[Modules] Enable the Modules language feature by default for the Clang builtins.

[eladcohen]

Summary

The motivation is to reduce the compile time of the Clang intrinsics header files. Furthermore, this should allow us to remove the feature guards from these headers D27896 and solve some MSVC compatibility issues regarding intrinsics.

See also the threads "The intrinsics headers (especially avx512) are too big. What to do about it?" and "clang-cl's <intrin.h>, _tzcnt_u32, and compatibility with MSVC's <intrin.h>" in cfe-dev.

[eladcohen]

ping

lib/Driver/Tools.cpp

   }
 
   if (Args.hasFlag(options::OPT_fcoroutines_ts, options::OPT_fno_coroutines_ts,
                    false) &&
       types::isCXX(InputType)) {
     CmdArgs.push_back("-fcoroutines-ts");
   }
 
+  // If modules is not asked for explicitly enable it exclusively
+  // for the Clang builtins in applicable cases.
+  bool HaveBuiltinModules = !Args.hasArg(options::OPT_fno_implicit_modules) &&
+                            !Args.hasArg(options::OPT_fmodules) &&
+                            !Args.hasArg(options::OPT_include) &&
+                            !isa<PreprocessJobAction>(JA) &&
+                            ((JA.getType() == types::TY_LLVM_BC) ||
+                             (JA.getType() == types::TY_LLVM_IR) ||
+                             (JA.getType() == types::TY_LTO_BC) ||
+                             (JA.getType() == types::TY_LTO_IR) ||
+                             (JA.getType() == types::TY_PP_Asm) ||
+                             (JA.getType() == types::TY_Object) ||
+                             (JA.getType() == types::TY_AST));
+
   // -fmodules enables the use of precompiled modules (off by default).
   // Users can pass -fno-cxx-modules to turn off modules support for
   // C++/Objective-C++ programs.
   bool HaveClangModules = false;
-  if (Args.hasFlag(options::OPT_fmodules, options::OPT_fno_modules, false)) {
+  if (Args.hasFlag(options::OPT_fmodules, options::OPT_fno_modules,
+                   HaveBuiltinModules)) {
     bool AllowedInCXX = Args.hasFlag(options::OPT_fcxx_modules,
                                      options::OPT_fno_cxx_modules, true);
     if (AllowedInCXX || !types::isCXX(InputType)) {
       CmdArgs.push_back("-fmodules");
       HaveClangModules = true;
     }
   }
 
   bool HaveAnyModules = HaveClangModules;
   if (Args.hasArg(options::OPT_fmodules_ts)) {
     CmdArgs.push_back("-fmodules-ts");
     HaveAnyModules = true;
   }
 
   // -fmodule-maps enables implicit reading of module map files. By default,
-  // this is enabled if we are using Clang's flavor of precompiled modules.
+  // this is enabled if Clang's flavor of precompiled modules is explicitly
+  // enabled.
   if (Args.hasFlag(options::OPT_fimplicit_module_maps,
-                   options::OPT_fno_implicit_module_maps, HaveClangModules)) {
+                   options::OPT_fno_implicit_module_maps,
+                   HaveClangModules && !HaveBuiltinModules)) {
     CmdArgs.push_back("-fimplicit-module-maps");
   }
 
   // -fmodules-decluse checks that modules used are declared so (off by
   // default).
   if (Args.hasFlag(options::OPT_fmodules_decluse,
                    options::OPT_fno_modules_decluse, false)) {
     CmdArgs.push_back("-fmodules-decluse");

   Args.AddLastArg(CmdArgs, options::OPT_fmodule_name_EQ);
 
   // -fmodule-map-file can be used to specify files containing module
   // definitions.
   Args.AddAllArgs(CmdArgs, options::OPT_fmodule_map_file);
 
   // -fbuiltin-module-map can be used to load the clang
   // builtin headers modulemap file.
-  if (Args.hasArg(options::OPT_fbuiltin_module_map)) {
+  if (Args.hasArg(options::OPT_fbuiltin_module_map) ||
+    (HaveClangModules && HaveBuiltinModules)) {
     SmallString<128> BuiltinModuleMap(getToolChain().getDriver().ResourceDir);
     llvm::sys::path::append(BuiltinModuleMap, "include");
     llvm::sys::path::append(BuiltinModuleMap, "module.modulemap");
     if (llvm::sys::fs::exists(BuiltinModuleMap)) {
       CmdArgs.push_back(Args.MakeArgString("-fmodule-map-file=" +
                                            BuiltinModuleMap));
     }
   }

test/Driver/modules.mm

-// RUN: %clang -### %s 2>&1 | FileCheck -check-prefix=CHECK-NO-MODULES %s
-// RUN: %clang -fcxx-modules -### %s 2>&1 | FileCheck -check-prefix=CHECK-NO-MODULES %s
+// RUN: %clang -### %s 2>&1 | FileCheck -check-prefix=CHECK-DEFAULT-MODULES %s
+// CHECK-DEFAULT-MODULES: -fmodules
+
+// RUN: %clang -fno-modules -### %s 2>&1 | FileCheck -check-prefix=CHECK-NO-MODULES %s
+// RUN: %clang -fno-cxx-modules -### %s 2>&1 | FileCheck -check-prefix=CHECK-NO-MODULES %s
 // RUN: %clang -fmodules -fno-cxx-modules -### %s 2>&1 | FileCheck -check-prefix=CHECK-NO-MODULES %s
 // CHECK-NO-MODULES-NOT: -fmodules
 
 // RUN: %clang -fmodules -### %s 2>&1 | FileCheck -check-prefix=CHECK-HAS-MODULES %s
 // RUN: %clang -fmodules -fno-cxx-modules -fcxx-modules -### %s 2>&1 | FileCheck -check-prefix=CHECK-HAS-MODULES %s
 // CHECK-HAS-MODULES: -fmodules
 
 // RUN: %clang -### %s 2>&1 | FileCheck -check-prefix=CHECK-NO-MAPS %s
 // RUN: %clang -fimplicit-module-maps -### %s 2>&1 | FileCheck -check-prefix=CHECK-HAS-MAPS %s
 // RUN: %clang -fmodules -### %s 2>&1 | FileCheck -check-prefix=CHECK-HAS-MAPS %s
 // RUN: %clang -fmodules -fno-implicit-module-maps -### %s 2>&1 | FileCheck -check-prefix=CHECK-NO-MAPS %s
 // CHECK-HAS-MAPS: -fimplicit-module-maps
 // CHECK-NO-MAPS-NOT: -fimplicit-module-maps