[DA] DivergenceAnalysis for unstructured, reducible CFGs

Summary:
This is patch 2 of the new DivergenceAnalysis (https://reviews.llvm.org/D50433).

This patch contains a generic divergence analysis implementation for
unstructured, reducible Control-Flow Graphs. It contains two new classes.
The `SyncDependenceAnalysis` class lazily computes sync dependences, which
relate divergent branches to points of joining divergent control. The
`DivergenceAnalysis` class contains the generic divergence analysis
implementation.

Reviewers: nhaehnle

Reviewed By: nhaehnle

Subscribers: sameerds, kristina, nhaehnle, xbolva00, tschuett, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D51491

llvm-svn: 344734

Author: nhaehnle

llvm/include/llvm/ADT/PostOrderIterator.h

@@ -296,12 +296,15 @@ class ReversePostOrderTraversal {
 
 public:
   using rpo_iterator = typename std::vector<NodeRef>::reverse_iterator;
+  using const_rpo_iterator = typename std::vector<NodeRef>::const_reverse_iterator;
 
   ReversePostOrderTraversal(GraphT G) { Initialize(GT::getEntryNode(G)); }
 
   // Because we want a reverse post order, use reverse iterators from the vector
   rpo_iterator begin() { return Blocks.rbegin(); }
+  const_rpo_iterator begin() const { return Blocks.crbegin(); }
   rpo_iterator end() { return Blocks.rend(); }
+  const_rpo_iterator end() const { return Blocks.crend(); }
 };
 
 } // end namespace llvm

llvm/include/llvm/Analysis/DivergenceAnalysis.h

@@ -0,0 +1,178 @@
+//===- llvm/Analysis/DivergenceAnalysis.h - Divergence Analysis -*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// \file
+// The divergence analysis determines which instructions and branches are
+// divergent given a set of divergent source instructions.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ANALYSIS_DIVERGENCE_ANALYSIS_H
+#define LLVM_ANALYSIS_DIVERGENCE_ANALYSIS_H
+
+#include "llvm/ADT/DenseSet.h"
+#include "llvm/Analysis/SyncDependenceAnalysis.h"
+#include "llvm/IR/Function.h"
+#include "llvm/Pass.h"
+#include <vector>
+
+namespace llvm {
+class Module;
+class Value;
+class Instruction;
+class Loop;
+class raw_ostream;
+class TargetTransformInfo;
+
+/// \brief Generic divergence analysis for reducible CFGs.
+///
+/// This analysis propagates divergence in a data-parallel context from sources
+/// of divergence to all users. It requires reducible CFGs. All assignments
+/// should be in SSA form.
+class DivergenceAnalysis {
+public:
+  /// \brief This instance will analyze the whole function \p F or the loop \p
+  /// RegionLoop.
+  ///
+  /// \param RegionLoop if non-null the analysis is restricted to \p RegionLoop.
+  /// Otherwise the whole function is analyzed.
+  /// \param IsLCSSAForm whether the analysis may assume that the IR in the
+  /// region in in LCSSA form.
+  DivergenceAnalysis(const Function &F, const Loop *RegionLoop,
+                     const DominatorTree &DT, const LoopInfo &LI,
+                     SyncDependenceAnalysis &SDA, bool IsLCSSAForm);
+
+  /// \brief The loop that defines the analyzed region (if any).
+  const Loop *getRegionLoop() const { return RegionLoop; }
+  const Function &getFunction() const { return F; }
+
+  /// \brief Whether \p BB is part of the region.
+  bool inRegion(const BasicBlock &BB) const;
+  /// \brief Whether \p I is part of the region.
+  bool inRegion(const Instruction &I) const;
+
+  /// \brief Mark \p UniVal as a value that is always uniform.
+  void addUniformOverride(const Value &UniVal);
+
+  /// \brief Mark \p DivVal as a value that is always divergent.
+  void markDivergent(const Value &DivVal);
+
+  /// \brief Propagate divergence to all instructions in the region.
+  /// Divergence is seeded by calls to \p markDivergent.
+  void compute();
+
+  /// \brief Whether any value was marked or analyzed to be divergent.
+  bool hasDetectedDivergence() const { return !DivergentValues.empty(); }
+
+  /// \brief Whether \p Val will always return a uniform value regardless of its
+  /// operands
+  bool isAlwaysUniform(const Value &Val) const;
+
+  /// \brief Whether \p Val is a divergent value
+  bool isDivergent(const Value &Val) const;
+
+  void print(raw_ostream &OS, const Module *) const;
+
+private:
+  bool updateTerminator(const TerminatorInst &Term) const;
+  bool updatePHINode(const PHINode &Phi) const;
+
+  /// \brief Computes whether \p Inst is divergent based on the
+  /// divergence of its operands.
+  ///
+  /// \returns Whether \p Inst is divergent.
+  ///
+  /// This should only be called for non-phi, non-terminator instructions.
+  bool updateNormalInstruction(const Instruction &Inst) const;
+
+  /// \brief Mark users of live-out users as divergent.
+  ///
+  /// \param LoopHeader the header of the divergent loop.
+  ///
+  /// Marks all users of live-out values of the loop headed by \p LoopHeader
+  /// as divergent and puts them on the worklist.
+  void taintLoopLiveOuts(const BasicBlock &LoopHeader);
+
+  /// \brief Push all users of \p Val (in the region) to the worklist
+  void pushUsers(const Value &I);
+
+  /// \brief Push all phi nodes in @block to the worklist
+  void pushPHINodes(const BasicBlock &Block);
+
+  /// \brief Mark \p Block as join divergent
+  ///
+  /// A block is join divergent if two threads may reach it from different
+  /// incoming blocks at the same time.
+  void markBlockJoinDivergent(const BasicBlock &Block) {
+    DivergentJoinBlocks.insert(&Block);
+  }
+
+  /// \brief Whether \p Val is divergent when read in \p ObservingBlock.
+  bool isTemporalDivergent(const BasicBlock &ObservingBlock,
+                           const Value &Val) const;
+
+  /// \brief Whether \p Block is join divergent
+  ///
+  /// (see markBlockJoinDivergent).
+  bool isJoinDivergent(const BasicBlock &Block) const {
+    return DivergentJoinBlocks.find(&Block) != DivergentJoinBlocks.end();
+  }
+
+  /// \brief Propagate control-induced divergence to users (phi nodes and
+  /// instructions).
+  //
+  // \param JoinBlock is a divergent loop exit or join point of two disjoint
+  // paths.
+  // \returns Whether \p JoinBlock is a divergent loop exit of \p TermLoop.
+  bool propagateJoinDivergence(const BasicBlock &JoinBlock,
+                               const Loop *TermLoop);
+
+  /// \brief Propagate induced value divergence due to control divergence in \p
+  /// Term.
+  void propagateBranchDivergence(const TerminatorInst &Term);
+
+  /// \brief Propagate divergent caused by a divergent loop exit.
+  ///
+  /// \param ExitingLoop is a divergent loop.
+  void propagateLoopDivergence(const Loop &ExitingLoop);
+
+private:
+  const Function &F;
+  // If regionLoop != nullptr, analysis is only performed within \p RegionLoop.
+  // Otw, analyze the whole function
+  const Loop *RegionLoop;
+
+  const DominatorTree &DT;
+  const LoopInfo &LI;
+
+  // Recognized divergent loops
+  DenseSet<const Loop *> DivergentLoops;
+
+  // The SDA links divergent branches to divergent control-flow joins.
+  SyncDependenceAnalysis &SDA;
+
+  // Use simplified code path for LCSSA form.
+  bool IsLCSSAForm;
+
+  // Set of known-uniform values.
+  DenseSet<const Value *> UniformOverrides;
+
+  // Blocks with joining divergent control from different predecessors.
+  DenseSet<const BasicBlock *> DivergentJoinBlocks;
+
+  // Detected/marked divergent values.
+  DenseSet<const Value *> DivergentValues;
+
+  // Internal worklist for divergence propagation.
+  std::vector<const Instruction *> Worklist;
+};
+
+} // namespace llvm
+
+#endif // LLVM_ANALYSIS_DIVERGENCE_ANALYSIS_H

llvm/include/llvm/Analysis/SyncDependenceAnalysis.h

@@ -0,0 +1,88 @@
+//===- SyncDependenceAnalysis.h - Divergent Branch Dependence -*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// \file
+// This file defines the SyncDependenceAnalysis class, which computes for
+// every divergent branch the set of phi nodes that the branch will make
+// divergent.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ANALYSIS_SYNC_DEPENDENCE_ANALYSIS_H
+#define LLVM_ANALYSIS_SYNC_DEPENDENCE_ANALYSIS_H
+
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include <memory>
+
+namespace llvm {
+
+class BasicBlock;
+class DominatorTree;
+class Loop;
+class PostDominatorTree;
+class TerminatorInst;
+class TerminatorInst;
+
+using ConstBlockSet = SmallPtrSet<const BasicBlock *, 4>;
+
+/// \brief Relates points of divergent control to join points in
+/// reducible CFGs.
+///
+/// This analysis relates points of divergent control to points of converging
+/// divergent control. The analysis requires all loops to be reducible.
+class SyncDependenceAnalysis {
+  void visitSuccessor(const BasicBlock &succBlock, const Loop *termLoop,
+                      const BasicBlock *defBlock);
+
+public:
+  bool inRegion(const BasicBlock &BB) const;
+
+  ~SyncDependenceAnalysis();
+  SyncDependenceAnalysis(const DominatorTree &DT, const PostDominatorTree &PDT,
+                         const LoopInfo &LI);
+
+  /// \brief Computes divergent join points and loop exits caused by branch
+  /// divergence in \p Term.
+  ///
+  /// The set of blocks which are reachable by disjoint paths from \p Term.
+  /// The set also contains loop exits if there two disjoint paths:
+  /// one from \p Term to the loop exit and another from \p Term to the loop
+  /// header. Those exit blocks are added to the returned set.
+  /// If L is the parent loop of \p Term and an exit of L is in the returned
+  /// set then L is a divergent loop.
+  const ConstBlockSet &join_blocks(const TerminatorInst &Term);
+
+  /// \brief Computes divergent join points and loop exits (in the surrounding
+  /// loop) caused by the divergent loop exits of\p Loop.
+  ///
+  /// The set of blocks which are reachable by disjoint paths from the
+  /// loop exits of \p Loop.
+  /// This treats the loop as a single node in \p Loop's parent loop.
+  /// The returned set has the same properties as for join_blocks(TermInst&).
+  const ConstBlockSet &join_blocks(const Loop &Loop);
+
+private:
+  static ConstBlockSet EmptyBlockSet;
+
+  ReversePostOrderTraversal<const Function *> FuncRPOT;
+  const DominatorTree &DT;
+  const PostDominatorTree &PDT;
+  const LoopInfo &LI;
+
+  std::map<const Loop *, std::unique_ptr<ConstBlockSet>> CachedLoopExitJoins;
+  std::map<const TerminatorInst *, std::unique_ptr<ConstBlockSet>>
+      CachedBranchJoins;
+};
+
+} // namespace llvm
+
+#endif // LLVM_ANALYSIS_SYNC_DEPENDENCE_ANALYSIS_H

llvm/lib/Analysis/CMakeLists.txt

@@ -25,6 +25,7 @@ add_llvm_library(LLVMAnalysis
   Delinearization.cpp
   DemandedBits.cpp
   DependenceAnalysis.cpp
+  DivergenceAnalysis.cpp
   DomPrinter.cpp
   DominanceFrontier.cpp
   EHPersonalities.cpp
@@ -80,6 +81,7 @@ add_llvm_library(LLVMAnalysis
   ScalarEvolutionAliasAnalysis.cpp
   ScalarEvolutionExpander.cpp
   ScalarEvolutionNormalization.cpp
+  SyncDependenceAnalysis.cpp
   SyntheticCountsUtils.cpp
   TargetLibraryInfo.cpp
   TargetTransformInfo.cpp