[LTO][NFC] Free the GlobalResolutions map after final use

llvm/include/llvm/LTO/LTO.h

@@ -404,7 +404,9 @@ class LTO {
   };
 
   // Global mapping from mangled symbol names to resolutions.
-  StringMap<GlobalResolution> GlobalResolutions;
+  // Make this an optional to guard against accessing after it has been reset
+  // (to reduce memory after we're done with it).
+  std::optional<StringMap<GlobalResolution>> GlobalResolutions;
 
   void addModuleToGlobalRes(ArrayRef<InputFile::Symbol> Syms,
                             ArrayRef<SymbolResolution> Res, unsigned Partition,

llvm/lib/LTO/LTO.cpp

@@ -592,7 +592,9 @@ LTO::LTO(Config Conf, ThinBackend Backend,
          unsigned ParallelCodeGenParallelismLevel, LTOKind LTOMode)
     : Conf(std::move(Conf)),
       RegularLTO(ParallelCodeGenParallelismLevel, this->Conf),
-      ThinLTO(std::move(Backend)), LTOMode(LTOMode) {}
+      ThinLTO(std::move(Backend)),
+      GlobalResolutions(std::make_optional<StringMap<GlobalResolution>>()),
+      LTOMode(LTOMode) {}
 
 // Requires a destructor for MapVector<BitcodeModule>.
 LTO::~LTO() = default;
@@ -610,7 +612,7 @@ void LTO::addModuleToGlobalRes(ArrayRef<InputFile::Symbol> Syms,
     assert(ResI != ResE);
     SymbolResolution Res = *ResI++;
 
-    auto &GlobalRes = GlobalResolutions[Sym.getName()];
+    auto &GlobalRes = (*GlobalResolutions)[Sym.getName()];
     GlobalRes.UnnamedAddr &= Sym.isUnnamedAddr();
     if (Res.Prevailing) {
       assert(!GlobalRes.Prevailing &&
@@ -1125,7 +1127,7 @@ Error LTO::run(AddStreamFn AddStream, FileCache Cache) {
   // Compute "dead" symbols, we don't want to import/export these!
   DenseSet<GlobalValue::GUID> GUIDPreservedSymbols;
   DenseMap<GlobalValue::GUID, PrevailingType> GUIDPrevailingResolutions;
-  for (auto &Res : GlobalResolutions) {
+  for (auto &Res : *GlobalResolutions) {
     // Normally resolution have IR name of symbol. We can do nothing here
     // otherwise. See comments in GlobalResolution struct for more details.
     if (Res.second.IRName.empty())
@@ -1169,6 +1171,8 @@ Error LTO::run(AddStreamFn AddStream, FileCache Cache) {
 
   Error Result = runRegularLTO(AddStream);
   if (!Result)
+    // This will reset the GlobalResolutions optional once done with it to
+    // reduce peak memory before importing.
     Result = runThinLTO(AddStream, Cache, GUIDPreservedSymbols);
 
   if (StatsFile)
@@ -1273,8 +1277,8 @@ Error LTO::runRegularLTO(AddStreamFn AddStream) {
   // This returns true when the name is local or not defined. Locals are
   // expected to be handled separately.
   auto IsVisibleToRegularObj = [&](StringRef name) {
-    auto It = GlobalResolutions.find(name);
-    return (It == GlobalResolutions.end() || It->second.VisibleOutsideSummary);
+    auto It = GlobalResolutions->find(name);
+    return (It == GlobalResolutions->end() || It->second.VisibleOutsideSummary);
   };
 
   // If allowed, upgrade public vcall visibility metadata to linkage unit
@@ -1291,7 +1295,7 @@ Error LTO::runRegularLTO(AddStreamFn AddStream) {
     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 
   if (!Conf.CodeGenOnly) {
-    for (const auto &R : GlobalResolutions) {
+    for (const auto &R : *GlobalResolutions) {
       GlobalValue *GV =
           RegularLTO.CombinedModule->getNamedValue(R.second.IRName);
       if (!R.second.isPrevailingIRSymbol())
@@ -1708,8 +1712,8 @@ Error LTO::runThinLTO(AddStreamFn AddStream, FileCache Cache,
     // This returns true when the name is local or not defined. Locals are
     // expected to be handled separately.
     auto IsVisibleToRegularObj = [&](StringRef name) {
-      auto It = GlobalResolutions.find(name);
-      return (It == GlobalResolutions.end() ||
+      auto It = GlobalResolutions->find(name);
+      return (It == GlobalResolutions->end() ||
               It->second.VisibleOutsideSummary);
     };
 
@@ -1739,15 +1743,11 @@ Error LTO::runThinLTO(AddStreamFn AddStream, FileCache Cache,
     ContextDisambiguation.run(ThinLTO.CombinedIndex, isPrevailing);
   }
 
-  if (Conf.OptLevel > 0)
-    ComputeCrossModuleImport(ThinLTO.CombinedIndex, ModuleToDefinedGVSummaries,
-                             isPrevailing, ImportLists, ExportLists);
-
   // Figure out which symbols need to be internalized. This also needs to happen
   // at -O0 because summary-based DCE is implemented using internalization, and
   // we must apply DCE consistently with the full LTO module in order to avoid
   // undefined references during the final link.
-  for (auto &Res : GlobalResolutions) {
+  for (auto &Res : *GlobalResolutions) {
     // If the symbol does not have external references or it is not prevailing,
     // then not need to mark it as exported from a ThinLTO partition.
     if (Res.second.Partition != GlobalResolution::External ||
@@ -1760,6 +1760,16 @@ Error LTO::runThinLTO(AddStreamFn AddStream, FileCache Cache,
       ExportedGUIDs.insert(GUID);
   }
 
+  // Reset the GlobalResolutions to deallocate the associated memory, as there
+  // are no further accesses. We specifically want to do this before computing
+  // cross module importing, which adds to peak memory via the computed import
+  // and export lists.
+  GlobalResolutions.reset();
+
+  if (Conf.OptLevel > 0)
+    ComputeCrossModuleImport(ThinLTO.CombinedIndex, ModuleToDefinedGVSummaries,
+                             isPrevailing, ImportLists, ExportLists);
+
   // Any functions referenced by the jump table in the regular LTO object must
   // be exported.
   for (auto &Def : ThinLTO.CombinedIndex.cfiFunctionDefs())