SwiftCompilerSources: support Undef.parentFunction and PlaceholderValue.parentFunction

After support in the C++ SIL, we can implement parentFunction for those values in swift, too.
It simplifies a few things.

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/Utilities/LifetimeDependenceUtils.swift

@@ -185,7 +185,7 @@ struct LifetimeDependence : CustomStringConvertible {
   var parentValue: Value { scope.parentValue }
 
   var function: Function {
-    dependentValue.parentFunction // dependentValue can't be undef
+    dependentValue.parentFunction
   }
 
   var description: String {

SwiftCompilerSources/Sources/Optimizer/Utilities/OptUtils.swift

@@ -47,9 +47,6 @@ extension Value {
   /// check, because it treats a value_to_bridge_object instruction as "trivial".
   /// It can also handle non-trivial enums with trivial cases.
   func isTrivial(_ context: some Context) -> Bool {
-    if self is Undef {
-      return true
-    }
     var worklist = ValueWorklist(context)
     defer { worklist.deinitialize() }
 
@@ -64,12 +61,12 @@ extension Value {
       switch v {
       case is ValueToBridgeObjectInst:
         break
-      case is StructInst, is TupleInst:
-        let inst = (v as! SingleValueInstruction)
-        worklist.pushIfNotVisited(contentsOf: inst.operands.values.filter { !($0 is Undef) })
+      case let si as StructInst:
+        worklist.pushIfNotVisited(contentsOf: si.operands.values)
+      case let ti as TupleInst:
+        worklist.pushIfNotVisited(contentsOf: ti.operands.values)
       case let en as EnumInst:
-        if let payload = en.payload,
-           !(payload is Undef) {
+        if let payload = en.payload {
           worklist.pushIfNotVisited(payload)
         }
       default:

SwiftCompilerSources/Sources/SIL/Value.swift

@@ -25,10 +25,13 @@ public protocol Value : AnyObject, CustomStringConvertible {
   var definingInstruction: Instruction? { get }
 
   /// The block where the value is defined.
-  ///
-  /// It's not legal to get the definingBlock of an `Undef` value.
   var parentBlock: BasicBlock { get }
 
+  /// The function where the value lives in.
+  ///
+  /// It's not legal to get the parentFunction of an instruction in a global initializer.
+  var parentFunction: Function { get }
+
   /// True if the value has a trivial type.
   var hasTrivialType: Bool { get }
 
@@ -113,6 +116,7 @@ extension Value {
 
   public var uses: UseList { UseList(bridged.getFirstUse()) }
 
+  // Default implementation for all values which have a parent block, like instructions and arguments.
   public var parentFunction: Function { parentBlock.parentFunction }
 
   public var type: Type { bridged.getType().type }
@@ -206,8 +210,11 @@ extension BridgedValue {
 public final class Undef : Value {
   public var definingInstruction: Instruction? { nil }
 
+  public var parentFunction: Function { bridged.SILUndef_getParentFunction().function }
+
   public var parentBlock: BasicBlock {
-    fatalError("undef has no defining block")
+    // By convention, undefs are considered to be defined at the entry of the function.
+    parentFunction.entryBlock
   }
 
   /// Undef has not parent function, therefore the default `hasTrivialType` does not work.
@@ -221,9 +228,12 @@ public final class Undef : Value {
 
 final class PlaceholderValue : Value {
   public var definingInstruction: Instruction? { nil }
+
   public var parentBlock: BasicBlock {
     fatalError("PlaceholderValue has no defining block")
   }
+
+  public var parentFunction: Function { bridged.PlaceholderValue_getParentFunction().function }
 }
 
 extension OptionalBridgedValue {

include/swift/SIL/SILBridging.h

@@ -414,6 +414,8 @@ struct BridgedValue {
   SWIFT_IMPORT_UNSAFE BRIDGED_INLINE OptionalBridgedOperand getFirstUse() const;
   SWIFT_IMPORT_UNSAFE BRIDGED_INLINE BridgedType getType() const;
   BRIDGED_INLINE Ownership getOwnership() const;
+  SWIFT_IMPORT_UNSAFE BRIDGED_INLINE BridgedFunction SILUndef_getParentFunction() const;
+  SWIFT_IMPORT_UNSAFE BRIDGED_INLINE BridgedFunction PlaceholderValue_getParentFunction() const;
 
   bool findPointerEscape() const;
 };

include/swift/SIL/SILBridgingImpl.h

@@ -418,6 +418,14 @@ BridgedValue::Ownership BridgedValue::getOwnership() const {
   return castOwnership(getSILValue()->getOwnershipKind());
 }
 
+BridgedFunction BridgedValue::SILUndef_getParentFunction() const {
+  return {llvm::cast<swift::SILUndef>(getSILValue())->getParent()};
+}
+
+BridgedFunction BridgedValue::PlaceholderValue_getParentFunction() const {
+  return {llvm::cast<swift::PlaceholderValue>(getSILValue())->getParent()};
+}
+
 //===----------------------------------------------------------------------===//
 //                                BridgedOperand
 //===----------------------------------------------------------------------===//