lld/MachO/ExportTrie.cpp

   1 //===- ExportTrie.cpp -----------------------------------------------------===//
   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 // This is a partial implementation of the Mach-O export trie format. It's
  10 // essentially a symbol table encoded as a compressed prefix trie, meaning that
  11 // the common prefixes of each symbol name are shared for a more compact
  12 // representation. The prefixes are stored on the edges of the trie, and one
  13 // edge can represent multiple characters. For example, given two exported
  14 // symbols _bar and _baz, we will have a trie like this (terminal nodes are
  15 // marked with an asterisk):
  16 //
  17 //              +-+-+
  18 //              |   | // root node
  19 //              +-+-+
  20 //                |
  21 //                | _ba
  22 //                |
  23 //              +-+-+
  24 //              |   |
  25 //              +-+-+
  26 //           r /     \ z
  27 //            /       \
  28 //        +-+-+       +-+-+
  29 //        | * |       | * |
  30 //        +-+-+       +-+-+
  31 //
  32 // More documentation of the format can be found in
  33 // llvm/tools/obj2yaml/macho2yaml.cpp.
  34 //
  35 //===----------------------------------------------------------------------===//
  36
  37 #include "ExportTrie.h"
  38 #include "Symbols.h"
  39
  40 #include "lld/Common/ErrorHandler.h"
  41 #include "lld/Common/Memory.h"
  42 #include "llvm/ADT/Optional.h"
  43 #include "llvm/BinaryFormat/MachO.h"
  44 #include "llvm/Support/LEB128.h"
  45
  46 using namespace llvm;
  47 using namespace lld;
  48 using namespace lld::macho;
  49
  50 namespace {
  51
  52 struct Edge {
  53   Edge(StringRef s, TrieNode *node) : substring(s), child(node) {}
  54
  55   StringRef substring;
  56   struct TrieNode *child;
  57 };
  58
  59 struct ExportInfo {
  60   uint64_t address;
  61   uint8_t flags = 0;
  62   ExportInfo(const Symbol &sym, uint64_t imageBase)
  63       : address(sym.getVA() - imageBase) {
  64     using namespace llvm::MachO;
  65     // Set the symbol type.
  66     if (sym.isWeakDef())
  67       flags |= EXPORT_SYMBOL_FLAGS_WEAK_DEFINITION;
  68     // TODO: Add proper support for re-exports & stub-and-resolver flags.
  69
  70     // Set the symbol kind.
  71     if (sym.isTlv()) {
  72       flags |= EXPORT_SYMBOL_FLAGS_KIND_THREAD_LOCAL;
  73     } else if (auto *defined = dyn_cast<Defined>(&sym)) {
  74       if (defined->isAbsolute())
  75         flags |= EXPORT_SYMBOL_FLAGS_KIND_ABSOLUTE;
  76     }
  77   }
  78 };
  79
  80 } // namespace
  81
  82 struct macho::TrieNode {
  83   std::vector<Edge> edges;
  84   Optional<ExportInfo> info;
  85   // Estimated offset from the start of the serialized trie to the current node.
  86   // This will converge to the true offset when updateOffset() is run to a
  87   // fixpoint.
  88   size_t offset = 0;
  89
  90   // Returns whether the new estimated offset differs from the old one.
  91   bool updateOffset(size_t &nextOffset);
  92   void writeTo(uint8_t *buf) const;
  93 };
  94
  95 bool TrieNode::updateOffset(size_t &nextOffset) {
  96   // Size of the whole node (including the terminalSize and the outgoing edges.)
  97   // In contrast, terminalSize only records the size of the other data in the
  98   // node.
  99   size_t nodeSize;
 100   if (info) {
 101     uint32_t terminalSize =
 102         getULEB128Size(info->flags) + getULEB128Size(info->address);
 103     // Overall node size so far is the uleb128 size of the length of the symbol
 104     // info + the symbol info itself.
 105     nodeSize = terminalSize + getULEB128Size(terminalSize);
 106   } else {
 107     nodeSize = 1; // Size of terminalSize (which has a value of 0)
 108   }
 109   // Compute size of all child edges.
 110   ++nodeSize; // Byte for number of children.
 111   for (const Edge &edge : edges) {
 112     nodeSize += edge.substring.size() + 1             // String length.
 113                 + getULEB128Size(edge.child->offset); // Offset len.
 114   }
 115   // On input, 'nextOffset' is the new preferred location for this node.
 116   bool result = (offset != nextOffset);
 117   // Store new location in node object for use by parents.
 118   offset = nextOffset;
 119   nextOffset += nodeSize;
 120   return result;
 121 }
 122
 123 void TrieNode::writeTo(uint8_t *buf) const {
 124   buf += offset;
 125   if (info) {
 126     // TrieNodes with Symbol info: size, flags address
 127     uint32_t terminalSize =
 128         getULEB128Size(info->flags) + getULEB128Size(info->address);
 129     buf += encodeULEB128(terminalSize, buf);
 130     buf += encodeULEB128(info->flags, buf);
 131     buf += encodeULEB128(info->address, buf);
 132   } else {
 133     // TrieNode with no Symbol info.
 134     *buf++ = 0; // terminalSize
 135   }
 136   // Add number of children. TODO: Handle case where we have more than 256.
 137   assert(edges.size() < 256);
 138   *buf++ = edges.size();
 139   // Append each child edge substring and node offset.
 140   for (const Edge &edge : edges) {
 141     memcpy(buf, edge.substring.data(), edge.substring.size());
 142     buf += edge.substring.size();
 143     *buf++ = '\0';
 144     buf += encodeULEB128(edge.child->offset, buf);
 145   }
 146 }
 147
 148 TrieBuilder::~TrieBuilder() {
 149   for (TrieNode *node : nodes)
 150     delete node;
 151 }
 152
 153 TrieNode *TrieBuilder::makeNode() {
 154   auto *node = new TrieNode();
 155   nodes.emplace_back(node);
 156   return node;
 157 }
 158
 159 static int charAt(const Symbol *sym, size_t pos) {
 160   StringRef str = sym->getName();
 161   if (pos >= str.size())
 162     return -1;
 163   return str[pos];
 164 }
 165
 166 // Build the trie by performing a three-way radix quicksort: We start by sorting
 167 // the strings by their first characters, then sort the strings with the same
 168 // first characters by their second characters, and so on recursively. Each
 169 // time the prefixes diverge, we add a node to the trie.
 170 //
 171 // node:    The most recently created node along this path in the trie (i.e.
 172 //          the furthest from the root.)
 173 // lastPos: The prefix length of the most recently created node, i.e. the number
 174 //          of characters along its path from the root.
 175 // pos:     The string index we are currently sorting on. Note that each symbol
 176 //          S contained in vec has the same prefix S[0...pos).
 177 void TrieBuilder::sortAndBuild(MutableArrayRef<const Symbol *> vec,
 178                                TrieNode *node, size_t lastPos, size_t pos) {
 179 tailcall:
 180   if (vec.empty())
 181     return;
 182
 183   // Partition items so that items in [0, i) are less than the pivot,
 184   // [i, j) are the same as the pivot, and [j, vec.size()) are greater than
 185   // the pivot.
 186   const Symbol *pivotSymbol = vec[vec.size() / 2];
 187   int pivot = charAt(pivotSymbol, pos);
 188   size_t i = 0;
 189   size_t j = vec.size();
 190   for (size_t k = 0; k < j;) {
 191     int c = charAt(vec[k], pos);
 192     if (c < pivot)
 193       std::swap(vec[i++], vec[k++]);
 194     else if (c > pivot)
 195       std::swap(vec[--j], vec[k]);
 196     else
 197       k++;
 198   }
 199
 200   bool isTerminal = pivot == -1;
 201   bool prefixesDiverge = i != 0 || j != vec.size();
 202   if (lastPos != pos && (isTerminal || prefixesDiverge)) {
 203     TrieNode *newNode = makeNode();
 204     node->edges.emplace_back(pivotSymbol->getName().slice(lastPos, pos),
 205                              newNode);
 206     node = newNode;
 207     lastPos = pos;
 208   }
 209
 210   sortAndBuild(vec.slice(0, i), node, lastPos, pos);
 211   sortAndBuild(vec.slice(j), node, lastPos, pos);
 212
 213   if (isTerminal) {
 214     assert(j - i == 1); // no duplicate symbols
 215     node->info = ExportInfo(*pivotSymbol, imageBase);
 216   } else {
 217     // This is the tail-call-optimized version of the following:
 218     // sortAndBuild(vec.slice(i, j - i), node, lastPos, pos + 1);
 219     vec = vec.slice(i, j - i);
 220     ++pos;
 221     goto tailcall;
 222   }
 223 }
 224
 225 size_t TrieBuilder::build() {
 226   if (exported.empty())
 227     return 0;
 228
 229   TrieNode *root = makeNode();
 230   sortAndBuild(exported, root, 0, 0);
 231
 232   // Assign each node in the vector an offset in the trie stream, iterating
 233   // until all uleb128 sizes have stabilized.
 234   size_t offset;
 235   bool more;
 236   do {
 237     offset = 0;
 238     more = false;
 239     for (TrieNode *node : nodes)
 240       more |= node->updateOffset(offset);
 241   } while (more);
 242
 243   return offset;
 244 }
 245
 246 void TrieBuilder::writeTo(uint8_t *buf) const {
 247   for (TrieNode *node : nodes)
 248     node->writeTo(buf);
 249 }
 250
 251 namespace {
 252
 253 // Parse a serialized trie and invoke a callback for each entry.
 254 class TrieParser {
 255 public:
 256   TrieParser(const uint8_t *buf, size_t size, const TrieEntryCallback &callback)
 257       : start(buf), end(start + size), callback(callback) {}
 258
 259   void parse(const uint8_t *buf, const Twine &cumulativeString);
 260
 261   void parse() { parse(start, ""); }
 262
 263   const uint8_t *start;
 264   const uint8_t *end;
 265   const TrieEntryCallback &callback;
 266 };
 267
 268 } // namespace
 269
 270 void TrieParser::parse(const uint8_t *buf, const Twine &cumulativeString) {
 271   if (buf >= end)
 272     fatal("Node offset points outside export section");
 273
 274   unsigned ulebSize;
 275   uint64_t terminalSize = decodeULEB128(buf, &ulebSize);
 276   buf += ulebSize;
 277   uint64_t flags = 0;
 278   size_t offset;
 279   if (terminalSize != 0) {
 280     flags = decodeULEB128(buf, &ulebSize);
 281     callback(cumulativeString, flags);
 282   }
 283   buf += terminalSize;
 284   uint8_t numEdges = *buf++;
 285   for (uint8_t i = 0; i < numEdges; ++i) {
 286     const char *cbuf = reinterpret_cast<const char *>(buf);
 287     StringRef substring = StringRef(cbuf, strnlen(cbuf, end - buf));
 288     buf += substring.size() + 1;
 289     offset = decodeULEB128(buf, &ulebSize);
 290     buf += ulebSize;
 291     parse(start + offset, cumulativeString + substring);
 292   }
 293 }
 294
 295 void macho::parseTrie(const uint8_t *buf, size_t size,
 296                       const TrieEntryCallback &callback) {
 297   if (size == 0)
 298     return;
 299
 300   TrieParser(buf, size, callback).parse();
 301 }