IRGen: Enable the new llvm pass manager

rdar://96687157

Author: aschwaighofer

include/swift/AST/IRGenOptions.h

@@ -446,7 +446,7 @@ class IRGenOptions {
   IRGenOptions()
       : DWARFVersion(2),
         OutputKind(IRGenOutputKind::LLVMAssemblyAfterOptimization),
-        Verify(true), LegacyPassManager(1), OptMode(OptimizationMode::NotSet),
+        Verify(true), LegacyPassManager(0), OptMode(OptimizationMode::NotSet),
         Sanitizers(OptionSet<SanitizerKind>()),
         SanitizersWithRecoveryInstrumentation(OptionSet<SanitizerKind>()),
         SanitizeAddressUseODRIndicator(false),

include/swift/Option/FrontendOptions.td

@@ -1079,5 +1079,8 @@ def concurrency_model_EQ :
 
 def enable_new_llvm_pass_manager :
   Flag<["-"], "enable-new-llvm-pass-manager">,
-  HelpText<"Enable new llvm pass manager">;
+  HelpText<"Enable the new llvm pass manager">;
+def disable_new_llvm_pass_manager :
+  Flag<["-"], "disable-new-llvm-pass-manager">,
+  HelpText<"Disable the new llvm pass manager">;
 } // end let Flags = [FrontendOption, NoDriverOption, HelpHidden]

lib/Frontend/CompilerInvocation.cpp

@@ -2389,8 +2389,10 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
       Opts.SwiftAsyncFramePointer = SwiftAsyncFramePointerKind::Never;
   }
 
-  if (Args.hasArg(OPT_enable_new_llvm_pass_manager))
-    Opts.LegacyPassManager = false;
+  Opts.LegacyPassManager =
+      Args.hasFlag(OPT_disable_new_llvm_pass_manager,
+                   OPT_enable_new_llvm_pass_manager,
+                   Opts.LegacyPassManager);
 
   return false;
 }