[IRGen] Added gate for metadata prespecialization.

The new frontend flag -prespecialize-generic-metadata must be passed in
order for generic metadata to be specialized statically.

rdar://problem/56984885

Author: nate-chandler

include/swift/AST/IRGenOptions.h

@@ -190,6 +190,10 @@ class IRGenOptions {
   /// Passing this flag completely disables this behavior.
   unsigned DisableLegacyTypeInfo : 1;
 
+  /// Create metadata specializations for generic types at statically known type
+  /// arguments.
+  unsigned PrespecializeGenericMetadata : 1;
+
   /// The path to load legacy type layouts from.
   StringRef ReadLegacyTypeInfoPath;
 
@@ -255,8 +259,9 @@ class IRGenOptions {
         EnableAnonymousContextMangledNames(false), ForcePublicLinkage(false),
         LazyInitializeClassMetadata(false),
         LazyInitializeProtocolConformances(false), DisableLegacyTypeInfo(false),
-        UseIncrementalLLVMCodeGen(true), UseSwiftCall(false),
-        GenerateProfile(false), EnableDynamicReplacementChaining(false),
+        PrespecializeGenericMetadata(false), UseIncrementalLLVMCodeGen(true),
+        UseSwiftCall(false), GenerateProfile(false),
+        EnableDynamicReplacementChaining(false),
         DisableRoundTripDebugTypes(false), DisableDebuggerShadowCopies(false),
         CmdArgs(), SanitizeCoverage(llvm::SanitizerCoverageOptions()),
         TypeInfoFilter(TypeInfoDumpFilter::All) {}

include/swift/Option/FrontendOptions.td

@@ -644,6 +644,10 @@ def disable_verify_exclusivity : Flag<["-"], "disable-verify-exclusivity">,
 def disable_legacy_type_info : Flag<["-"], "disable-legacy-type-info">,
   HelpText<"Completely disable legacy type layout">;
 
+def prespecialize_generic_metadata : Flag<["-"], "prespecialize-generic-metadata">,
+  HelpText<"Statically specialize metadata for generic types at types that "
+           "are known to be used in source.">;
+
 def read_legacy_type_info_path_EQ : Joined<["-"], "read-legacy-type-info-path=">,
   HelpText<"Read legacy type layout from the given path instead of default path">;
 

lib/Frontend/CompilerInvocation.cpp

@@ -1249,6 +1249,10 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     Opts.DisableLegacyTypeInfo = true;
   }
 
+  if (Args.hasArg(OPT_prespecialize_generic_metadata)) {
+    Opts.PrespecializeGenericMetadata = true;
+  }
+
   if (const Arg *A = Args.getLastArg(OPT_read_legacy_type_info_path_EQ)) {
     Opts.ReadLegacyTypeInfoPath = A->getValue();
   }