[LTO] Introduce a helper function collectImportStatistics (NFC) (#106179)

This patch introduces a helper function collectImportStatistics.  The
new function computes statistics of imports for
ComputeCrossModuleImport and dumpImportListForModule with no
functional change.

The background is as follows.  I'm planning to reduce the memory
footprint of ThinLTO indexing by changing ImportMapTy, the data
structure used for an import list.  The new list will be a hash set of
tuples (SourceModule, GUID, ImportType) represented in a space
efficient manner.  That means that obtaining statistics like the
number of definitions per source module requires us to go through the
entire import list (for a given destination module).

Introducing a helper function now makes the callers more independent
of the underlying data structures used in ImportMapT.

Author: kazutakahirata

llvm/lib/Transforms/IPO/FunctionImport.cpp

@@ -1082,21 +1082,29 @@ numGlobalVarSummaries(const ModuleSummaryIndex &Index,
   return NumGVS;
 }
 
-// Given ImportMap, return the number of global variable summaries and record
-// the number of defined function summaries as output parameter.
-static unsigned
-numGlobalVarSummaries(const ModuleSummaryIndex &Index,
-                      const FunctionImporter::FunctionsToImportTy &ImportMap,
-                      unsigned &DefinedFS) {
+struct ImportStatistics {
   unsigned NumGVS = 0;
-  DefinedFS = 0;
-  for (auto &[GUID, Type] : ImportMap) {
-    if (isGlobalVarSummary(Index, GUID))
-      ++NumGVS;
-    else if (Type == GlobalValueSummary::Definition)
-      ++DefinedFS;
+  unsigned DefinedFS = 0;
+  unsigned Count = 0;
+};
+
+// Compute import statistics for each source module in ImportList.
+static DenseMap<StringRef, ImportStatistics>
+collectImportStatistics(const ModuleSummaryIndex &Index,
+                        const FunctionImporter::ImportMapTy &ImportList) {
+  DenseMap<StringRef, ImportStatistics> Histogram;
+
+  for (const auto &[FromModule, GUIDs] : ImportList.getImportMap()) {
+    ImportStatistics &Entry = Histogram[FromModule];
+    for (const auto &[GUID, Type] : GUIDs) {
+      ++Entry.Count;
+      if (isGlobalVarSummary(Index, GUID))
+        ++Entry.NumGVS;
+      else if (Type == GlobalValueSummary::Definition)
+        ++Entry.DefinedFS;
+    }
   }
-  return NumGVS;
+  return Histogram;
 }
 #endif
 
@@ -1217,21 +1225,19 @@ void llvm::ComputeCrossModuleImport(
     auto ModName = ModuleImports.first;
     auto &Exports = ExportLists[ModName];
     unsigned NumGVS = numGlobalVarSummaries(Index, Exports);
+    DenseMap<StringRef, ImportStatistics> Histogram =
+        collectImportStatistics(Index, ModuleImports.second);
     LLVM_DEBUG(dbgs() << "* Module " << ModName << " exports "
                       << Exports.size() - NumGVS << " functions and " << NumGVS
-                      << " vars. Imports from "
-                      << ModuleImports.second.getImportMap().size()
+                      << " vars. Imports from " << Histogram.size()
                       << " modules.\n");
-    for (const auto &Src : ModuleImports.second.getImportMap()) {
-      auto SrcModName = Src.first;
-      unsigned DefinedFS = 0;
-      unsigned NumGVSPerMod =
-          numGlobalVarSummaries(Index, Src.second, DefinedFS);
-      LLVM_DEBUG(dbgs() << " - " << DefinedFS << " function definitions and "
-                        << Src.second.size() - NumGVSPerMod - DefinedFS
+    for (const auto &[SrcModName, Stats] : Histogram) {
+      LLVM_DEBUG(dbgs() << " - " << Stats.DefinedFS
+                        << " function definitions and "
+                        << Stats.Count - Stats.NumGVS - Stats.DefinedFS
                         << " function declarations imported from " << SrcModName
                         << "\n");
-      LLVM_DEBUG(dbgs() << " - " << NumGVSPerMod
+      LLVM_DEBUG(dbgs() << " - " << Stats.NumGVS
                         << " global vars imported from " << SrcModName << "\n");
     }
   }
@@ -1242,17 +1248,17 @@ void llvm::ComputeCrossModuleImport(
 static void dumpImportListForModule(const ModuleSummaryIndex &Index,
                                     StringRef ModulePath,
                                     FunctionImporter::ImportMapTy &ImportList) {
+  DenseMap<StringRef, ImportStatistics> Histogram =
+      collectImportStatistics(Index, ImportList);
   LLVM_DEBUG(dbgs() << "* Module " << ModulePath << " imports from "
-                    << ImportList.getImportMap().size() << " modules.\n");
-  for (const auto &Src : ImportList.getImportMap()) {
-    auto SrcModName = Src.first;
-    unsigned DefinedFS = 0;
-    unsigned NumGVSPerMod = numGlobalVarSummaries(Index, Src.second, DefinedFS);
-    LLVM_DEBUG(dbgs() << " - " << DefinedFS << " function definitions and "
-                      << Src.second.size() - DefinedFS - NumGVSPerMod
+                    << Histogram.size() << " modules.\n");
+  for (const auto &[SrcModName, Stats] : Histogram) {
+    LLVM_DEBUG(dbgs() << " - " << Stats.DefinedFS
+                      << " function definitions and "
+                      << Stats.Count - Stats.DefinedFS - Stats.NumGVS
                       << " function declarations imported from " << SrcModName
                       << "\n");
-    LLVM_DEBUG(dbgs() << " - " << NumGVSPerMod << " vars imported from "
+    LLVM_DEBUG(dbgs() << " - " << Stats.NumGVS << " vars imported from "
                       << SrcModName << "\n");
   }
 }