Merge pull request #80137 from eeckstein/fix-cast-optimizer

CastOptimizer: fix optimization of casts from non-existentials to existentials

Author: eeckstein

include/swift/AST/Types.h

@@ -752,6 +752,10 @@ class alignas(1 << TypeAlignInBits) TypeBase
     return getRecursiveProperties().hasOpenedExistential();
   }
 
+  /// True if this type is an existential or an archetype which may be an
+  /// existential.
+  bool canBeExistential();
+
   /// Determine whether the type involves an opened element archetype.
   bool hasElementArchetype() const {
     return getRecursiveProperties().hasElementArchetype();

lib/AST/Type.cpp

@@ -616,6 +616,29 @@ bool TypeBase::isTypeVariableOrMember() {
   return getDependentMemberRoot()->is<TypeVariableType>();
 }
 
+bool TypeBase::canBeExistential() {
+  if (isAnyExistentialType())
+    return true;
+
+  Type ty(this);
+  // Unwrap (potentially multiple levels of) metatypes.
+  while (auto *mt = ty->getAs<MetatypeType>())
+    ty = mt->getInstanceType();
+
+  if (auto *archeTy = ty->getAs<ArchetypeType>()) {
+    // Only if all conformances are self-conforming protocols, the archetype
+    // may be an existential.
+    for (auto *proto : archeTy->getConformsTo()) {
+      if (!proto->existentialConformsToSelf())
+        return false;
+    }
+    // If there are no requirements on the archetype at all (`getConformsTo`
+    // is empty), the archetype can still be `Any` and we have to return true.
+    return true;
+  }
+  return false;
+}
+
 bool TypeBase::isTypeParameter() {
   return getDependentMemberRoot()->is<GenericTypeParamType>();
 }

lib/SILOptimizer/Utils/CastOptimizer.cpp

@@ -1450,7 +1450,7 @@ static bool optimizeStaticallyKnownProtocolConformance(
   auto &Mod = Inst->getModule();
 
   if (TargetType->isAnyExistentialType() &&
-      !SourceType->isAnyExistentialType()) {
+      !SourceType->canBeExistential()) {
     auto &Ctx = Mod.getASTContext();
 
     auto *Proto = dyn_cast_or_null<ProtocolDecl>(TargetType->getAnyNominal());

test/SILOptimizer/constant_propagation.sil

@@ -1425,6 +1425,33 @@ bb0(%0 : $*Error, %1 : $*E1):
   return %2 : $()
 }
 
+// CHECK-LABEL: sil [ossa] @generic_to_existential_cast
+// CHECK:         init_existential_addr
+// CHECK:       } // end sil function 'generic_to_existential_cast'
+sil [ossa] @generic_to_existential_cast : $@convention(thin) <T where T : P> (@in_guaranteed T) -> @out any P {
+bb0(%0: $*any P, %1 : $*T):
+  %29 = alloc_stack $T
+  copy_addr %1 to [init] %29
+  unconditional_checked_cast_addr T in %29 to any P in %0
+  dealloc_stack %29
+  %10 = tuple ()
+  return %10
+}
+
+// CHECK-LABEL: sil [ossa] @existential_generic_to_existential_cast
+// CHECK:         unconditional_checked_cast_addr
+// CHECK:       } // end sil function 'existential_generic_to_existential_cast'
+sil [ossa] @existential_generic_to_existential_cast : $@convention(thin) <T where T : Error> (@in_guaranteed T) -> @owned any Error {
+bb0(%0 : $*T):
+  %29 = alloc_stack $T
+  copy_addr %0 to [init] %29
+  %31 = alloc_stack $any Error
+  unconditional_checked_cast_addr T in %29 to any Error in %31
+  %33 = load [take] %31
+  dealloc_stack %31
+  dealloc_stack %29
+  return %33
+}
 // Test constant folding of Builtin.globalStringTablePointer.
 
 // CHECK-LABEL: sil @replace_global_string_table_pointer