Home | History | Annotate | Download | only in Analysis 
      1 //===- IteratedDominanceFrontier.h - Calculate IDF --------------*- C++ -*-===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 /// \brief Compute iterated dominance frontiers using a linear time algorithm.
     11 ///
     12 /// The algorithm used here is based on:
     13 ///
     14 ///   Sreedhar and Gao. A linear time algorithm for placing phi-nodes.
     15 ///   In Proceedings of the 22nd ACM SIGPLAN-SIGACT Symposium on Principles of
     16 ///   Programming Languages
     17 ///   POPL '95. ACM, New York, NY, 62-73.
     18 ///
     19 /// It has been modified to not explicitly use the DJ graph data structure and
     20 /// to directly compute pruned SSA using per-variable liveness information.
     21 //
     22 //===----------------------------------------------------------------------===//
     23 
     24 #ifndef LLVM_ANALYSIS_IDF_H
     25 #define LLVM_ANALYSIS_IDF_H
     26 
     27 #include "llvm/ADT/ArrayRef.h"
     28 #include "llvm/ADT/DenseMap.h"
     29 #include "llvm/ADT/SmallPtrSet.h"
     30 #include "llvm/ADT/SmallVector.h"
     31 
     32 namespace llvm {
     33 
     34 class BasicBlock;
     35 template <class T> class DomTreeNodeBase;
     36 typedef DomTreeNodeBase<BasicBlock> DomTreeNode;
     37 template <class T> class DominatorTreeBase;
     38 
     39 /// \brief Determine the iterated dominance frontier, given a set of defining
     40 /// blocks, and optionally, a set of live-in blocks.
     41 ///
     42 /// In turn, the results can be used to place phi nodes.
     43 ///
     44 /// This algorithm is a linear time computation of Iterated Dominance Frontiers,
     45 /// pruned using the live-in set.
     46 /// By default, liveness is not used to prune the IDF computation.
     47 class IDFCalculator {
     48 
     49 public:
     50   IDFCalculator(DominatorTreeBase<BasicBlock> &DT) : DT(DT), useLiveIn(false) {}
     51 
     52   /// \brief Give the IDF calculator the set of blocks in which the value is
     53   /// defined.  This is equivalent to the set of starting blocks it should be
     54   /// calculating the IDF for (though later gets pruned based on liveness).
     55   ///
     56   /// Note: This set *must* live for the entire lifetime of the IDF calculator.
     57   void setDefiningBlocks(const SmallPtrSetImpl<BasicBlock *> &Blocks) {
     58     DefBlocks = &Blocks;
     59   }
     60 
     61   /// \brief Give the IDF calculator the set of blocks in which the value is
     62   /// live on entry to the block.   This is used to prune the IDF calculation to
     63   /// not include blocks where any phi insertion would be dead.
     64   ///
     65   /// Note: This set *must* live for the entire lifetime of the IDF calculator.
     66 
     67   void setLiveInBlocks(const SmallPtrSetImpl<BasicBlock *> &Blocks) {
     68     LiveInBlocks = &Blocks;
     69     useLiveIn = true;
     70   }
     71 
     72   /// \brief Reset the live-in block set to be empty, and tell the IDF
     73   /// calculator to not use liveness anymore.
     74   void resetLiveInBlocks() {
     75     LiveInBlocks = nullptr;
     76     useLiveIn = false;
     77   }
     78 
     79   /// \brief Calculate iterated dominance frontiers
     80   ///
     81   /// This uses the linear-time phi algorithm based on DJ-graphs mentioned in
     82   /// the file-level comment.  It performs DF->IDF pruning using the live-in
     83   /// set, to avoid computing the IDF for blocks where an inserted PHI node
     84   /// would be dead.
     85   void calculate(SmallVectorImpl<BasicBlock *> &IDFBlocks);
     86 
     87 private:
     88   DominatorTreeBase<BasicBlock> &DT;
     89   bool useLiveIn;
     90   DenseMap<DomTreeNode *, unsigned> DomLevels;
     91   const SmallPtrSetImpl<BasicBlock *> *LiveInBlocks;
     92   const SmallPtrSetImpl<BasicBlock *> *DefBlocks;
     93   SmallVector<BasicBlock *, 32> PHIBlocks;
     94 };
     95 }
     96 #endif
     97