1 //===- LowerBitSets.h - Bitset lowering pass --------------------*- 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 file defines parts of the bitset lowering pass implementation that may 11 // be usefully unit tested. 12 // 13 //===----------------------------------------------------------------------===// 14 15 #ifndef LLVM_TRANSFORMS_IPO_LOWERBITSETS_H 16 #define LLVM_TRANSFORMS_IPO_LOWERBITSETS_H 17 18 #include "llvm/ADT/DenseMap.h" 19 #include "llvm/ADT/SmallVector.h" 20 21 #include <stdint.h> 22 #include <limits> 23 #include <set> 24 #include <vector> 25 26 namespace llvm { 27 28 class DataLayout; 29 class GlobalObject; 30 class Value; 31 class raw_ostream; 32 33 struct BitSetInfo { 34 // The indices of the set bits in the bitset. 35 std::set<uint64_t> Bits; 36 37 // The byte offset into the combined global represented by the bitset. 38 uint64_t ByteOffset; 39 40 // The size of the bitset in bits. 41 uint64_t BitSize; 42 43 // Log2 alignment of the bit set relative to the combined global. 44 // For example, a log2 alignment of 3 means that bits in the bitset 45 // represent addresses 8 bytes apart. 46 unsigned AlignLog2; 47 48 bool isSingleOffset() const { 49 return Bits.size() == 1; 50 } 51 52 bool isAllOnes() const { 53 return Bits.size() == BitSize; 54 } 55 56 bool containsGlobalOffset(uint64_t Offset) const; 57 58 bool containsValue(const DataLayout &DL, 59 const DenseMap<GlobalObject *, uint64_t> &GlobalLayout, 60 Value *V, uint64_t COffset = 0) const; 61 62 void print(raw_ostream &OS) const; 63 }; 64 65 struct BitSetBuilder { 66 SmallVector<uint64_t, 16> Offsets; 67 uint64_t Min, Max; 68 69 BitSetBuilder() : Min(std::numeric_limits<uint64_t>::max()), Max(0) {} 70 71 void addOffset(uint64_t Offset) { 72 if (Min > Offset) 73 Min = Offset; 74 if (Max < Offset) 75 Max = Offset; 76 77 Offsets.push_back(Offset); 78 } 79 80 BitSetInfo build(); 81 }; 82 83 /// This class implements a layout algorithm for globals referenced by bit sets 84 /// that tries to keep members of small bit sets together. This can 85 /// significantly reduce bit set sizes in many cases. 86 /// 87 /// It works by assembling fragments of layout from sets of referenced globals. 88 /// Each set of referenced globals causes the algorithm to create a new 89 /// fragment, which is assembled by appending each referenced global in the set 90 /// into the fragment. If a referenced global has already been referenced by an 91 /// fragment created earlier, we instead delete that fragment and append its 92 /// contents into the fragment we are assembling. 93 /// 94 /// By starting with the smallest fragments, we minimize the size of the 95 /// fragments that are copied into larger fragments. This is most intuitively 96 /// thought about when considering the case where the globals are virtual tables 97 /// and the bit sets represent their derived classes: in a single inheritance 98 /// hierarchy, the optimum layout would involve a depth-first search of the 99 /// class hierarchy (and in fact the computed layout ends up looking a lot like 100 /// a DFS), but a naive DFS would not work well in the presence of multiple 101 /// inheritance. This aspect of the algorithm ends up fitting smaller 102 /// hierarchies inside larger ones where that would be beneficial. 103 /// 104 /// For example, consider this class hierarchy: 105 /// 106 /// A B 107 /// \ / | \ 108 /// C D E 109 /// 110 /// We have five bit sets: bsA (A, C), bsB (B, C, D, E), bsC (C), bsD (D) and 111 /// bsE (E). If we laid out our objects by DFS traversing B followed by A, our 112 /// layout would be {B, C, D, E, A}. This is optimal for bsB as it needs to 113 /// cover the only 4 objects in its hierarchy, but not for bsA as it needs to 114 /// cover 5 objects, i.e. the entire layout. Our algorithm proceeds as follows: 115 /// 116 /// Add bsC, fragments {{C}} 117 /// Add bsD, fragments {{C}, {D}} 118 /// Add bsE, fragments {{C}, {D}, {E}} 119 /// Add bsA, fragments {{A, C}, {D}, {E}} 120 /// Add bsB, fragments {{B, A, C, D, E}} 121 /// 122 /// This layout is optimal for bsA, as it now only needs to cover two (i.e. 3 123 /// fewer) objects, at the cost of bsB needing to cover 1 more object. 124 /// 125 /// The bit set lowering pass assigns an object index to each object that needs 126 /// to be laid out, and calls addFragment for each bit set passing the object 127 /// indices of its referenced globals. It then assembles a layout from the 128 /// computed layout in the Fragments field. 129 struct GlobalLayoutBuilder { 130 /// The computed layout. Each element of this vector contains a fragment of 131 /// layout (which may be empty) consisting of object indices. 132 std::vector<std::vector<uint64_t>> Fragments; 133 134 /// Mapping from object index to fragment index. 135 std::vector<uint64_t> FragmentMap; 136 137 GlobalLayoutBuilder(uint64_t NumObjects) 138 : Fragments(1), FragmentMap(NumObjects) {} 139 140 /// Add F to the layout while trying to keep its indices contiguous. 141 /// If a previously seen fragment uses any of F's indices, that 142 /// fragment will be laid out inside F. 143 void addFragment(const std::set<uint64_t> &F); 144 }; 145 146 /// This class is used to build a byte array containing overlapping bit sets. By 147 /// loading from indexed offsets into the byte array and applying a mask, a 148 /// program can test bits from the bit set with a relatively short instruction 149 /// sequence. For example, suppose we have 15 bit sets to lay out: 150 /// 151 /// A (16 bits), B (15 bits), C (14 bits), D (13 bits), E (12 bits), 152 /// F (11 bits), G (10 bits), H (9 bits), I (7 bits), J (6 bits), K (5 bits), 153 /// L (4 bits), M (3 bits), N (2 bits), O (1 bit) 154 /// 155 /// These bits can be laid out in a 16-byte array like this: 156 /// 157 /// Byte Offset 158 /// 0123456789ABCDEF 159 /// Bit 160 /// 7 HHHHHHHHHIIIIIII 161 /// 6 GGGGGGGGGGJJJJJJ 162 /// 5 FFFFFFFFFFFKKKKK 163 /// 4 EEEEEEEEEEEELLLL 164 /// 3 DDDDDDDDDDDDDMMM 165 /// 2 CCCCCCCCCCCCCCNN 166 /// 1 BBBBBBBBBBBBBBBO 167 /// 0 AAAAAAAAAAAAAAAA 168 /// 169 /// For example, to test bit X of A, we evaluate ((bits[X] & 1) != 0), or to 170 /// test bit X of I, we evaluate ((bits[9 + X] & 0x80) != 0). This can be done 171 /// in 1-2 machine instructions on x86, or 4-6 instructions on ARM. 172 /// 173 /// This is a byte array, rather than (say) a 2-byte array or a 4-byte array, 174 /// because for one thing it gives us better packing (the more bins there are, 175 /// the less evenly they will be filled), and for another, the instruction 176 /// sequences can be slightly shorter, both on x86 and ARM. 177 struct ByteArrayBuilder { 178 /// The byte array built so far. 179 std::vector<uint8_t> Bytes; 180 181 enum { BitsPerByte = 8 }; 182 183 /// The number of bytes allocated so far for each of the bits. 184 uint64_t BitAllocs[BitsPerByte]; 185 186 ByteArrayBuilder() { 187 memset(BitAllocs, 0, sizeof(BitAllocs)); 188 } 189 190 /// Allocate BitSize bits in the byte array where Bits contains the bits to 191 /// set. AllocByteOffset is set to the offset within the byte array and 192 /// AllocMask is set to the bitmask for those bits. This uses the LPT (Longest 193 /// Processing Time) multiprocessor scheduling algorithm to lay out the bits 194 /// efficiently; the pass allocates bit sets in decreasing size order. 195 void allocate(const std::set<uint64_t> &Bits, uint64_t BitSize, 196 uint64_t &AllocByteOffset, uint8_t &AllocMask); 197 }; 198 199 } // namespace llvm 200 201 #endif 202
