[DTU] Refine the document of mutation APIs [NFC] (PR40528)

Summary:
It was pointed out in [[ https://bugs.llvm.org/show_bug.cgi?id=40528 | Bug 40528 ]] that it is not clear whether insert/deleteEdge can be used to perform multiple updates and [[ https://reviews.llvm.org/D57316#1388344 | a comment in D57316 ]] reveals that the difference between several ways to update the DominatorTree is confusing.

This patch tries to address issues above.

Reviewers: mkazantsev, kuhar, asbirlea, chandlerc, brzycki

Reviewed By: mkazantsev, kuhar, brzycki

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57881

llvm-svn: 354437

llvm/include/llvm/Analysis/DomTreeUpdater.h

@@ -82,60 +82,89 @@ public:
   /// Returns false under Eager UpdateStrategy or PDT is nullptr.
   bool hasPendingPostDomTreeUpdates() const;
 
-  /// Apply updates on all available trees. Under Eager UpdateStrategy with
+  ///@{
+  /// \name Mutation APIs
+  ///
+  /// These methods provide APIs for submitting updates to the DominatorTree and
+  /// the PostDominatorTree.
+  ///
+  /// Note: There are two strategies to update the DominatorTree and the
+  /// PostDominatorTree:
+  /// 1. Eager UpdateStrategy: Updates are submitted and then flushed
+  /// immediately.
+  /// 2. Lazy UpdateStrategy: Updates are submitted but only flushed when you
+  /// explicitly call Flush APIs. It is recommended to use this update strategy
+  /// when you submit a bunch of updates multiple times which can then
+  /// add up to a large number of updates between two queries on the
+  /// DominatorTree. The incremental updater can reschedule the updates or
+  /// decide to recalculate the dominator tree in order to speedup the updating
+  /// process depending on the number of updates.
+  ///
+  /// 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
-  /// discard duplicated updates and self-dominance updates. If both DT and PDT
-  /// are nullptrs, this function discards all updates. The Eager Strategy
-  /// applies the updates immediately while the Lazy Strategy queues the
-  /// updates. It is required for the state of the LLVM IR to be updated
-  /// *before* applying the Updates because the internal update routine will
-  /// analyze the current state of the relationship between a pair of (From, To)
-  /// BasicBlocks to determine whether a single update needs to be discarded.
+  /// 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.)
+  /// 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,
                     bool ForceRemoveDuplicates = false);
 
-  /// Notify all available trees on an edge insertion. If both DT and PDT are
-  /// nullptrs, this function discards the update. Under either Strategy,
-  /// self-dominance update will be removed. The Eager Strategy applies
-  /// the update immediately while the Lazy Strategy queues the update.
-  /// It is recommended to only use this method when you have exactly one
-  /// insertion (and no deletions). It is recommended to use applyUpdates() in
-  /// all other cases. This function has to be called *after* making the update
-  /// on the actual CFG. An internal functions checks if the edge exists in the
-  /// CFG in DEBUG mode.
+  /// Notify DTU that the entry block was replaced.
+  /// Recalculate all available trees and flush all BasicBlocks
+  /// awaiting deletion immediately.
+  void recalculate(Function &F);
+
+  /// \deprecated { Submit an edge insertion to all available trees. The Eager
+  /// Strategy flushes this update immediately while the Lazy Strategy queues
+  /// the update. An internal function checks if the edge exists in the CFG in
+  /// DEBUG mode. CAUTION! This function has to be called *after* making the
+  /// update on the actual CFG. It is illegal to submit any update that has
+  /// already been applied. }
   void insertEdge(BasicBlock *From, BasicBlock *To);
 
-  /// Notify all available trees on an edge insertion.
-  /// Under either Strategy, the following updates will be discard silently
-  /// 1. Invalid - Inserting an edge that does not exist in the CFG.
-  /// 2. Self-dominance update.
-  /// 3. Both DT and PDT are nullptrs.
-  /// The Eager Strategy applies the update immediately while the Lazy Strategy
-  /// queues the update. It is recommended to only use this method when you have
-  /// exactly one insertion (and no deletions) and want to discard an invalid
-  /// update.
+  /// \deprecated {Submit an edge insertion to all available trees.
+  /// Under either Strategy, an invalid update will be discard silently.
+  /// Invalid update means inserting an edge that does not exist in the CFG.
+  /// The Eager Strategy flushes this update immediately while the Lazy Strategy
+  /// queues the update. It is only recommended to use this method when you
+  /// want to discard an invalid update.
+  /// CAUTION! It is illegal to submit any update that has already been
+  /// submitted. }
   void insertEdgeRelaxed(BasicBlock *From, BasicBlock *To);
 
-  /// Notify all available trees on an edge deletion. If both DT and PDT are
-  /// nullptrs, this function discards the update. Under either Strategy,
-  /// self-dominance update will be removed. The Eager Strategy applies
-  /// the update immediately while the Lazy Strategy queues the update.
-  /// It is recommended to only use this method when you have exactly one
-  /// deletion (and no insertions). It is recommended to use applyUpdates() in
-  /// all other cases. This function has to be called *after* making the update
-  /// on the actual CFG. An internal functions checks if the edge doesn't exist
-  /// in the CFG in DEBUG mode.
+  /// \deprecated { Submit an edge deletion to all available trees. The Eager
+  /// Strategy flushes this update immediately while the Lazy Strategy queues
+  /// the update. An internal function checks if the edge doesn't exist in the
+  /// CFG in DEBUG mode.
+  /// CAUTION! This function has to be called *after* making the update on the
+  /// actual CFG. It is illegal to submit any update that has already been
+  /// submitted. }
   void deleteEdge(BasicBlock *From, BasicBlock *To);
 
-  /// Notify all available trees on an edge deletion.
-  /// Under either Strategy, the following updates will be discard silently
-  /// 1. Invalid - Deleting an edge that still exists in the CFG.
-  /// 2. Self-dominance update.
-  /// 3. Both DT and PDT are nullptrs.
-  /// The Eager Strategy applies the update immediately while the Lazy Strategy
-  /// queues the update. It is recommended to only use this method when you have
-  /// exactly one deletion (and no insertions) and want to discard an invalid
-  /// update.
+  /// \deprecated { Submit an edge deletion to all available trees.
+  /// Under either Strategy, an invalid update will be discard silently.
+  /// Invalid update means deleting an edge that exists in the CFG.
+  /// The Eager Strategy flushes this update immediately while the Lazy Strategy
+  /// queues the update. It is only recommended to use this method when you
+  /// want to discard an invalid update.
+  /// CAUTION! It is illegal to submit any update that has already been
+  /// submitted. }
   void deleteEdgeRelaxed(BasicBlock *From, BasicBlock *To);
 
   /// Delete DelBB. DelBB will be removed from its Parent and
@@ -158,27 +187,32 @@ public:
   void callbackDeleteBB(BasicBlock *DelBB,
                         std::function<void(BasicBlock *)> Callback);
 
-  /// Recalculate all available trees and flush all BasicBlocks
-  /// awaiting deletion immediately.
-  void recalculate(Function &F);
+  ///@}
+
+  ///@{
+  /// \name Flush APIs
+  ///
+  /// CAUTION! By the moment these flush APIs are called, the current CFG needs
+  /// to be the same as the CFG which DTU is in sync with + all updates
+  /// submitted.
 
   /// Flush DomTree updates and return DomTree.
-  /// It also flush out of date updates applied by all available trees
-  /// and flush Deleted BBs if both trees are up-to-date.
+  /// It flushes Deleted BBs if both trees are up-to-date.
   /// It must only be called when it has a DomTree.
   DominatorTree &getDomTree();
 
   /// Flush PostDomTree updates and return PostDomTree.
-  /// It also flush out of date updates applied by all available trees
-  /// and flush Deleted BBs if both trees are up-to-date.
+  /// It flushes Deleted BBs if both trees are up-to-date.
   /// It must only be called when it has a PostDomTree.
   PostDominatorTree &getPostDomTree();
 
   /// Apply all pending updates to available trees and flush all BasicBlocks
   /// awaiting deletion.
-  /// Does nothing under Eager UpdateStrategy.
+
   void flush();
 
+  ///@}
+
   /// Debug method to help view the internal state of this class.
   LLVM_DUMP_METHOD void dump() const;