[clang][deps] Modules don't contribute to search path usage

[jansvoboda11]

Summary

To reduce the number of modules we build in explicit builds (which use strict context hash), we prune unused header search paths. This essentially merges parts of the dependency graph.

Determining whether a search path was used to discover a module (through implicit module maps) proved to be somewhat complicated. Initial support landed in D102923, while D113676 attempts to fix some bugs.

However, now that we don't use implicit module maps in explicit builds (since D120465), we don't need to consider such search paths as used anymore. Modules are no longer discovered through the header search mechanism, so we can drop such search paths (provided they are not needed for other reasons).

This patch removes whatever support for detecting such usage we had, since it's buggy and not required anymore.

Depends on D120465.

[dexonsmith]

clang/lib/Lex/HeaderSearch.cpp

307

This triggers a bunch of noise in this patch. Is it possible to make this change either before or after, to avoid the noise, giving that there's a functional change hiding here?

[jansvoboda11]

Undo unrelated changes

[dexonsmith]

Nice! LGTM.

clang/include/clang/Lex/HeaderSearch.h

public:
   /// Determine whether the given file is known to have ever been \#imported.
   bool hasFileBeenImported(const FileEntry *File) {
     const HeaderFileInfo *FI = getExistingFileInfo(File);
     return FI && FI->isImport;
   }
 
   /// Determine which HeaderSearchOptions::UserEntries have been successfully
   /// used so far and mark their index with 'true' in the resulting bit vector.
+  /// Note: implicit module maps don't contribute to entry usage.
   std::vector<bool> computeUserEntryUsage() const;
 
   /// This method returns a HeaderMap for the specified
   /// FileEntry, uniquing them through the 'HeaderMaps' datastructure.
   const HeaderMap *CreateHeaderMap(const FileEntry *FE);
 
   /// Get filenames for all registered header maps.
   void getHeaderMapFileNames(SmallVectorImpl<std::string> &Names) const;

clang/lib/Lex/HeaderSearch.cpp

Module *HeaderSearch::lookupModule(StringRef ModuleName, StringRef SearchName,
                                    SourceLocation ImportLoc,
                                    bool AllowExtraModuleMapSearch) {
   Module *Module = nullptr;
   SearchDirIterator It = nullptr;
 
   // Look through the various header search paths to load any available module
   // maps, searching for a module map that describes this module.
   for (It = search_dir_begin(); It != search_dir_end(); ++It) {
     if (It->isFramework()) {
       // Search for or infer a module map for a framework. Here we use
       // SearchName rather than ModuleName, to permit finding private modules
       // named FooPrivate in buggy frameworks named Foo.
       SmallString<128> FrameworkDirName;
       FrameworkDirName += It->getFrameworkDir()->getName();
       llvm::sys::path::append(FrameworkDirName, SearchName + ".framework");
       if (auto FrameworkDir = FileMgr.getDirectory(FrameworkDirName)) {
         bool IsSystem = It->getDirCharacteristic() != SrcMgr::C_User;
if (AllowExtraModuleMapSearch)
       loadSubdirectoryModuleMaps(*It);
 
     // Look again for the module.
     Module = ModMap.findModule(ModuleName);
     if (Module)
       break;
   }
 
-  if (Module)
-    noteLookupUsage(It.Idx, ImportLoc);
-
   return Module;
 }
 
 //===----------------------------------------------------------------------===//
 // File lookup within a DirectoryLookup scope
 //===----------------------------------------------------------------------===//
 
 /// getName - Return the directory or filename corresponding to this lookup

clang/test/Preprocessor/search-path-usage.m

 // RUN:   -I b                                   \
 // RUN:   -DHMAP_MATCHED_BUT_MISSING_IN_HAS_INCLUDE -verify
 #ifdef HMAP_MATCHED_BUT_MISSING_IN_HAS_INCLUDE
 #if __has_include("b.h") // \
 // expected-remark-re {{search path used: '{{.*}}/b-missing.hmap'}}
 #endif
 #endif
 
-// Check that search paths with module maps are reported.
+// Check that search paths with module maps are NOT reported.
 //
 // RUN: mkdir %t/modulemap_abs
 // RUN: sed "s|DIR|%/S/Inputs/search-path-usage|g"                            \
 // RUN:   %S/Inputs/search-path-usage/modulemap_abs/module.modulemap.template \
 // RUN:     > %t/modulemap_abs/module.modulemap
 // RUN: %clang_cc1 -Eonly %s -Rsearch-path-usage                           \
 // RUN:   -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules \
 // RUN:   -I %t/modulemap_abs                                              \
 // RUN:   -I %S/Inputs/search-path-usage/a                                 \
 // RUN:   -DMODMAP_ABS -verify
 #ifdef MODMAP_ABS
 @import b; // \
-// expected-remark-re {{search path used: '{{.*}}/modulemap_abs'}}
+// expected-no-diagnostics
 #endif