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1 //===-- SpillPlacement.h - Optimal Spill Code Placement --------*- 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 // This analysis computes the optimal spill code placement between basic blocks.
11 //
12 // The runOnMachineFunction() method only precomputes some profiling information
13 // about the CFG. The real work is done by prepare(), addConstraints(), and
14 // finish() which are called by the register allocator.
15 //
16 // Given a variable that is live across multiple basic blocks, and given
17 // constraints on the basic blocks where the variable is live, determine which
18 // edge bundles should have the variable in a register and which edge bundles
19 // should have the variable in a stack slot.
20 //
21 // The returned bit vector can be used to place optimal spill code at basic
22 // block entries and exits. Spill code placement inside a basic block is not
23 // considered.
24 //
25 //===----------------------------------------------------------------------===//
26 
27 #ifndef LLVM_CODEGEN_SPILLPLACEMENT_H
28 #define LLVM_CODEGEN_SPILLPLACEMENT_H
29 
30 #include "llvm/ADT/ArrayRef.h"
31 #include "llvm/ADT/SmallVector.h"
32 #include "llvm/CodeGen/MachineFunctionPass.h"
33 
34 namespace llvm {
35 
36 class BitVector;
37 class EdgeBundles;
38 class MachineBasicBlock;
39 class MachineLoopInfo;
40 
41 class SpillPlacement  : public MachineFunctionPass {
42   struct Node;
43   const MachineFunction *MF;
44   const EdgeBundles *bundles;
45   const MachineLoopInfo *loops;
46   Node *nodes;
47 
48   // Nodes that are active in the current computation. Owned by the prepare()
49   // caller.
50   BitVector *ActiveNodes;
51 
52   // Nodes with active links. Populated by scanActiveBundles.
53   SmallVector<unsigned, 8> Linked;
54 
55   // Nodes that went positive during the last call to scanActiveBundles or
56   // iterate.
57   SmallVector<unsigned, 8> RecentPositive;
58 
59   // Block frequencies are computed once. Indexed by block number.
60   SmallVector<float, 4> BlockFrequency;
61 
62 public:
63   static char ID; // Pass identification, replacement for typeid.
64 
SpillPlacement()65   SpillPlacement() : MachineFunctionPass(ID), nodes(0) {}
~SpillPlacement()66   ~SpillPlacement() { releaseMemory(); }
67 
68   /// BorderConstraint - A basic block has separate constraints for entry and
69   /// exit.
70   enum BorderConstraint {
71     DontCare,  ///< Block doesn't care / variable not live.
72     PrefReg,   ///< Block entry/exit prefers a register.
73     PrefSpill, ///< Block entry/exit prefers a stack slot.
74     MustSpill  ///< A register is impossible, variable must be spilled.
75   };
76 
77   /// BlockConstraint - Entry and exit constraints for a basic block.
78   struct BlockConstraint {
79     unsigned Number;            ///< Basic block number (from MBB::getNumber()).
80     BorderConstraint Entry : 8; ///< Constraint on block entry.
81     BorderConstraint Exit : 8;  ///< Constraint on block exit.
82   };
83 
84   /// prepare - Reset state and prepare for a new spill placement computation.
85   /// @param RegBundles Bit vector to receive the edge bundles where the
86   ///                   variable should be kept in a register. Each bit
87   ///                   corresponds to an edge bundle, a set bit means the
88   ///                   variable should be kept in a register through the
89   ///                   bundle. A clear bit means the variable should be
90   ///                   spilled. This vector is retained.
91   void prepare(BitVector &RegBundles);
92 
93   /// addConstraints - Add constraints and biases. This method may be called
94   /// more than once to accumulate constraints.
95   /// @param LiveBlocks Constraints for blocks that have the variable live in or
96   ///                   live out.
97   void addConstraints(ArrayRef<BlockConstraint> LiveBlocks);
98 
99   /// addLinks - Add transparent blocks with the given numbers.
100   void addLinks(ArrayRef<unsigned> Links);
101 
102   /// scanActiveBundles - Perform an initial scan of all bundles activated by
103   /// addConstraints and addLinks, updating their state. Add all the bundles
104   /// that now prefer a register to RecentPositive.
105   /// Prepare internal data structures for iterate.
106   /// Return true is there are any positive nodes.
107   bool scanActiveBundles();
108 
109   /// iterate - Update the network iteratively until convergence, or new bundles
110   /// are found.
111   void iterate();
112 
113   /// getRecentPositive - Return an array of bundles that became positive during
114   /// the previous call to scanActiveBundles or iterate.
getRecentPositive()115   ArrayRef<unsigned> getRecentPositive() { return RecentPositive; }
116 
117   /// finish - Compute the optimal spill code placement given the
118   /// constraints. No MustSpill constraints will be violated, and the smallest
119   /// possible number of PrefX constraints will be violated, weighted by
120   /// expected execution frequencies.
121   /// The selected bundles are returned in the bitvector passed to prepare().
122   /// @return True if a perfect solution was found, allowing the variable to be
123   ///         in a register through all relevant bundles.
124   bool finish();
125 
126   /// getBlockFrequency - Return the estimated block execution frequency per
127   /// function invocation.
getBlockFrequency(unsigned Number)128   float getBlockFrequency(unsigned Number) const {
129     return BlockFrequency[Number];
130   }
131 
132 private:
133   virtual bool runOnMachineFunction(MachineFunction&);
134   virtual void getAnalysisUsage(AnalysisUsage&) const;
135   virtual void releaseMemory();
136 
137   void activate(unsigned);
138 };
139 
140 } // end namespace llvm
141 
142 #endif
143