Add a Flag to Force Type Layouts for Structs

Added ForceStructTypeLayouts. When enabled, IRGen will lower structs using the
aligned group of TypeLayout rather than using TypeInfos. This potentially leads
to a size increase as TypeInfos currently produce better code than the
TypeLayout route.

Author: Gwen Mittertreiner

include/swift/AST/IRGenOptions.h

@@ -335,6 +335,10 @@ class IRGenOptions {
   /// vw functions instead of outlined copy/destroy functions.
   unsigned UseTypeLayoutValueHandling : 1;
 
+  /// Also force structs to be lowered to aligned group TypeLayouts rather than
+  /// using TypeInfo entries.
+  unsigned ForceStructTypeLayouts : 1;
+
   /// Instrument code to generate profiling information.
   unsigned GenerateProfile : 1;
 
@@ -413,7 +417,7 @@ class IRGenOptions {
         ForcePublicLinkage(false), LazyInitializeClassMetadata(false),
         LazyInitializeProtocolConformances(false), DisableLegacyTypeInfo(false),
         PrespecializeGenericMetadata(false), UseIncrementalLLVMCodeGen(true),
-        UseTypeLayoutValueHandling(true),
+        UseTypeLayoutValueHandling(true), ForceStructTypeLayouts(false),
         GenerateProfile(false), EnableDynamicReplacementChaining(false),
         DisableRoundTripDebugTypes(false), DisableDebuggerShadowCopies(false),
         DisableConcreteTypeMetadataMangledNameAccessors(false),

include/swift/Option/FrontendOptions.td

@@ -830,6 +830,9 @@ def enable_type_layouts : Flag<["-"], "enable-type-layout">,
 def disable_type_layouts : Flag<["-"], "disable-type-layout">,
   HelpText<"Disable type layout based lowering">;
 
+def force_struct_type_layouts : Flag<["-"], "force-struct-type-layouts">,
+  HelpText<"Force type layout based lowering for structs">;
+
 def disable_interface_lockfile : Flag<["-"], "disable-interface-lock">,
   HelpText<"Don't lock interface file when building module">;
 

lib/Frontend/CompilerInvocation.cpp

@@ -1724,6 +1724,9 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     Opts.UseTypeLayoutValueHandling = false;
   }
 
+  Opts.ForceStructTypeLayouts = Args.hasArg(OPT_force_struct_type_layouts) &&
+                                Opts.UseTypeLayoutValueHandling;
+
   // This is set to true by default.
   Opts.UseIncrementalLLVMCodeGen &=
     !Args.hasArg(OPT_disable_incremental_llvm_codegeneration);