[lld-macho] Reduce memory usage of prining thunks in map file

Author: alx32

lld/MachO/ConcatOutputSection.h

@@ -72,7 +72,7 @@ class TextOutputSection : public ConcatOutputSection {
   void finalizeContents() override {}
   void finalize() override;
   bool needsThunks() const;
-  ArrayRef<ConcatInputSection *> getThunks() const { return thunks; }
+  const std::vector<ConcatInputSection *> &getThunks() const { return thunks; }
   void writeTo(uint8_t *buf) const override;
 
   static bool classof(const OutputSection *sec) {

lld/MachO/MapFile.cpp

@@ -161,20 +161,6 @@ static uint64_t getSymSizeForMap(Defined *sym) {
   return sym->size;
 }
 
-// Merges two vectors of input sections in order of their outSecOff values.
-// This approach creates a new (temporary) vector which is not ideal but the
-// ideal approach leads to a lot of code duplication.
-static std::vector<ConcatInputSection *>
-mergeOrderedInputs(ArrayRef<ConcatInputSection *> inputs1,
-                   ArrayRef<ConcatInputSection *> inputs2) {
-  std::vector<ConcatInputSection *> vec(inputs1.size() + inputs2.size());
-  std::merge(inputs1.begin(), inputs1.end(), inputs2.begin(), inputs2.end(),
-             vec.begin(), [](ConcatInputSection *a, ConcatInputSection *b) {
-               return a->outSecOff < b->outSecOff;
-             });
-  return vec;
-}
-
 void macho::writeMapFile() {
   if (config->mapFile.empty())
     return;
@@ -217,15 +203,42 @@ void macho::writeMapFile() {
                    seg->name.str().c_str(), osec->name.str().c_str());
     }
 
-  // Shared function to print an array of symbols.
-  auto printIsecArrSyms = [&](const std::vector<ConcatInputSection *> &arr) {
-    for (const ConcatInputSection *isec : arr) {
+  // Shared function to print one or two arrays of ConcatInputSection in
+  // ascending outSecOff order. The second array is optional; if provided, we
+  // interleave the printing in sorted order without allocating a merged temp
+  // array.
+  auto printIsecArrSyms = [&](const std::vector<ConcatInputSection *> &arr1,
+                              const std::vector<ConcatInputSection *> *arr2 =
+                                  nullptr) {
+    // Helper lambda that prints all symbols from one ConcatInputSection.
+    auto printOne = [&](const ConcatInputSection *isec) {
       for (Defined *sym : isec->symbols) {
-        if (!(isPrivateLabel(sym->getName()) && getSymSizeForMap(sym) == 0))
+        if (!(isPrivateLabel(sym->getName()) && getSymSizeForMap(sym) == 0)) {
           os << format("0x%08llX\t0x%08llX\t[%3u] %s\n", sym->getVA(),
                        getSymSizeForMap(sym),
-                       readerToFileOrdinal[sym->getFile()],
+                       readerToFileOrdinal.lookup(sym->getFile()),
                        sym->getName().str().data());
+        }
+      }
+    };
+
+    // If there is only one array, print all symbols from it and return.
+    // This simplifies the logic for the merge case below.
+    if (!arr2) {
+      for (const ConcatInputSection *isec : arr1)
+        printOne(isec);
+      return;
+    }
+
+    size_t i = 0, j = 0;
+    size_t size1 = arr1.size();
+    size_t size2 = arr2->size();
+    while (i < size1 || j < size2) {
+      if (i < size1 &&
+          (j >= size2 || arr1[i]->outSecOff <= (*arr2)[j]->outSecOff)) {
+        printOne(arr1[i++]);
+      } else if (j < size2) {
+        printOne((*arr2)[j++]);
       }
     }
   };
@@ -235,9 +248,7 @@ void macho::writeMapFile() {
   for (const OutputSegment *seg : outputSegments) {
     for (const OutputSection *osec : seg->getSections()) {
       if (auto *textOsec = dyn_cast<TextOutputSection>(osec)) {
-        auto inputsAndThunks =
-            mergeOrderedInputs(textOsec->inputs, textOsec->getThunks());
-        printIsecArrSyms(inputsAndThunks);
+        printIsecArrSyms(textOsec->inputs, &textOsec->getThunks());
       } else if (auto *concatOsec = dyn_cast<ConcatOutputSection>(osec)) {
         printIsecArrSyms(concatOsec->inputs);
       } else if (osec == in.cStringSection || osec == in.objcMethnameSection) {