[DTU] Refine the interface and logic of applyUpdates

Summary:
This patch separates two semantics of `applyUpdates`:
1. User provides an accurate CFG diff and the dominator tree is updated according to the difference of `the number of edge insertions` and `the number of edge deletions` to infer the status of an edge before and after the update.
2. User provides a sequence of hints. Updates mentioned in this sequence might never happened and even duplicated.

Logic changes:

Previously, removing invalid updates is considered a side-effect of deduplication and is not guaranteed to be reliable. To handle the second semantic, `applyUpdates` does validity checking before deduplication, which can cause updates that have already been applied to be submitted again. Then, different calls to `applyUpdates` might cause unintended consequences, for example,
```
DTU(Lazy) and Edge A->B exists.
1. DTU.applyUpdates({{Delete, A, B}, {Insert, A, B}}) // User expects these 2 updates result in a no-op, but {Insert, A, B} is queued
2. Remove A->B
3. DTU.applyUpdates({{Delete, A, B}}) // DTU cancels this update with {Insert, A, B} mentioned above together (Unintended)
```
But by restricting the precondition that updates of an edge need to be strictly ordered as how CFG changes were made, we can infer the initial status of this edge to resolve this issue.

Interface changes:
The second semantic of `applyUpdates`  is separated to `applyUpdatesPermissive`.
These changes enable DTU(Lazy) to use the first semantic if needed, which is quite useful in `transforms/utils`.

Reviewers: kuhar, brzycki, dmgreen, grosser

Reviewed By: brzycki

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58170

llvm-svn: 354669

Author: NutshellySima

llvm/include/llvm/Analysis/DomTreeUpdater.h

@@ -103,8 +103,24 @@ class DomTreeUpdater {
   /// Although GenericDomTree provides several update primitives,
   /// it is not encouraged to use these APIs directly.
 
-  /// Submit updates to all available trees. Under Eager UpdateStrategy with
-  /// ForceRemoveDuplicates enabled or under Lazy UpdateStrategy, it will
+  /// Submit updates to all available trees.
+  /// The Eager Strategy flushes updates immediately while the Lazy Strategy
+  /// queues the updates.
+  ///
+  /// Note: The "existence" of an edge in a CFG refers to the CFG which DTU is
+  /// in sync with + all updates before that single update.
+  ///
+  /// CAUTION!
+  /// 1. It is required for the state of the LLVM IR to be updated
+  /// *before* submitting the updates because the internal update routine will
+  /// analyze the current state of the CFG to determine whether an update
+  /// is valid.
+  /// 2. It is illegal to submit any update that has already been submitted,
+  /// i.e., you are supposed not to insert an existent edge or delete a
+  /// nonexistent edge.
+  void applyUpdates(ArrayRef<DominatorTree::UpdateType> Updates);
+
+  /// Submit updates to all available trees. It will also
   /// 1. discard duplicated updates,
   /// 2. remove invalid updates. (Invalid updates means deletion of an edge that
   /// still exists or insertion of an edge that does not exist.)
@@ -122,8 +138,10 @@ class DomTreeUpdater {
   /// 2. It is illegal to submit any update that has already been submitted,
   /// i.e., you are supposed not to insert an existent edge or delete a
   /// nonexistent edge.
-  void applyUpdates(ArrayRef<DominatorTree::UpdateType> Updates,
-                    bool ForceRemoveDuplicates = false);
+  /// 3. It is only legal to submit updates to an edge in the order CFG changes
+  /// are made. The order you submit updates on different edges is not
+  /// restricted.
+  void applyUpdatesPermissive(ArrayRef<DominatorTree::UpdateType> Updates);
 
   /// Notify DTU that the entry block was replaced.
   /// Recalculate all available trees and flush all BasicBlocks
@@ -149,7 +167,7 @@ class DomTreeUpdater {
   /// submitted. }
   LLVM_ATTRIBUTE_DEPRECATED(void insertEdgeRelaxed(BasicBlock *From,
                                                    BasicBlock *To),
-                            "Use applyUpdates() instead.");
+                            "Use applyUpdatesPermissive() instead.");
 
   /// \deprecated { Submit an edge deletion to all available trees. The Eager
   /// Strategy flushes this update immediately while the Lazy Strategy queues
@@ -171,7 +189,7 @@ class DomTreeUpdater {
   /// submitted. }
   LLVM_ATTRIBUTE_DEPRECATED(void deleteEdgeRelaxed(BasicBlock *From,
                                                    BasicBlock *To),
-                            "Use applyUpdates() instead.");
+                            "Use applyUpdatesPermissive() instead.");
 
   /// Delete DelBB. DelBB will be removed from its Parent and
   /// erased from available trees if it exists and finally get deleted.
@@ -260,11 +278,6 @@ class DomTreeUpdater {
   /// Returns true if at least one BasicBlock is deleted.
   bool forceFlushDeletedBB();
 
-  /// Deduplicate and remove unnecessary updates (no-ops) when using Lazy
-  /// UpdateStrategy. Returns true if the update is queued for update.
-  bool applyLazyUpdate(DominatorTree::UpdateKind Kind, BasicBlock *From,
-                       BasicBlock *To);
-
   /// Helper function to apply all pending DomTree updates.
   void applyDomTreeUpdates();
 

llvm/lib/Analysis/DomTreeUpdater.cpp

@@ -12,11 +12,13 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Analysis/DomTreeUpdater.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/Analysis/PostDominators.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/Support/GenericDomTree.h"
 #include <algorithm>
 #include <functional>
+#include <utility>
 
 namespace llvm {
 
@@ -53,41 +55,6 @@ bool DomTreeUpdater::isSelfDominance(
   return Update.getFrom() == Update.getTo();
 }
 
-bool DomTreeUpdater::applyLazyUpdate(DominatorTree::UpdateKind Kind,
-                                     BasicBlock *From, BasicBlock *To) {
-  assert((DT || PDT) &&
-         "Call applyLazyUpdate() when both DT and PDT are nullptrs.");
-  assert(Strategy == DomTreeUpdater::UpdateStrategy::Lazy &&
-         "Call applyLazyUpdate() with Eager strategy error");
-  // Analyze pending updates to determine if the update is unnecessary.
-  const DominatorTree::UpdateType Update = {Kind, From, To};
-  const DominatorTree::UpdateType Invert = {Kind != DominatorTree::Insert
-                                                ? DominatorTree::Insert
-                                                : DominatorTree::Delete,
-                                            From, To};
-  // Only check duplicates in updates that are not applied by both trees.
-  auto I =
-      PendUpdates.begin() + std::max(PendDTUpdateIndex, PendPDTUpdateIndex);
-  const auto E = PendUpdates.end();
-
-  assert(I <= E && "Iterator out of range.");
-
-  for (; I != E; ++I) {
-    if (Update == *I)
-      return false; // Discard duplicate updates.
-
-    if (Invert == *I) {
-      // Update and Invert are both valid (equivalent to a no-op). Remove
-      // Invert from PendUpdates and discard the Update.
-      PendUpdates.erase(I);
-      return false;
-    }
-  }
-
-  PendUpdates.push_back(Update); // Save the valid update.
-  return true;
-}
-
 void DomTreeUpdater::applyDomTreeUpdates() {
   // No pending DomTreeUpdates.
   if (Strategy != UpdateStrategy::Lazy || !DT)
@@ -261,31 +228,15 @@ void DomTreeUpdater::validateDeleteBB(BasicBlock *DelBB) {
   new UnreachableInst(DelBB->getContext(), DelBB);
 }
 
-void DomTreeUpdater::applyUpdates(ArrayRef<DominatorTree::UpdateType> Updates,
-                                  bool ForceRemoveDuplicates) {
+void DomTreeUpdater::applyUpdates(ArrayRef<DominatorTree::UpdateType> Updates) {
   if (!DT && !PDT)
     return;
 
-  if (Strategy == UpdateStrategy::Lazy || ForceRemoveDuplicates) {
-    SmallVector<DominatorTree::UpdateType, 8> Seen;
+  if (Strategy == UpdateStrategy::Lazy) {
     for (const auto U : Updates)
-      // For Lazy UpdateStrategy, avoid duplicates to applyLazyUpdate() to save
-      // on analysis.
-      if (llvm::none_of(
-              Seen,
-              [U](const DominatorTree::UpdateType S) { return S == U; }) &&
-          isUpdateValid(U) && !isSelfDominance(U)) {
-        Seen.push_back(U);
-        if (Strategy == UpdateStrategy::Lazy)
-          applyLazyUpdate(U.getKind(), U.getFrom(), U.getTo());
-      }
-    if (Strategy == UpdateStrategy::Lazy)
-      return;
+      if (!isSelfDominance(U))
+        PendUpdates.push_back(U);
 
-    if (DT)
-      DT->applyUpdates(Seen);
-    if (PDT)
-      PDT->applyUpdates(Seen);
     return;
   }
 
@@ -295,6 +246,60 @@ void DomTreeUpdater::applyUpdates(ArrayRef<DominatorTree::UpdateType> Updates,
     PDT->applyUpdates(Updates);
 }
 
+void DomTreeUpdater::applyUpdatesPermissive(
+    ArrayRef<DominatorTree::UpdateType> Updates) {
+  if (!DT && !PDT)
+    return;
+
+  SmallSet<std::pair<BasicBlock *, BasicBlock *>, 8> Seen;
+  SmallVector<DominatorTree::UpdateType, 8> DeduplicatedUpdates;
+  for (const auto U : Updates) {
+    auto Edge = std::make_pair(U.getFrom(), U.getTo());
+    // Because it is illegal to submit updates that have already been applied
+    // and updates to an edge need to be strictly ordered,
+    // it is safe to infer the existence of an edge from the first update
+    // to this edge.
+    // If the first update to an edge is "Delete", it means that the edge
+    // existed before. If the first update to an edge is "Insert", it means
+    // that the edge didn't exist before.
+    //
+    // For example, if the user submits {{Delete, A, B}, {Insert, A, B}},
+    // because
+    // 1. it is illegal to submit updates that have already been applied,
+    // i.e., user cannot delete an nonexistent edge,
+    // 2. updates to an edge need to be strictly ordered,
+    // So, initially edge A -> B existed.
+    // We can then safely ignore future updates to this edge and directly
+    // inspect the current CFG:
+    // a. If the edge still exists, because the user cannot insert an existent
+    // edge, so both {Delete, A, B}, {Insert, A, B} actually happened and
+    // resulted in a no-op. DTU won't submit any update in this case.
+    // b. If the edge doesn't exist, we can then infer that {Delete, A, B}
+    // actually happened but {Insert, A, B} was an invalid update which never
+    // happened. DTU will submit {Delete, A, B} in this case.
+    if (!isSelfDominance(U) && Seen.count(Edge) == 0) {
+      Seen.insert(Edge);
+      // If the update doesn't appear in the CFG, it means that
+      // either the change isn't made or relevant operations
+      // result in a no-op.
+      if (isUpdateValid(U)) {
+        if (isLazy())
+          PendUpdates.push_back(U);
+        else
+          DeduplicatedUpdates.push_back(U);
+      }
+    }
+  }
+
+  if (Strategy == UpdateStrategy::Lazy)
+    return;
+
+  if (DT)
+    DT->applyUpdates(DeduplicatedUpdates);
+  if (PDT)
+    PDT->applyUpdates(DeduplicatedUpdates);
+}
+
 DominatorTree &DomTreeUpdater::getDomTree() {
   assert(DT && "Invalid acquisition of a null DomTree");
   applyDomTreeUpdates();
@@ -331,7 +336,7 @@ void DomTreeUpdater::insertEdge(BasicBlock *From, BasicBlock *To) {
     return;
   }
 
-  applyLazyUpdate(DominatorTree::Insert, From, To);
+  PendUpdates.push_back({DominatorTree::Insert, From, To});
 }
 
 void DomTreeUpdater::insertEdgeRelaxed(BasicBlock *From, BasicBlock *To) {
@@ -352,7 +357,7 @@ void DomTreeUpdater::insertEdgeRelaxed(BasicBlock *From, BasicBlock *To) {
     return;
   }
 
-  applyLazyUpdate(DominatorTree::Insert, From, To);
+  PendUpdates.push_back({DominatorTree::Insert, From, To});
 }
 
 void DomTreeUpdater::deleteEdge(BasicBlock *From, BasicBlock *To) {
@@ -377,7 +382,7 @@ void DomTreeUpdater::deleteEdge(BasicBlock *From, BasicBlock *To) {
     return;
   }
 
-  applyLazyUpdate(DominatorTree::Delete, From, To);
+  PendUpdates.push_back({DominatorTree::Delete, From, To});
 }
 
 void DomTreeUpdater::deleteEdgeRelaxed(BasicBlock *From, BasicBlock *To) {
@@ -398,7 +403,7 @@ void DomTreeUpdater::deleteEdgeRelaxed(BasicBlock *From, BasicBlock *To) {
     return;
   }
 
-  applyLazyUpdate(DominatorTree::Delete, From, To);
+  PendUpdates.push_back({DominatorTree::Delete, From, To});
 }
 
 void DomTreeUpdater::dropOutOfDateUpdates() {

llvm/lib/Transforms/Scalar/CallSiteSplitting.cpp

@@ -366,7 +366,7 @@ static void splitCallSite(
     assert(Splits.size() == 2 && "Expected exactly 2 splits!");
     for (unsigned i = 0; i < Splits.size(); i++) {
       Splits[i]->getTerminator()->eraseFromParent();
-      DTU.applyUpdates({{DominatorTree::Delete, Splits[i], TailBB}});
+      DTU.applyUpdatesPermissive({{DominatorTree::Delete, Splits[i], TailBB}});
     }
 
     // Erase the tail block once done with musttail patching

llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

@@ -373,7 +373,7 @@ static bool processSwitch(SwitchInst *SI, LazyValueInfo *LVI,
       ++NumDeadCases;
       Changed = true;
       if (--SuccessorsCount[Succ] == 0)
-        DTU.applyUpdates({{DominatorTree::Delete, BB, Succ}});
+        DTU.applyUpdatesPermissive({{DominatorTree::Delete, BB, Succ}});
       continue;
     }
     if (State == LazyValueInfo::True) {

llvm/lib/Transforms/Scalar/JumpThreading.cpp

@@ -1091,7 +1091,7 @@ bool JumpThreadingPass::ProcessBlock(BasicBlock *BB) {
                       << "' folding undef terminator: " << *BBTerm << '\n');
     BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
     BBTerm->eraseFromParent();
-    DTU->applyUpdates(Updates);
+    DTU->applyUpdatesPermissive(Updates);
     return true;
   }
 
@@ -1159,7 +1159,8 @@ bool JumpThreadingPass::ProcessBlock(BasicBlock *BB) {
             ConstantInt::getFalse(CondCmp->getType());
           ReplaceFoldableUses(CondCmp, CI);
         }
-        DTU->applyUpdates({{DominatorTree::Delete, BB, ToRemoveSucc}});
+        DTU->applyUpdatesPermissive(
+            {{DominatorTree::Delete, BB, ToRemoveSucc}});
         return true;
       }
 
@@ -1246,7 +1247,7 @@ bool JumpThreadingPass::ProcessImpliedCondition(BasicBlock *BB) {
       RemoveSucc->removePredecessor(BB);
       BranchInst::Create(KeepSucc, BI);
       BI->eraseFromParent();
-      DTU->applyUpdates({{DominatorTree::Delete, BB, RemoveSucc}});
+      DTU->applyUpdatesPermissive({{DominatorTree::Delete, BB, RemoveSucc}});
       return true;
     }
     CurrentBB = CurrentPred;
@@ -1676,7 +1677,7 @@ bool JumpThreadingPass::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
       Instruction *Term = BB->getTerminator();
       BranchInst::Create(OnlyDest, Term);
       Term->eraseFromParent();
-      DTU->applyUpdates(Updates);
+      DTU->applyUpdatesPermissive(Updates);
 
       // If the condition is now dead due to the removal of the old terminator,
       // erase it.
@@ -2018,9 +2019,9 @@ bool JumpThreadingPass::ThreadEdge(BasicBlock *BB,
     }
 
   // Enqueue required DT updates.
-  DTU->applyUpdates({{DominatorTree::Insert, NewBB, SuccBB},
-                     {DominatorTree::Insert, PredBB, NewBB},
-                     {DominatorTree::Delete, PredBB, BB}});
+  DTU->applyUpdatesPermissive({{DominatorTree::Insert, NewBB, SuccBB},
+                               {DominatorTree::Insert, PredBB, NewBB},
+                               {DominatorTree::Delete, PredBB, BB}});
 
   // If there were values defined in BB that are used outside the block, then we
   // now have to update all uses of the value to use either the original value,
@@ -2114,7 +2115,7 @@ BasicBlock *JumpThreadingPass::SplitBlockPreds(BasicBlock *BB,
       BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
   }
 
-  DTU->applyUpdates(Updates);
+  DTU->applyUpdatesPermissive(Updates);
   return NewBBs[0];
 }
 
@@ -2387,7 +2388,7 @@ bool JumpThreadingPass::DuplicateCondBranchOnPHIIntoPred(
 
   // Remove the unconditional branch at the end of the PredBB block.
   OldPredBranch->eraseFromParent();
-  DTU->applyUpdates(Updates);
+  DTU->applyUpdatesPermissive(Updates);
 
   ++NumDupes;
   return true;
@@ -2423,8 +2424,8 @@ void JumpThreadingPass::UnfoldSelectInstr(BasicBlock *Pred, BasicBlock *BB,
 
   // The select is now dead.
   SI->eraseFromParent();
-  DTU->applyUpdates({{DominatorTree::Insert, NewBB, BB},
-                    {DominatorTree::Insert, Pred, NewBB}});
+  DTU->applyUpdatesPermissive({{DominatorTree::Insert, NewBB, BB},
+                               {DominatorTree::Insert, Pred, NewBB}});
 
   // Update any other PHI nodes in BB.
   for (BasicBlock::iterator BI = BB->begin();
@@ -2601,7 +2602,7 @@ bool JumpThreadingPass::TryToUnfoldSelectInCurrBB(BasicBlock *BB) {
       Updates.push_back({DominatorTree::Delete, BB, Succ});
       Updates.push_back({DominatorTree::Insert, SplitBB, Succ});
     }
-    DTU->applyUpdates(Updates);
+    DTU->applyUpdatesPermissive(Updates);
     return true;
   }
   return false;

llvm/lib/Transforms/Utils/BasicBlockUtils.cpp

@@ -101,7 +101,7 @@ void llvm::DeleteDeadBlocks(ArrayRef <BasicBlock *> BBs, DomTreeUpdater *DTU,
   DetatchDeadBlocks(BBs, DTU ? &Updates : nullptr, KeepOneInputPHIs);
 
   if (DTU)
-    DTU->applyUpdates(Updates, /*ForceRemoveDuplicates*/ true);
+    DTU->applyUpdatesPermissive(Updates);
 
   for (BasicBlock *BB : BBs)
     if (DTU)
@@ -235,7 +235,7 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
            isa<UnreachableInst>(BB->getTerminator()) &&
            "The successor list of BB isn't empty before "
            "applying corresponding DTU updates.");
-    DTU->applyUpdates(Updates, /*ForceRemoveDuplicates*/ true);
+    DTU->applyUpdatesPermissive(Updates);
     DTU->deleteBB(BB);
   }