[DDG] Data Dependence Graph - Topological Sort (Memory Leak Fix)

Summary:
This fixes the memory leak in bec37c3fc766a7b97f8c52c181c325fd47b75259
and re-delivers the reverted patch.
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

2 files changed, 45 insertions, 5 deletions

diff --git a/llvm/lib/Analysis/DDG.cpp b/llvm/lib/Analysis/DDG.cpp
index 82ccea06f28..90ce13e6f65 100644
--- a/llvm/lib/Analysis/DDG.cpp
+++ b/llvm/lib/Analysis/DDG.cpp
@@ -9,7 +9,9 @@
 // The implementation for the data dependence graph.
 //===----------------------------------------------------------------------===//
 #include "llvm/Analysis/DDG.h"
+#include "llvm/ADT/SCCIterator.h"
 #include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/LoopIterator.h"
 #include "llvm/Support/CommandLine.h"
 
 using namespace llvm;
@@ -179,19 +181,28 @@ using BasicBlockListType = SmallVector<BasicBlock *, 8>;
 
 DataDependenceGraph::DataDependenceGraph(Function &F, DependenceInfo &D)
     : DependenceGraphInfo(F.getName().str(), D) {
+  // Put the basic blocks in program order for correct dependence
+  // directions.
   BasicBlockListType BBList;
-  for (auto &BB : F.getBasicBlockList())
-    BBList.push_back(&BB);
+  for (auto &SCC : make_range(scc_begin(&F), scc_end(&F)))
+    for (BasicBlock * BB : SCC)
+      BBList.push_back(BB);
+  std::reverse(BBList.begin(), BBList.end());
   DDGBuilder(*this, D, BBList).populate();
 }
 
-DataDependenceGraph::DataDependenceGraph(const Loop &L, DependenceInfo &D)
+DataDependenceGraph::DataDependenceGraph(Loop &L, LoopInfo &LI,
+                                         DependenceInfo &D)
     : DependenceGraphInfo(Twine(L.getHeader()->getParent()->getName() + "." +
                                 L.getHeader()->getName())
                               .str(),
                           D) {
+  // Put the basic blocks in program order for correct dependence
+  // directions.
+  LoopBlocksDFS DFS(&L);
+  DFS.perform(&LI);
   BasicBlockListType BBList;
-  for (BasicBlock *BB : L.blocks())
+  for (BasicBlock *BB : make_range(DFS.beginRPO(), DFS.endRPO()))
     BBList.push_back(BB);
   DDGBuilder(*this, D, BBList).populate();
 }
@@ -259,7 +270,7 @@ DDGAnalysis::Result DDGAnalysis::run(Loop &L, LoopAnalysisManager &AM,
                                      LoopStandardAnalysisResults &AR) {
   Function *F = L.getHeader()->getParent();
   DependenceInfo DI(F, &AR.AA, &AR.SE, &AR.LI);
-  return std::make_unique<DataDependenceGraph>(L, DI);
+  return std::make_unique<DataDependenceGraph>(L, AR.LI, DI);
 }
 AnalysisKey DDGAnalysis::Key;
 
diff --git a/llvm/lib/Analysis/DependenceGraphBuilder.cpp b/llvm/lib/Analysis/DependenceGraphBuilder.cpp
index 115f5d6e814..98bb09d792b 100644
--- a/llvm/lib/Analysis/DependenceGraphBuilder.cpp
+++ b/llvm/lib/Analysis/DependenceGraphBuilder.cpp
@@ -353,5 +353,34 @@ 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;
+  using NodeKind = typename NodeType::NodeKind;
+  for (NodeType *N : post_order(&Graph)) {
+    if (N->getKind() == NodeKind::PiBlock) {
+      // Put members of the pi-block right after the pi-block itself, for
+      // convenience.
+      const NodeListType &PiBlockMembers = getNodesInPiBlock(*N);
+      NodesInPO.insert(NodesInPO.end(), PiBlockMembers.begin(),
+                       PiBlockMembers.end());
+    }
+    NodesInPO.push_back(N);
+  }
+
+  size_t OldSize = Graph.Nodes.size();
+  Graph.Nodes.clear();
+  for (NodeType *N : reverse(NodesInPO))
+    Graph.Nodes.push_back(N);
+  if (Graph.Nodes.size() != OldSize)
+    assert(false &&
+           "Expected the number of nodes to stay the same after the sort");
+}
+
 template class llvm::AbstractDependenceGraphBuilder<DataDependenceGraph>;
 template class llvm::DependenceGraphInfo<DDGNode>;