[IDF] Enforce the returned blocks to be sorted.

Summary:
Currently the order of blocks returned by `IDF::calculate` can be
non-deterministic. This was discovered in several attempts to enable
SSAUpdaterBulk for JumpThreading (which led to miscompare in bootstrap between
stage 3 and stage4). Originally, the blocks were put into a priority queue with
a depth level as their key, and this patch adds a DFSIn number as a second key
to specify a deterministic order across blocks from one level.

The solution was suggested by Daniel Berlin.

Reviewers: dberlin, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46646

llvm-svn: 332167

1 files changed, 11 insertions, 5 deletions

diff --git a/llvm/lib/Analysis/IteratedDominanceFrontier.cpp b/llvm/lib/Analysis/IteratedDominanceFrontier.cpp
index 3992657417c..e7751d32aab 100644
--- a/llvm/lib/Analysis/IteratedDominanceFrontier.cpp
+++ b/llvm/lib/Analysis/IteratedDominanceFrontier.cpp
@@ -21,15 +21,20 @@ template <class NodeTy, bool IsPostDom>
 void IDFCalculator<NodeTy, IsPostDom>::calculate(
     SmallVectorImpl<BasicBlock *> &PHIBlocks) {
   // Use a priority queue keyed on dominator tree level so that inserted nodes
-  // are handled from the bottom of the dominator tree upwards.
-  typedef std::pair<DomTreeNode *, unsigned> DomTreeNodePair;
+  // are handled from the bottom of the dominator tree upwards. We also augment
+  // the level with a DFS number to ensure that the blocks are ordered in a
+  // deterministic way.
+  typedef std::pair<DomTreeNode *, std::pair<unsigned, unsigned>>
+      DomTreeNodePair;
   typedef std::priority_queue<DomTreeNodePair, SmallVector<DomTreeNodePair, 32>,
                               less_second> IDFPriorityQueue;
   IDFPriorityQueue PQ;
 
+  DT.updateDFSNumbers();
+
   for (BasicBlock *BB : *DefBlocks) {
     if (DomTreeNode *Node = DT.getNode(BB))
-      PQ.push({Node, Node->getLevel()});
+      PQ.push({Node, std::make_pair(Node->getLevel(), Node->getDFSNumIn())});
   }
 
   SmallVector<DomTreeNode *, 32> Worklist;
@@ -40,7 +45,7 @@ void IDFCalculator<NodeTy, IsPostDom>::calculate(
     DomTreeNodePair RootPair = PQ.top();
     PQ.pop();
     DomTreeNode *Root = RootPair.first;
-    unsigned RootLevel = RootPair.second;
+    unsigned RootLevel = RootPair.second.first;
 
     // Walk all dominator tree children of Root, inspecting their CFG edges with
     // targets elsewhere on the dominator tree. Only targets whose level is at
@@ -77,7 +82,8 @@ void IDFCalculator<NodeTy, IsPostDom>::calculate(
 
         PHIBlocks.emplace_back(SuccBB);
         if (!DefBlocks->count(SuccBB))
-          PQ.push(std::make_pair(SuccNode, SuccLevel));
+          PQ.push(std::make_pair(
+              SuccNode, std::make_pair(SuccLevel, SuccNode->getDFSNumIn())));
       }
 
       for (auto DomChild : *Node) {