[LTO][NFC] Free the GlobalResolutions map after final use (#76780)

The GlobalResolutions map was found to contribute ~9% of the peak
memory of a large thin link. However, we are essentially done with it
when we are about to compute cross module imports, which itself adds to
the peak memory due to the import and export lists (there is one use
just after importing but it can easily be moved before importing).

Move the last use up above importing, and free the GlobalResolutions
map after that (and before importing). To help guard against future
inadvertent use after it has been released, change it to a
std::optional.

Author: teresajohnson

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())