[lld][macho] Support order cstrings with -order_file

lld/MachO/SectionPriorities.cpp

@@ -247,15 +247,13 @@ DenseMap<const InputSection *, int> CallGraphSort::run() {
 }
 
 std::optional<int>
-macho::PriorityBuilder::getSymbolPriority(const Defined *sym) {
-  if (sym->isAbsolute())
-    return std::nullopt;
+macho::PriorityBuilder::getSymbolOrCStringPriority(const StringRef key,
+                                                   InputFile *f) {
 
-  auto it = priorities.find(utils::getRootSymbol(sym->getName()));
+  auto it = priorities.find(key);
   if (it == priorities.end())
     return std::nullopt;
   const SymbolPriorityEntry &entry = it->second;
-  const InputFile *f = sym->isec()->getFile();
   if (!f)
     return entry.anyObjectFile;
   // We don't use toString(InputFile *) here because it returns the full path
@@ -269,6 +267,14 @@ macho::PriorityBuilder::getSymbolPriority(const Defined *sym) {
   return std::min(entry.objectFiles.lookup(filename), entry.anyObjectFile);
 }
 
+std::optional<int>
+macho::PriorityBuilder::getSymbolPriority(const Defined *sym) {
+  if (sym->isAbsolute())
+    return std::nullopt;
+  return getSymbolOrCStringPriority(utils::getRootSymbol(sym->getName()),
+                                    sym->isec()->getFile());
+}
+
 void macho::PriorityBuilder::extractCallGraphProfile() {
   TimeTraceScope timeScope("Extract call graph profile");
   bool hasOrderFile = !priorities.empty();
@@ -301,7 +307,7 @@ void macho::PriorityBuilder::parseOrderFile(StringRef path) {
   int prio = std::numeric_limits<int>::min();
   MemoryBufferRef mbref = *buffer;
   for (StringRef line : args::getLines(mbref)) {
-    StringRef objectFile, symbol;
+    StringRef objectFile, symbolOrCStrHash;
     line = line.take_until([](char c) { return c == '#'; }); // ignore comments
     line = line.ltrim();
 
@@ -316,7 +322,6 @@ void macho::PriorityBuilder::parseOrderFile(StringRef path) {
 
     if (cpuType != CPU_TYPE_ANY && cpuType != target->cpuType)
       continue;
-
     // Drop the CPU type as well as the colon
     if (cpuType != CPU_TYPE_ANY)
       line = line.drop_until([](char c) { return c == ':'; }).drop_front();
@@ -331,10 +336,20 @@ void macho::PriorityBuilder::parseOrderFile(StringRef path) {
         break;
       }
     }
-    symbol = utils::getRootSymbol(line.trim());
 
-    if (!symbol.empty()) {
-      SymbolPriorityEntry &entry = priorities[symbol];
+    // The rest of the line is either <symbol name> or
+    // CStringEntryPrefix<cstring hash>
+    line = line.trim();
+    if (line.starts_with(CStringEntryPrefix)) {
+      StringRef possibleHash = line.drop_front(CStringEntryPrefix.size());
+      uint32_t hash = 0;
+      if (to_integer(possibleHash, hash))
+        symbolOrCStrHash = possibleHash;
+    } else
+      symbolOrCStrHash = utils::getRootSymbol(line);
+
+    if (!symbolOrCStrHash.empty()) {
+      SymbolPriorityEntry &entry = priorities[symbolOrCStrHash];
       if (!objectFile.empty())
         entry.objectFiles.insert(std::make_pair(objectFile, prio));
       else
@@ -389,3 +404,41 @@ macho::PriorityBuilder::buildInputSectionPriorities() {
 
   return sectionPriorities;
 }
+
+std::vector<StringPiecePair> macho::PriorityBuilder::buildCStringPriorities(
+    ArrayRef<CStringInputSection *> inputs) {
+  // Split the input strings into hold and cold sets.
+  // Order hot set based on -order_file_cstring for performance improvement;
+  // TODO: Order cold set of cstrings for compression via BP.
+  std::vector<std::pair<int, StringPiecePair>>
+      hotStringPrioritiesAndStringPieces;
+  std::vector<StringPiecePair> coldStringPieces;
+  std::vector<StringPiecePair> orderedStringPieces;
+
+  for (CStringInputSection *isec : inputs) {
+    for (const auto &[stringPieceIdx, piece] : llvm::enumerate(isec->pieces)) {
+      if (!piece.live)
+        continue;
+
+      std::optional<int> priority = getSymbolOrCStringPriority(
+          std::to_string(piece.hash), isec->getFile());
+      if (!priority)
+        coldStringPieces.emplace_back(isec, stringPieceIdx);
+      else
+        hotStringPrioritiesAndStringPieces.emplace_back(
+            *priority, std::make_pair(isec, stringPieceIdx));
+    }
+  }
+
+  // Order hot set for perf
+  llvm::stable_sort(hotStringPrioritiesAndStringPieces);
+  for (auto &[priority, stringPiecePair] : hotStringPrioritiesAndStringPieces)
+    orderedStringPieces.push_back(stringPiecePair);
+
+  // TODO: Order cold set for compression
+
+  orderedStringPieces.insert(orderedStringPieces.end(),
+                             coldStringPieces.begin(), coldStringPieces.end());
+
+  return orderedStringPieces;
+}

lld/MachO/SectionPriorities.h

@@ -16,6 +16,7 @@
 namespace lld::macho {
 
 using SectionPair = std::pair<const InputSection *, const InputSection *>;
+using StringPiecePair = std::pair<CStringInputSection *, size_t>;
 
 class PriorityBuilder {
 public:
@@ -28,17 +29,28 @@ class PriorityBuilder {
   //
   // An order file has one entry per line, in the following format:
   //
-  //   <cpu>:<object file>:<symbol name>
+  //   <cpu>:<object file>:[<symbol name> | CStringEntryPrefix <cstring hash>]
   //
-  // <cpu> and <object file> are optional. If not specified, then that entry
-  // matches any symbol of that name. Parsing this format is not quite
-  // straightforward because the symbol name itself can contain colons, so when
-  // encountering a colon, we consider the preceding characters to decide if it
-  // can be a valid CPU type or file path.
+  // <cpu> and <object file> are optional.
+  // If not specified, then that entry tries to match either,
   //
+  // 1) any symbol of the <symbol name>;
+  // Parsing this format is not quite straightforward because the symbol name
+  // itself can contain colons, so when encountering a colon, we consider the
+  // preceding characters to decide if it can be a valid CPU type or file path.
   // If a symbol is matched by multiple entries, then it takes the
   // lowest-ordered entry (the one nearest to the front of the list.)
   //
+  // or 2) any cstring literal with the given hash, if the entry has the
+  // CStringEntryPrefix prefix defined below in the file. <cstring hash> is the
+  // hash of cstring literal content.
+  //
+  // Cstring literals are not symbolized, we can't identify them by name
+  // However, cstrings are deduplicated, hence unique, so we use the hash of
+  // the content of cstring literals to identify them and assign priority to it.
+  // We use the same hash as used in StringPiece, i.e. 31 bit:
+  // xxh3_64bits(string) & 0x7fffffff
+  //
   // The file can also have line comments that start with '#'.
   void parseOrderFile(StringRef path);
 
@@ -54,6 +66,8 @@ class PriorityBuilder {
   // Each section gets assigned the priority of the highest-priority symbol it
   // contains.
   llvm::DenseMap<const InputSection *, int> buildInputSectionPriorities();
+  std::vector<StringPiecePair>
+      buildCStringPriorities(ArrayRef<CStringInputSection *>);
 
 private:
   // The symbol with the smallest priority should be ordered first in the output
@@ -65,8 +79,11 @@ class PriorityBuilder {
     // The priority given to a matching symbol from a particular object file.
     llvm::DenseMap<llvm::StringRef, int> objectFiles;
   };
+  const llvm::StringRef CStringEntryPrefix = "CSTR;";
 
   std::optional<int> getSymbolPriority(const Defined *sym);
+  std::optional<int> getSymbolOrCStringPriority(const StringRef key,
+                                                InputFile *f);
   llvm::DenseMap<llvm::StringRef, SymbolPriorityEntry> priorities;
   llvm::MapVector<SectionPair, uint64_t> callGraphProfile;
 };

lld/MachO/SyntheticSections.cpp

@@ -14,6 +14,7 @@
 #include "InputFiles.h"
 #include "ObjC.h"
 #include "OutputSegment.h"
+#include "SectionPriorities.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
 
@@ -1760,26 +1761,25 @@ void DeduplicatedCStringSection::finalizeContents() {
     }
   }
 
-  // Assign an offset for each string and save it to the corresponding
+  // Sort the strings for performance and compression size win, and then
+  // assign an offset for each string and save it to the corresponding
   // StringPieces for easy access.
-  for (CStringInputSection *isec : inputs) {
-    for (const auto &[i, piece] : llvm::enumerate(isec->pieces)) {
-      if (!piece.live)
-        continue;
-      auto s = isec->getCachedHashStringRef(i);
-      auto it = stringOffsetMap.find(s);
-      assert(it != stringOffsetMap.end());
-      StringOffset &offsetInfo = it->second;
-      if (offsetInfo.outSecOff == UINT64_MAX) {
-        offsetInfo.outSecOff =
-            alignToPowerOf2(size, 1ULL << offsetInfo.trailingZeros);
-        size =
-            offsetInfo.outSecOff + s.size() + 1; // account for null terminator
-      }
-      piece.outSecOff = offsetInfo.outSecOff;
+  for (auto &[isec, i] : priorityBuilder.buildCStringPriorities(inputs)) {
+    auto &piece = isec->pieces[i];
+    auto s = isec->getCachedHashStringRef(i);
+    auto it = stringOffsetMap.find(s);
+    assert(it != stringOffsetMap.end());
+    lld::macho::DeduplicatedCStringSection::StringOffset &offsetInfo =
+        it->second;
+    if (offsetInfo.outSecOff == UINT64_MAX) {
+      offsetInfo.outSecOff =
+          alignToPowerOf2(size, 1ULL << offsetInfo.trailingZeros);
+      size = offsetInfo.outSecOff + s.size() + 1; // account for null terminator
     }
-    isec->isFinal = true;
+    piece.outSecOff = offsetInfo.outSecOff;
   }
+  for (CStringInputSection *isec : inputs)
+    isec->isFinal = true;
 }
 
 void DeduplicatedCStringSection::writeTo(uint8_t *buf) const {