Add a flag -enable-default-cmo to enable default cross-module-optimization.

So far, the swift-frontend decided by itself if CMO can be enabled. This caused problems when used with an old driver, which doesn't consider CMO.
Now, the driver decides when to use default CMO by passing this flag to swift-frontend.

Author: eeckstein

include/swift/AST/SILOptions.h

@@ -66,6 +66,12 @@ enum class DestroyHoistingOption : uint8_t {
   On = 1
 };
 
+enum class CrossModuleOptimizationMode : uint8_t {
+  Off = 0,
+  Default = 1,
+  Aggressive = 2
+};
+
 class SILModule;
 
 class SILOptions {
@@ -117,7 +123,7 @@ class SILOptions {
   bool DisableSILPerfOptimizations = false;
 
   /// Controls whether cross module optimization is enabled.
-  bool CrossModuleOptimization = false;
+  CrossModuleOptimizationMode CMOMode = CrossModuleOptimizationMode::Off;
 
   /// Enables experimental performance annotations.
   bool EnablePerformanceAnnotations = false;

include/swift/Option/Options.td

@@ -812,6 +812,10 @@ def Oplayground : Flag<["-"], "Oplayground">, Group<O_Group>,
   Flags<[HelpHidden, FrontendOption, ModuleInterfaceOption]>,
   HelpText<"Compile with optimizations appropriate for a playground">;
 
+def EnbaleDefaultCMO : Flag<["-"], "enable-default-cmo">,
+  Flags<[HelpHidden, FrontendOption]>,
+  HelpText<"Perform conservative cross-module optimization">;
+
 def CrossModuleOptimization : Flag<["-"], "cross-module-optimization">,
   Flags<[HelpHidden, FrontendOption]>,
   HelpText<"Perform cross-module optimization">;

lib/Frontend/CompilerInvocation.cpp

@@ -1745,7 +1745,11 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
       OPT_enable_actor_data_race_checks,
       OPT_disable_actor_data_race_checks, /*default=*/false);
   Opts.DisableSILPerfOptimizations |= Args.hasArg(OPT_disable_sil_perf_optzns);
-  Opts.CrossModuleOptimization |= Args.hasArg(OPT_CrossModuleOptimization);
+  if (Args.hasArg(OPT_CrossModuleOptimization)) {
+    Opts.CMOMode = CrossModuleOptimizationMode::Aggressive;
+  } else if (Args.hasArg(OPT_EnbaleDefaultCMO)) {
+    Opts.CMOMode = CrossModuleOptimizationMode::Default;  
+  }
   Opts.EnablePerformanceAnnotations |=
       Args.hasArg(OPT_ExperimentalPerformanceAnnotations);
   Opts.VerifyAll |= Args.hasArg(OPT_sil_verify_all);

lib/FrontendTool/FrontendTool.cpp

@@ -702,7 +702,8 @@ static bool writeTBDIfNeeded(CompilerInstance &Instance) {
     return false;
   }
 
-  if (Invocation.getSILOptions().CrossModuleOptimization) {
+  if (Invocation.getSILOptions().CMOMode ==
+      CrossModuleOptimizationMode::Aggressive) {
     Instance.getDiags().diagnose(SourceLoc(),
                                  diag::tbd_not_supported_with_cmo);
     return false;
@@ -1406,7 +1407,7 @@ static bool validateTBDIfNeeded(const CompilerInvocation &Invocation,
     }
 
     // Cross-module optimization does not support TBD.
-    if (Invocation.getSILOptions().CrossModuleOptimization) {
+    if (Invocation.getSILOptions().CMOMode == CrossModuleOptimizationMode::Aggressive) {
       return false;
     }
 

lib/SILOptimizer/IPO/CrossModuleOptimization.cpp

@@ -613,9 +613,20 @@ class CrossModuleOptimizationPass: public SILModuleTransform {
       return;
     if (!M.isWholeModule())
       return;
+      
+    bool conservative = false;
+    switch (M.getOptions().CMOMode) {
+      case swift::CrossModuleOptimizationMode::Off:
+        return;
+      case swift::CrossModuleOptimizationMode::Default:
+        conservative = true;
+        break;
+      case swift::CrossModuleOptimizationMode::Aggressive:
+        conservative = false;
+        break;
+    }
 
-    CrossModuleOptimization CMO(M,
-      /*conservative*/ !M.getOptions().CrossModuleOptimization);
+    CrossModuleOptimization CMO(M, conservative);
     CMO.serializeFunctionsInModule();
   }
 };

test/SILOptimizer/default-cmo.swift

@@ -1,9 +1,9 @@
 
 // RUN: %empty-directory(%t) 
 
-// RUN: %target-build-swift -O -wmo -parse-as-library -emit-module -emit-module-path=%t/Submodule.swiftmodule -module-name=Submodule %S/Inputs/cross-module/default-submodule.swift -c -o %t/submodule.o
-// RUN: %target-build-swift -O -wmo -parse-as-library -emit-module -emit-module-path=%t/Module.swiftmodule -module-name=Module -I%t -I%S/Inputs/cross-module %S/Inputs/cross-module/default-module.swift -c -o %t/module.o
-// RUN: %target-build-swift -O -wmo -parse-as-library -emit-tbd -emit-tbd-path %t/ModuleTBD.tbd -emit-module -emit-module-path=%t/ModuleTBD.swiftmodule -module-name=ModuleTBD -I%t -I%S/Inputs/cross-module %S/Inputs/cross-module/default-module.swift -c -o %t/moduletbd.o
+// RUN: %target-build-swift -O -wmo -Xfrontend -enable-default-cmo -parse-as-library -emit-module -emit-module-path=%t/Submodule.swiftmodule -module-name=Submodule %S/Inputs/cross-module/default-submodule.swift -c -o %t/submodule.o
+// RUN: %target-build-swift -O -wmo -Xfrontend -enable-default-cmo -parse-as-library -emit-module -emit-module-path=%t/Module.swiftmodule -module-name=Module -I%t -I%S/Inputs/cross-module %S/Inputs/cross-module/default-module.swift -c -o %t/module.o
+// RUN: %target-build-swift -O -wmo -Xfrontend -enable-default-cmo -parse-as-library -emit-tbd -emit-tbd-path %t/ModuleTBD.tbd -emit-module -emit-module-path=%t/ModuleTBD.swiftmodule -module-name=ModuleTBD -I%t -I%S/Inputs/cross-module %S/Inputs/cross-module/default-module.swift -c -o %t/moduletbd.o
 
 // RUN: %target-build-swift -O -wmo -module-name=Main -I%t %s -emit-sil | %FileCheck %s