // For example, if we have a build vector (i.e., insertelement sequence)
+    // that is used by more than one vector instruction, we only need to
+    // compute the cost of the insertelement instructions once. The redundent
+    // instructions will be eliminated by CSE.
+    //
+    // We should consider not creating duplicate tree entries for gather
+    // sequences, and instead add additional edges to the tree representing
+    // their uses. Since such an approach results in fewer total entries,
+    // existing heuristics based on tree size may yeild different results.
+    //
+    if (TE.NeedToGather &&
+        std::any_of(std::next(VectorizableTree.begin(), I + 1),
+                    VectorizableTree.end(), [TE](TreeEntry &Entry) {
+                      return Entry.NeedToGather && Entry.isSame(TE.Scalars);
+                    }))
+      continue;
+
     int C = getEntryCost(&TE);
     DEBUG(dbgs() << "SLP: Adding cost " << C << " for bundle that starts with "
                  << *TE.Scalars[0] << ".\n");