1 //===- StringTableBuilder.cpp - String table building utility -------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #include "llvm/MC/StringTableBuilder.h"
10 #include "llvm/ADT/CachedHashString.h"
11 #include "llvm/ADT/SmallString.h"
12 #include "llvm/ADT/StringRef.h"
13 #include "llvm/BinaryFormat/COFF.h"
14 #include "llvm/Support/Endian.h"
15 #include "llvm/Support/MathExtras.h"
16 #include "llvm/Support/raw_ostream.h"
17 #include <cassert>
18 #include <cstddef>
19 #include <cstdint>
20 #include <cstring>
21 #include <utility>
22 #include <vector>
23 
24 using namespace llvm;
25 
26 StringTableBuilder::~StringTableBuilder() = default;
27 
28 void StringTableBuilder::initSize() {
29   // Account for leading bytes in table so that offsets returned from add are
30   // correct.
31   switch (K) {
32   case RAW:
33   case DWARF:
34     Size = 0;
35     break;
36   case MachO:
37   case ELF:
38     // Start the table with a NUL byte.
39     Size = 1;
40     break;
41   case XCOFF:
42   case WinCOFF:
43     // Make room to write the table size later.
44     Size = 4;
45     break;
46   }
47 }
48 
49 StringTableBuilder::StringTableBuilder(Kind K, unsigned Alignment)
50     : K(K), Alignment(Alignment) {
51   initSize();
52 }
53 
54 void StringTableBuilder::write(raw_ostream &OS) const {
55   assert(isFinalized());
56   SmallString<0> Data;
57   Data.resize(getSize());
58   write((uint8_t *)Data.data());
59   OS << Data;
60 }
61 
62 using StringPair = std::pair<CachedHashStringRef, size_t>;
63 
64 void StringTableBuilder::write(uint8_t *Buf) const {
65   assert(isFinalized());
66   for (const StringPair &P : StringIndexMap) {
67     StringRef Data = P.first.val();
68     if (!Data.empty())
69       memcpy(Buf + P.second, Data.data(), Data.size());
70   }
71   // The COFF formats store the size of the string table in the first 4 bytes.
72   // For Windows, the format is little-endian; for AIX, it is big-endian.
73   if (K == WinCOFF)
74     support::endian::write32le(Buf, Size);
75   else if (K == XCOFF)
76     support::endian::write32be(Buf, Size);
77 }
78 
79 // Returns the character at Pos from end of a string.
80 static int charTailAt(StringPair *P, size_t Pos) {
81   StringRef S = P->first.val();
82   if (Pos >= S.size())
83     return -1;
84   return (unsigned char)S[S.size() - Pos - 1];
85 }
86 
87 // Three-way radix quicksort. This is much faster than std::sort with strcmp
88 // because it does not compare characters that we already know the same.
89 static void multikeySort(MutableArrayRef<StringPair *> Vec, int Pos) {
90 tailcall:
91   if (Vec.size() <= 1)
92     return;
93 
94   // Partition items so that items in [0, I) are greater than the pivot,
95   // [I, J) are the same as the pivot, and [J, Vec.size()) are less than
96   // the pivot.
97   int Pivot = charTailAt(Vec[0], Pos);
98   size_t I = 0;
99   size_t J = Vec.size();
100   for (size_t K = 1; K < J;) {
101     int C = charTailAt(Vec[K], Pos);
102     if (C > Pivot)
103       std::swap(Vec[I++], Vec[K++]);
104     else if (C < Pivot)
105       std::swap(Vec[--J], Vec[K]);
106     else
107       K++;
108   }
109 
110   multikeySort(Vec.slice(0, I), Pos);
111   multikeySort(Vec.slice(J), Pos);
112 
113   // multikeySort(Vec.slice(I, J - I), Pos + 1), but with
114   // tail call optimization.
115   if (Pivot != -1) {
116     Vec = Vec.slice(I, J - I);
117     ++Pos;
118     goto tailcall;
119   }
120 }
121 
122 void StringTableBuilder::finalize() {
123   assert(K != DWARF);
124   finalizeStringTable(/*Optimize=*/true);
125 }
126 
127 void StringTableBuilder::finalizeInOrder() {
128   finalizeStringTable(/*Optimize=*/false);
129 }
130 
131 void StringTableBuilder::finalizeStringTable(bool Optimize) {
132   Finalized = true;
133 
134   if (Optimize) {
135     std::vector<StringPair *> Strings;
136     Strings.reserve(StringIndexMap.size());
137     for (StringPair &P : StringIndexMap)
138       Strings.push_back(&P);
139 
140     multikeySort(Strings, 0);
141     initSize();
142 
143     StringRef Previous;
144     for (StringPair *P : Strings) {
145       StringRef S = P->first.val();
146       if (Previous.endswith(S)) {
147         size_t Pos = Size - S.size() - (K != RAW);
148         if (!(Pos & (Alignment - 1))) {
149           P->second = Pos;
150           continue;
151         }
152       }
153 
154       Size = alignTo(Size, Alignment);
155       P->second = Size;
156 
157       Size += S.size();
158       if (K != RAW)
159         ++Size;
160       Previous = S;
161     }
162   }
163 
164   if (K == MachO)
165     Size = alignTo(Size, 4); // Pad to multiple of 4.
166 
167   // The first byte in an ELF string table must be null, according to the ELF
168   // specification. In 'initSize()' we reserved the first byte to hold null for
169   // this purpose and here we actually add the string to allow 'getOffset()' to
170   // be called on an empty string.
171   if (K == ELF)
172     StringIndexMap[CachedHashStringRef("")] = 0;
173 }
174 
175 void StringTableBuilder::clear() {
176   Finalized = false;
177   StringIndexMap.clear();
178 }
179 
180 size_t StringTableBuilder::getOffset(CachedHashStringRef S) const {
181   assert(isFinalized());
182   auto I = StringIndexMap.find(S);
183   assert(I != StringIndexMap.end() && "String is not in table!");
184   return I->second;
185 }
186 
187 size_t StringTableBuilder::add(CachedHashStringRef S) {
188   if (K == WinCOFF)
189     assert(S.size() > COFF::NameSize && "Short string in COFF string table!");
190 
191   assert(!isFinalized());
192   auto P = StringIndexMap.insert(std::make_pair(S, 0));
193   if (P.second) {
194     size_t Start = alignTo(Size, Alignment);
195     P.first->second = Start;
196     Size = Start + S.size() + (K != RAW);
197   }
198   return P.first->second;
199 }
200