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 /// \file
10 /// 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/DenseMap.h"
28 #include "llvm/ADT/SmallPtrSet.h"
29 #include "llvm/ADT/SmallVector.h"
30 #include "llvm/IR/BasicBlock.h"
31 #include "llvm/IR/CFGDiff.h"
32 #include "llvm/IR/Dominators.h"
33 
34 namespace llvm {
35 
36 /// Determine the iterated dominance frontier, given a set of defining
37 /// blocks, and optionally, a set of live-in blocks.
38 ///
39 /// In turn, the results can be used to place phi nodes.
40 ///
41 /// This algorithm is a linear time computation of Iterated Dominance Frontiers,
42 /// pruned using the live-in set.
43 /// By default, liveness is not used to prune the IDF computation.
44 /// The template parameters should be either BasicBlock* or Inverse<BasicBlock
45 /// *>, depending on if you want the forward or reverse IDF.
46 template <class NodeTy, bool IsPostDom>
47 class IDFCalculator {
48  public:
IDFCalculator(DominatorTreeBase<BasicBlock,IsPostDom> & DT)49    IDFCalculator(DominatorTreeBase<BasicBlock, IsPostDom> &DT)
50        : DT(DT), GD(nullptr), useLiveIn(false) {}
51 
IDFCalculator(DominatorTreeBase<BasicBlock,IsPostDom> & DT,const GraphDiff<BasicBlock *,IsPostDom> * GD)52    IDFCalculator(DominatorTreeBase<BasicBlock, IsPostDom> &DT,
53                  const GraphDiff<BasicBlock *, IsPostDom> *GD)
54        : DT(DT), GD(GD), useLiveIn(false) {}
55 
56    /// Give the IDF calculator the set of blocks in which the value is
57    /// defined.  This is equivalent to the set of starting blocks it should be
58    /// calculating the IDF for (though later gets pruned based on liveness).
59    ///
60    /// Note: This set *must* live for the entire lifetime of the IDF calculator.
setDefiningBlocks(const SmallPtrSetImpl<BasicBlock * > & Blocks)61    void setDefiningBlocks(const SmallPtrSetImpl<BasicBlock *> &Blocks) {
62      DefBlocks = &Blocks;
63    }
64 
65   /// Give the IDF calculator the set of blocks in which the value is
66   /// live on entry to the block.   This is used to prune the IDF calculation to
67   /// not include blocks where any phi insertion would be dead.
68   ///
69   /// Note: This set *must* live for the entire lifetime of the IDF calculator.
70 
setLiveInBlocks(const SmallPtrSetImpl<BasicBlock * > & Blocks)71   void setLiveInBlocks(const SmallPtrSetImpl<BasicBlock *> &Blocks) {
72     LiveInBlocks = &Blocks;
73     useLiveIn = true;
74   }
75 
76   /// Reset the live-in block set to be empty, and tell the IDF
77   /// calculator to not use liveness anymore.
resetLiveInBlocks()78   void resetLiveInBlocks() {
79     LiveInBlocks = nullptr;
80     useLiveIn = false;
81   }
82 
83   /// Calculate iterated dominance frontiers
84   ///
85   /// This uses the linear-time phi algorithm based on DJ-graphs mentioned in
86   /// the file-level comment.  It performs DF->IDF pruning using the live-in
87   /// set, to avoid computing the IDF for blocks where an inserted PHI node
88   /// would be dead.
89   void calculate(SmallVectorImpl<BasicBlock *> &IDFBlocks);
90 
91 private:
92  DominatorTreeBase<BasicBlock, IsPostDom> &DT;
93  const GraphDiff<BasicBlock *, IsPostDom> *GD;
94  bool useLiveIn;
95  const SmallPtrSetImpl<BasicBlock *> *LiveInBlocks;
96  const SmallPtrSetImpl<BasicBlock *> *DefBlocks;
97 };
98 typedef IDFCalculator<BasicBlock *, false> ForwardIDFCalculator;
99 typedef IDFCalculator<Inverse<BasicBlock *>, true> ReverseIDFCalculator;
100 }
101 #endif
102