cmMakefile: Improve performance of GetSource for known files

Store "Known" files separately in KnownFileSearchIndex. This avoids
creating the rather expensive cmSourceFileLocation object for source
files that are already known. For large projects this results in a
factor 3-4 speedup of cmGlobalGenerator::Compute().

Source/cmMakefile.cxx

@@ -3138,6 +3138,14 @@ void cmMakefile::SetArgcArgv(const std::vector<std::string>& args)
 cmSourceFile* cmMakefile::GetSource(const std::string& sourceName,
                                     cmSourceFileLocationKind kind) const
 {
+  // First check "Known" paths (avoids the creation of cmSourceFileLocation)
+  if (kind == cmSourceFileLocationKind::Known) {
+    auto sfsi = this->KnownFileSearchIndex.find(sourceName);
+    if (sfsi != this->KnownFileSearchIndex.end()) {
+      return sfsi->second;
+    }
+  }
+
   cmSourceFileLocation sfl(this, sourceName, kind);
   auto name = this->GetCMakeInstance()->StripExtension(sfl.GetName());
 #if defined(_WIN32) || defined(__APPLE__)
@@ -3170,6 +3178,10 @@ cmSourceFile* cmMakefile::CreateSource(const std::string& sourceName,
   name = cmSystemTools::LowerCase(name);
 #endif
   this->SourceFileSearchIndex[name].push_back(sf);
+  // for "Known" paths add direct lookup (used for faster lookup in GetSource)
+  if (kind == cmSourceFileLocationKind::Known) {
+    this->KnownFileSearchIndex[sourceName] = sf;
+  }
 
   return sf;
 }

Source/cmMakefile.h

@@ -861,6 +861,9 @@ protected:
   typedef std::unordered_map<std::string, SourceFileVec> SourceFileMap;
   SourceFileMap SourceFileSearchIndex;
 
+  // For "Known" paths we can store a direct filename to cmSourceFile map
+  std::unordered_map<std::string, cmSourceFile*> KnownFileSearchIndex;
+
   // Tests
   std::map<std::string, cmTest*> Tests;