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      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 MachOLinked:
     37   case MachO64Linked:
     38     Size = 2;
     39     break;
     40   case MachO:
     41   case MachO64:
     42   case ELF:
     43     // Start the table with a NUL byte.
     44     Size = 1;
     45     break;
     46   case XCOFF:
     47   case WinCOFF:
     48     // Make room to write the table size later.
     49     Size = 4;
     50     break;
     51   }
     52 }
     53 
     54 StringTableBuilder::StringTableBuilder(Kind K, unsigned Alignment)
     55     : K(K), Alignment(Alignment) {
     56   initSize();
     57 }
     58 
     59 void StringTableBuilder::write(raw_ostream &OS) const {
     60   assert(isFinalized());
     61   SmallString<0> Data;
     62   Data.resize(getSize());
     63   write((uint8_t *)Data.data());
     64   OS << Data;
     65 }
     66 
     67 using StringPair = std::pair<CachedHashStringRef, size_t>;
     68 
     69 void StringTableBuilder::write(uint8_t *Buf) const {
     70   assert(isFinalized());
     71   for (const StringPair &P : StringIndexMap) {
     72     StringRef Data = P.first.val();
     73     if (!Data.empty())
     74       memcpy(Buf + P.second, Data.data(), Data.size());
     75   }
     76   // The COFF formats store the size of the string table in the first 4 bytes.
     77   // For Windows, the format is little-endian; for AIX, it is big-endian.
     78   if (K == WinCOFF)
     79     support::endian::write32le(Buf, Size);
     80   else if (K == XCOFF)
     81     support::endian::write32be(Buf, Size);
     82 }
     83 
     84 // Returns the character at Pos from end of a string.
     85 static int charTailAt(StringPair *P, size_t Pos) {
     86   StringRef S = P->first.val();
     87   if (Pos >= S.size())
     88     return -1;
     89   return (unsigned char)S[S.size() - Pos - 1];
     90 }
     91 
     92 // Three-way radix quicksort. This is much faster than std::sort with strcmp
     93 // because it does not compare characters that we already know the same.
     94 static void multikeySort(MutableArrayRef<StringPair *> Vec, int Pos) {
     95 tailcall:
     96   if (Vec.size() <= 1)
     97     return;
     98 
     99   // Partition items so that items in [0, I) are greater than the pivot,
    100   // [I, J) are the same as the pivot, and [J, Vec.size()) are less than
    101   // the pivot.
    102   int Pivot = charTailAt(Vec[0], Pos);
    103   size_t I = 0;
    104   size_t J = Vec.size();
    105   for (size_t K = 1; K < J;) {
    106     int C = charTailAt(Vec[K], Pos);
    107     if (C > Pivot)
    108       std::swap(Vec[I++], Vec[K++]);
    109     else if (C < Pivot)
    110       std::swap(Vec[--J], Vec[K]);
    111     else
    112       K++;
    113   }
    114 
    115   multikeySort(Vec.slice(0, I), Pos);
    116   multikeySort(Vec.slice(J), Pos);
    117 
    118   // multikeySort(Vec.slice(I, J - I), Pos + 1), but with
    119   // tail call optimization.
    120   if (Pivot != -1) {
    121     Vec = Vec.slice(I, J - I);
    122     ++Pos;
    123     goto tailcall;
    124   }
    125 }
    126 
    127 void StringTableBuilder::finalize() {
    128   assert(K != DWARF);
    129   finalizeStringTable(/*Optimize=*/true);
    130 }
    131 
    132 void StringTableBuilder::finalizeInOrder() {
    133   finalizeStringTable(/*Optimize=*/false);
    134 }
    135 
    136 void StringTableBuilder::finalizeStringTable(bool Optimize) {
    137   Finalized = true;
    138 
    139   if (Optimize) {
    140     std::vector<StringPair *> Strings;
    141     Strings.reserve(StringIndexMap.size());
    142     for (StringPair &P : StringIndexMap)
    143       Strings.push_back(&P);
    144 
    145     multikeySort(Strings, 0);
    146     initSize();
    147 
    148     StringRef Previous;
    149     for (StringPair *P : Strings) {
    150       StringRef S = P->first.val();
    151       if (Previous.endswith(S)) {
    152         size_t Pos = Size - S.size() - (K != RAW);
    153         if (!(Pos & (Alignment - 1))) {
    154           P->second = Pos;
    155           continue;
    156         }
    157       }
    158 
    159       Size = alignTo(Size, Alignment);
    160       P->second = Size;
    161 
    162       Size += S.size();
    163       if (K != RAW)
    164         ++Size;
    165       Previous = S;
    166     }
    167   }
    168 
    169   if (K == MachO || K == MachOLinked)
    170     Size = alignTo(Size, 4); // Pad to multiple of 4.
    171   if (K == MachO64 || K == MachO64Linked)
    172     Size = alignTo(Size, 8); // Pad to multiple of 8.
    173 
    174   // According to ld64 the string table of a final linked Mach-O binary starts
    175   // with " ", i.e. the first byte is ' ' and the second byte is zero. In
    176   // 'initSize()' we reserved the first two bytes for holding this string.
    177   if (K == MachOLinked || K == MachO64Linked)
    178     StringIndexMap[CachedHashStringRef(" ")] = 0;
    179 
    180   // The first byte in an ELF string table must be null, according to the ELF
    181   // specification. In 'initSize()' we reserved the first byte to hold null for
    182   // this purpose and here we actually add the string to allow 'getOffset()' to
    183   // be called on an empty string.
    184   if (K == ELF)
    185     StringIndexMap[CachedHashStringRef("")] = 0;
    186 }
    187 
    188 void StringTableBuilder::clear() {
    189   Finalized = false;
    190   StringIndexMap.clear();
    191 }
    192 
    193 size_t StringTableBuilder::getOffset(CachedHashStringRef S) const {
    194   assert(isFinalized());
    195   auto I = StringIndexMap.find(S);
    196   assert(I != StringIndexMap.end() && "String is not in table!");
    197   return I->second;
    198 }
    199 
    200 size_t StringTableBuilder::add(CachedHashStringRef S) {
    201   if (K == WinCOFF)
    202     assert(S.size() > COFF::NameSize && "Short string in COFF string table!");
    203 
    204   assert(!isFinalized());
    205   auto P = StringIndexMap.insert(std::make_pair(S, 0));
    206   if (P.second) {
    207     size_t Start = alignTo(Size, Alignment);
    208     P.first->second = Start;
    209     Size = Start + S.size() + (K != RAW);
    210   }
    211   return P.first->second;
    212 }
    213