1 //===-- SpillPlacement.h - Optimal Spill Code Placement --------*- C++ -*--===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This analysis computes the optimal spill code placement between basic blocks.
12 // The runOnMachineFunction() method only precomputes some profiling information
13 // about the CFG. The real work is done by placeSpills() which is called by the
14 // register allocator.
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.
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
25 //===----------------------------------------------------------------------===//
27 #ifndef LLVM_CODEGEN_SPILLPLACEMENT_H
28 #define LLVM_CODEGEN_SPILLPLACEMENT_H
30 #include "llvm/ADT/SmallVector.h"
31 #include "llvm/CodeGen/MachineFunctionPass.h"
37 class MachineBasicBlock;
38 class MachineLoopInfo;
40 class SpillPlacement : public MachineFunctionPass {
42 const MachineFunction *MF;
43 const EdgeBundles *bundles;
44 const MachineLoopInfo *loops;
47 // Nodes that are active in the current computation. Owned by the placeSpills
49 BitVector *ActiveNodes;
51 // Block frequencies are computed once. Indexed by block number.
52 SmallVector<float, 4> BlockFrequency;
55 static char ID; // Pass identification, replacement for typeid.
57 SpillPlacement() : MachineFunctionPass(ID), nodes(0) {}
58 ~SpillPlacement() { releaseMemory(); }
60 /// BorderConstraint - A basic block has separate constraints for entry and
62 enum BorderConstraint {
63 DontCare, ///< Block doesn't care / variable not live.
64 PrefReg, ///< Block entry/exit prefers a register.
65 PrefSpill, ///< Block entry/exit prefers a stack slot.
66 MustSpill ///< A register is impossible, variable must be spilled.
69 /// BlockConstraint - Entry and exit constraints for a basic block.
70 struct BlockConstraint {
71 unsigned Number; ///< Basic block number (from MBB::getNumber()).
72 BorderConstraint Entry : 8; ///< Constraint on block entry.
73 BorderConstraint Exit : 8; ///< Constraint on block exit.
76 /// placeSpills - Compute the optimal spill code placement given the
77 /// constraints. No MustSpill constraints will be violated, and the smallest
78 /// possible number of PrefX constraints will be violated, weighted by
79 /// expected execution frequencies.
80 /// @param LiveBlocks Constraints for blocks that have the variable live in or
81 /// live out. DontCare/DontCare means the variable is live
82 /// through the block. DontCare/X means the variable is live
83 /// out, but not live in.
84 /// @param RegBundles Bit vector to receive the edge bundles where the
85 /// variable should be kept in a register. Each bit
86 /// corresponds to an edge bundle, a set bit means the
87 /// variable should be kept in a register through the
88 /// bundle. A clear bit means the variable should be
90 /// @return True if a perfect solution was found, allowing the variable to be
91 /// in a register through all relevant bundles.
92 bool placeSpills(const SmallVectorImpl<BlockConstraint> &LiveBlocks,
93 BitVector &RegBundles);
95 /// getBlockFrequency - Return the estimated block execution frequency per
96 /// function invocation.
97 float getBlockFrequency(unsigned Number) const {
98 return BlockFrequency[Number];
102 virtual bool runOnMachineFunction(MachineFunction&);
103 virtual void getAnalysisUsage(AnalysisUsage&) const;
104 virtual void releaseMemory();
106 void activate(unsigned);
107 void prepareNodes(const SmallVectorImpl<BlockConstraint>&);
108 void iterate(const SmallVectorImpl<unsigned>&);
111 } // end namespace llvm