[DDG] Data Dependence Graph - Topological Sort

Summary:
In this patch the DDG DAG is sorted topologically to put the
nodes in the graph in the order that would satisfy all
dependencies. This helps transformations that would like to
generate code based on the DDG. Since the DDG is a DAG a
reverse-post-order traversal would give us the topological
ordering. This patch also sorts the basic blocks passed to
the builder based on program order to ensure that the
dependencies are computed in the correct direction.

Authored By: bmahjour

Reviewer: Meinersbur, fhahn, myhsu, xtian, dmgreen, kbarton, jdoerfert

Reviewed By: Meinersbur

Subscribers: ychen, arphaman, simoll, a.elovikov, mgorny, hiraditya, jfb, wuzish, llvm-commits, jsji, Whitney, etiotto, ppc-slack

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70609

1 files changed, 16 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/DependenceGraphBuilder.cpp b/llvm/lib/Analysis/DependenceGraphBuilder.cpp
index 115f5d6e814..a1ebe38b578 100644
--- a/llvm/lib/Analysis/DependenceGraphBuilder.cpp
+++ b/llvm/lib/Analysis/DependenceGraphBuilder.cpp
@@ -353,5 +353,21 @@ void AbstractDependenceGraphBuilder<G>::createMemoryDependencyEdges() {
   }
 }
 
+template <class G>
+void AbstractDependenceGraphBuilder<G>::sortNodesTopologically() {
+
+  // If we don't create pi-blocks, then we may not have a DAG.
+  if (!shouldCreatePiBlocks())
+    return;
+
+  SmallVector<NodeType *, 64> NodesInPO;
+  for (NodeType *N : post_order(&Graph))
+    NodesInPO.push_back(N);
+  
+  Graph.Nodes.clear();
+  for (auto &N : make_range(NodesInPO.rbegin(), NodesInPO.rend()))
+    Graph.Nodes.push_back(N);
+}
+
 template class llvm::AbstractDependenceGraphBuilder<DataDependenceGraph>;
 template class llvm::DependenceGraphInfo<DDGNode>;