cmSourceFileLocation: Avoid checking the disk repeatedly

Source file lookups with unknown extensions checked the disk every
time.  Skip the check when neither the directory nor the extension is
ambiguous, and remember files found to exist until files may have been
removed or renamed.

Source/cmGlobalGenerator.cxx

@@ -2603,6 +2603,7 @@ void cmGlobalGenerator::ClearGeneratorMembers()
   this->ProjectMap.clear();
   this->RuleHashes.clear();
   this->DirectoryContentMap.clear();
+  this->ExistingSourceFiles.clear();
   this->XcFrameworkPListContentMap.clear();
   this->BinaryDirectories.clear();
   this->GeneratedFiles.clear();
@@ -4518,6 +4519,18 @@ void cmGlobalGenerator::AddToManifest(std::string const& f)
   dc.All.insert(file);
 }
 
+bool cmGlobalGenerator::SourceFileExists(std::string const& path)
+{
+  if (this->ExistingSourceFiles.count(path)) {
+    return true;
+  }
+  if (cmSystemTools::FileExists(path, true)) {
+    this->ExistingSourceFiles.insert(path);
+    return true;
+  }
+  return false;
+}
+
 std::set<std::string> const& cmGlobalGenerator::GetDirectoryContent(
   std::string const& dir, bool needDisk)
 {

Source/cmGlobalGenerator.h

@@ -584,6 +584,15 @@ public:
   std::set<std::string> const& GetDirectoryContent(std::string const& dir,
                                                    bool needDisk = true);
 
+  /** Check whether a file exists for the purpose of resolving the
+      location of a source file.  Files found to exist are remembered until
+      the next configure, or until cmake::InvalidateFileSystemCaches,
+      because the same source file is typically looked up many times.
+      Missing files are always checked again so that files created later
+      are still found.  */
+  bool SourceFileExists(std::string const& path);
+  void ClearSourceFileExistsCache() { this->ExistingSourceFiles.clear(); }
+
   void IndexTarget(cmTarget* t);
   void IndexGeneratorTarget(cmGeneratorTarget* gt);
 
@@ -1136,6 +1145,9 @@ private:
   };
   std::map<std::string, DirectoryContent> DirectoryContentMap;
 
+  // Files known to exist, see SourceFileExists.
+  std::unordered_set<std::string> ExistingSourceFiles;
+
   // Cache parsed PList files
   std::map<std::string, cmXcFrameworkPlist> XcFrameworkPListContentMap;
 

Source/cmSourceFileLocation.cxx

@@ -117,9 +117,11 @@ void cmSourceFileLocation::UpdateExtension(std::string const& name)
     // This is a known extension.  Use the given filename with extension.
     this->Name = cmSystemTools::GetFilenameName(name);
     this->AmbiguousExtension = false;
-  } else {
+  } else if (this->AmbiguousExtension || this->AmbiguousDirectory) {
     // This is not a known extension.  See if the file exists on disk as
-    // named.
+    // named.  This is not needed if neither the extension nor the
+    // directory is ambiguous, because finding the file would not change
+    // the location.
     std::string tryPath;
     if (this->AmbiguousDirectory) {
       // Check the source tree only because a file in the build tree should
@@ -132,7 +134,7 @@ void cmSourceFileLocation::UpdateExtension(std::string const& name)
       tryPath += "/";
     }
     tryPath += this->Name;
-    if (cmSystemTools::FileExists(tryPath, true)) {
+    if (gg->SourceFileExists(tryPath)) {
       // We found a source file named by the user on disk.  Trust it's
       // extension.
       this->Name = cmSystemTools::GetFilenameName(name);

Source/cmake.cxx

@@ -2577,7 +2577,9 @@ int cmake::Configure()
 
 void cmake::InvalidateFileSystemCaches()
 {
-  // Caches of file system state are cleared here as they are added.
+  if (this->GlobalGenerator) {
+    this->GlobalGenerator->ClearSourceFileExistsCache();
+  }
 }
 
 int cmake::ActualConfigure()