[Frontend] Add flag for pack metadata stack promo.

Author: nate-chandler

include/swift/AST/SILOptions.h

@@ -176,6 +176,12 @@ class SILOptions {
   /// Require linear OSSA lifetimes after SILGen
   bool OSSACompleteLifetimes = false;
 
+  /// Enable pack metadata stack "promotion".
+  ///
+  /// More accurately, enable skipping mandatory heapification of pack metadata
+  /// when possible.
+  bool EnablePackMetadataStackPromotion = false;
+
   // The kind of function bodies to skip emitting.
   FunctionBodySkipping SkipFunctionBodies = FunctionBodySkipping::None;
 

include/swift/Option/FrontendOptions.td

@@ -1021,6 +1021,16 @@ def force_struct_type_layouts : Flag<["-"], "force-struct-type-layouts">,
 def enable_layout_string_value_witnesses : Flag<["-"], "enable-layout-string-value-witnesses">,
   HelpText<"Enable layout string based value witnesses">;
 
+let Flags = [FrontendOption, NoDriverOption, HelpHidden, ModuleInterfaceOptionIgnorable] in {
+  def enable_pack_metadata_stack_promotion :
+    Joined<["-"], "enable-pack-metadata-stack-promotion=">,
+    HelpText<"Whether to skip heapifying stack metadata packs when possible.">,
+    MetaVarName<"true|false">;
+  def enable_pack_metadata_stack_promotion_noArg :
+    Flag<["-"], "enable-pack-metadata-stack-promotion">,
+    HelpText<"Skip heapifying stack metadata packs when possible.">;
+}
+
 def disable_layout_string_value_witnesses : Flag<["-"], "disable-layout-string-value-witnesses">,
   HelpText<"Disable layout string based value witnesses">;
 

lib/Frontend/CompilerInvocation.cpp

@@ -2024,6 +2024,30 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
   if (specifiedDestroyHoistingOption)
     Opts.DestroyHoisting = *specifiedDestroyHoistingOption;
 
+  Optional<bool> enablePackMetadataStackPromotionFlag;
+  if (Arg *A = Args.getLastArg(OPT_enable_pack_metadata_stack_promotion)) {
+    enablePackMetadataStackPromotionFlag =
+        llvm::StringSwitch<Optional<bool>>(A->getValue())
+            .Case("true", true)
+            .Case("false", false)
+            .Default(None);
+  }
+  if (Args.getLastArg(OPT_enable_pack_metadata_stack_promotion_noArg)) {
+    if (!enablePackMetadataStackPromotionFlag.value_or(true)) {
+      // Error if pack metadata stack promotion has been disabled via the
+      // meta-var form and enabled via the flag.
+      Diags.diagnose(SourceLoc(), diag::error_invalid_arg_combination,
+                     "enable-pack-metadata-stack-promotion",
+                     "enable-pack-metadata-stack-promotion=false");
+      return true;
+    } else {
+      enablePackMetadataStackPromotionFlag = true;
+    }
+  }
+  if (enablePackMetadataStackPromotionFlag)
+    Opts.EnablePackMetadataStackPromotion =
+        enablePackMetadataStackPromotionFlag.value();
+
   Opts.EnableARCOptimizations &= !Args.hasArg(OPT_disable_arc_opts);
   Opts.EnableOSSAModules |= Args.hasArg(OPT_enable_ossa_modules);
   Opts.EnableOSSAOptimizations &= !Args.hasArg(OPT_disable_ossa_opts);

tools/sil-opt/SILOpt.cpp

@@ -144,6 +144,12 @@ static llvm::cl::opt<llvm::cl::boolOrDefault> EnableExperimentalMoveOnly(
     "enable-experimental-move-only", llvm::cl::init(llvm::cl::BOU_UNSET),
     llvm::cl::desc("Enable experimental move-only semantics."));
 
+static llvm::cl::opt<bool> EnablePackMetadataStackPromotion =
+    llvm::cl::opt<bool>(
+        "enable-pack-metadata-stack-promotion", llvm::cl::init(false),
+        llvm::cl::desc(
+            "Whether to skip heapifying stack metadata packs when possible."));
+
 static llvm::cl::opt<bool>
 EnableExperimentalDistributed("enable-experimental-distributed",
                    llvm::cl::desc("Enable experimental distributed actors."));
@@ -708,6 +714,8 @@ int main(int argc, char **argv) {
             ? LexicalLifetimesOption::DiagnosticMarkersOnly
             : LexicalLifetimesOption::Off;
 
+  SILOpts.EnablePackMetadataStackPromotion = EnablePackMetadataStackPromotion;
+
   if (OptModeFlag == OptimizationMode::NotSet) {
     if (OptimizationGroup == OptGroup::Diagnostics)
       SILOpts.OptMode = OptimizationMode::NoOptimization;