[IRGen] Skip metadata heapification on flag.

If the -enable-pack-metadata-stack-promotion[=true] flag is passed, skip
eager heapification.

Author: nate-chandler

lib/IRGen/GenPack.cpp

@@ -512,10 +512,12 @@ irgen::emitTypeMetadataPackRef(IRGenFunction &IGF, CanPackType packType,
   metadata = IGF.Builder.CreatePointerCast(
       metadata, IGF.IGM.TypeMetadataPtrTy->getPointerTo());
 
-  metadata = IGF.Builder.CreateCall(
-      IGF.IGM.getAllocateMetadataPackFunctionPointer(), {metadata, shape});
+  if (!IGF.canStackPromotePackMetadata()) {
+    metadata = IGF.Builder.CreateCall(
+        IGF.IGM.getAllocateMetadataPackFunctionPointer(), {metadata, shape});
 
-  cleanupTypeMetadataPack(IGF, pack, shape);
+    cleanupTypeMetadataPack(IGF, pack, shape);
+  }
 
   auto response = MetadataResponse::forComplete(metadata);
   IGF.setScopedLocalTypeMetadata(packType, response);
@@ -674,10 +676,12 @@ llvm::Value *irgen::emitWitnessTablePackRef(IRGenFunction &IGF,
   result = IGF.Builder.CreatePointerCast(
       result, IGF.IGM.WitnessTablePtrTy->getPointerTo());
 
-  result = IGF.Builder.CreateCall(
-      IGF.IGM.getAllocateWitnessTablePackFunctionPointer(), {result, shape});
+  if (!IGF.canStackPromotePackMetadata()) {
+    result = IGF.Builder.CreateCall(
+        IGF.IGM.getAllocateWitnessTablePackFunctionPointer(), {result, shape});
 
-  cleanupWitnessTablePack(IGF, pack, shape);
+    cleanupWitnessTablePack(IGF, pack, shape);
+  }
 
   IGF.setScopedLocalTypeData(packType, localDataKind, result);
 

lib/IRGen/IRGenFunction.cpp

@@ -76,6 +76,11 @@ OptimizationMode IRGenFunction::getEffectiveOptimizationMode() const {
   return IGM.getOptions().OptMode;
 }
 
+bool IRGenFunction::canStackPromotePackMetadata() const {
+  return IGM.getSILModule().getOptions().EnablePackMetadataStackPromotion &&
+         !packMetadataStackPromotionDisabled;
+}
+
 ModuleDecl *IRGenFunction::getSwiftModule() const {
   return IGM.getSwiftModule();
 }

lib/IRGen/IRGenFunction.h

@@ -201,6 +201,10 @@ class IRGenFunction {
   llvm::Value *AsyncCoroutineCurrentResume = nullptr;
   llvm::Value *AsyncCoroutineCurrentContinuationContext = nullptr;
 
+  // Whether pack metadata stack promotion is disabled for this function in
+  // particular.
+  bool packMetadataStackPromotionDisabled = false;
+
   Address asyncContextLocation;
 
   /// The unique block that calls @llvm.coro.end.
@@ -229,6 +233,10 @@ class IRGenFunction {
     return getEffectiveOptimizationMode() == OptimizationMode::ForSize;
   }
 
+  /// Whether metadata/wtable packs allocated on the stack must be eagerly
+  /// heapified.
+  bool canStackPromotePackMetadata() const;
+
   void setupAsync(unsigned asyncContextIndex);
   bool isAsync() const { return asyncContextLocation.isValid(); }