[clang][modules][pch] Allow loading PCH with different modules cache path

It's useful to be able to load explicitly-built PCH files into an implicit build (e.g. during dependency scanning). That's currently impossible, since the explicitly-built PCH has an empty modules cache path, while the current compilation has (and needs to have) a valid path, triggering an error in the `PCHValidator`.

This patch adds a preprocessor option and command-line flag that can be used to omit this check.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D103802

clang/include/clang/Driver/Options.td

@@ -5499,6 +5499,10 @@ def fallow_pch_with_errors : Flag<["-"], "fallow-pch-with-compiler-errors">,
   HelpText<"Accept a PCH file that was created with compiler errors">,
   MarshallingInfoFlag<PreprocessorOpts<"AllowPCHWithCompilerErrors">>,
   ImpliedByAnyOf<[fallow_pcm_with_errors.KeyPath]>;
+def fallow_pch_with_different_modules_cache_path :
+  Flag<["-"], "fallow-pch-with-different-modules-cache-path">,
+  HelpText<"Accept a PCH file that was created with a different modules cache path">,
+  MarshallingInfoFlag<PreprocessorOpts<"AllowPCHWithDifferentModulesCachePath">>;
 def dump_deserialized_pch_decls : Flag<["-"], "dump-deserialized-decls">,
   HelpText<"Dump declarations that are deserialized from PCH, for testing">,
   MarshallingInfoFlag<PreprocessorOpts<"DumpDeserializedPCHDecls">>;

clang/include/clang/Lex/PreprocessorOptions.h

@@ -106,6 +106,10 @@ public:
   /// When true, a PCH with compiler errors will not be rejected.
   bool AllowPCHWithCompilerErrors = false;
 
+  /// When true, a PCH with modules cache path different to the current
+  /// compilation will not be rejected.
+  bool AllowPCHWithDifferentModulesCachePath = false;
+
   /// Dump declarations that are deserialized from PCH, for testing.
   bool DumpDeserializedPCHDecls = false;
 

clang/lib/Serialization/ASTReader.cpp

@@ -783,9 +783,11 @@ static bool checkHeaderSearchOptions(const HeaderSearchOptions &HSOpts,
                                      StringRef SpecificModuleCachePath,
                                      StringRef ExistingModuleCachePath,
                                      DiagnosticsEngine *Diags,
-                                     const LangOptions &LangOpts) {
+                                     const LangOptions &LangOpts,
+                                     const PreprocessorOptions &PPOpts) {
   if (LangOpts.Modules) {
-    if (SpecificModuleCachePath != ExistingModuleCachePath) {
+    if (SpecificModuleCachePath != ExistingModuleCachePath &&
+        !PPOpts.AllowPCHWithDifferentModulesCachePath) {
       if (Diags)
         Diags->Report(diag::err_pch_modulecache_mismatch)
           << SpecificModuleCachePath << ExistingModuleCachePath;
@@ -802,7 +804,7 @@ bool PCHValidator::ReadHeaderSearchOptions(const HeaderSearchOptions &HSOpts,
   return checkHeaderSearchOptions(HSOpts, SpecificModuleCachePath,
                                   PP.getHeaderSearchInfo().getModuleCachePath(),
                                   Complain ? &Reader.Diags : nullptr,
-                                  PP.getLangOpts());
+                                  PP.getLangOpts(), PP.getPreprocessorOpts());
 }
 
 void PCHValidator::ReadCounter(const ModuleFile &M, unsigned Value) {
@@ -5142,8 +5144,8 @@ namespace {
                                  StringRef SpecificModuleCachePath,
                                  bool Complain) override {
       return checkHeaderSearchOptions(HSOpts, SpecificModuleCachePath,
-                                      ExistingModuleCachePath,
-                                      nullptr, ExistingLangOpts);
+                                      ExistingModuleCachePath, nullptr,
+                                      ExistingLangOpts, ExistingPPOpts);
     }
 
     bool ReadPreprocessorOptions(const PreprocessorOptions &PPOpts,

clang/test/Modules/Inputs/pch-typedef.h

@@ -0,0 +1 @@
+typedef int pch_int;

clang/test/Modules/module-pch-different-cache-path.c

@@ -0,0 +1,18 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+//
+// RUN: %clang_cc1 -x objective-c-header -emit-pch %S/Inputs/pch-typedef.h -o %t/pch-typedef.h.gch \
+// RUN:   -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/one
+//
+// RUN: not %clang_cc1 -x objective-c -fsyntax-only %s -include-pch %t/pch-typedef.h.gch \
+// RUN:   -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/two 2>&1 \
+// RUN:   | FileCheck %s -check-prefixes=CHECK-ERROR
+//
+// RUN: %clang_cc1 -x objective-c -fsyntax-only %s -include-pch %t/pch-typedef.h.gch \
+// RUN:   -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/two 2>&1 -fallow-pch-with-different-modules-cache-path \
+// RUN:   | FileCheck %s -check-prefixes=CHECK-SUCCESS -allow-empty
+
+pch_int x = 0;
+
+// CHECK-ERROR: PCH was compiled with module cache path '{{.*}}', but the path is currently '{{.*}}'
+// CHECK-SUCCESS-NOT: PCH was compiled with module cache path '{{.*}}', but the path is currently '{{.*}}'