[lld] [lld][macho] Support order cstrings with -order_file_cstring (PR #140307)

[Ellis Hoag via llvm-commits]

================
@@ -388,3 +388,74 @@ macho::PriorityBuilder::buildInputSectionPriorities() {
 
   return sectionPriorities;
 }
+
+void macho::PriorityBuilder::parseOrderFileCString(StringRef path) {
+  std::optional<MemoryBufferRef> buffer = readFile(path);
+  if (!buffer) {
+    error("Could not read cstring order file at " + path);
+    return;
+  }
+  MemoryBufferRef mbref = *buffer;
+  int priority = std::numeric_limits<int>::min();
+  for (StringRef line : args::getLines(mbref)) {
+    if (line.empty())
+      continue;
+    uint32_t hash = 0;
+    if (!to_integer(line, hash))
+      continue;
+    auto it = cStringPriorities.find(hash);
+    if (it == cStringPriorities.end())
+      cStringPriorities[hash] = ++priority;
+    else
+      assert(it->second <= priority);
+  }
+}
+
+std::vector<StringPiecePair> macho::PriorityBuilder::buildCStringPriorities(
+    ArrayRef<CStringInputSection *> inputs) {
+  std::vector<StringPiecePair> orderedStringPieces;
+  if (config->cStringOrderFilePath.empty()) {
+    for (CStringInputSection *isec : inputs) {
+      for (const auto &[stringPieceIdx, piece] :
+           llvm::enumerate(isec->pieces)) {
+        if (!piece.live)
+          continue;
+        orderedStringPieces.emplace_back(isec, stringPieceIdx);
+      }
+    }
+    return orderedStringPieces;
+  }
----------------
ellishg wrote:

`DenseMap::find()` returns pretty quickly if it's empty. I don't think it would impact performance

https://github.com/llvm/llvm-project/blob/286ab11dc65de8175e2bce69fd7e36acd3be7ce0/llvm/include/llvm/ADT/DenseMap.h#L650-L654

https://github.com/llvm/llvm-project/pull/140307