IRGen: fix emission of constant single-case enums

Enums which have a single case with a payload were emitted with wrong field alignement into the data section.

Fixes a miscompile:
rdar://115251963

Author: eeckstein

lib/IRGen/GenConstant.cpp

@@ -273,6 +273,11 @@ Explosion irgen::emitConstantValue(IRGenModule &IGM, SILValue operand,
                                      irgen::getPhysicalTupleElementStructIndex,
                                      flatten);
   } else if (auto *ei = dyn_cast<EnumInst>(operand)) {
+    auto &strategy = getEnumImplStrategy(IGM, ei->getType());
+    if (strategy.emitPayloadDirectlyIntoConstant()) {
+      return emitConstantValue(IGM, ei->getOperand(), flatten);
+    }
+
     Explosion data;
     if (ei->hasOperand()) {
       data = emitConstantValue(IGM, ei->getOperand(), /*flatten=*/ true);
@@ -282,10 +287,9 @@ Explosion irgen::emitConstantValue(IRGenModule &IGM, SILValue operand,
     // arguments to the enum are constant, the builder never has to emit an
     // instruction. Instead it can constant fold everything and just returns
     // the final constant.
-    Explosion out;
     IRBuilder builder(IGM.getLLVMContext(), false);
-    getEnumImplStrategy(IGM, ei->getType())
-      .emitValueInjection(IGM, builder, ei->getElement(), data, out);
+    Explosion out;
+    strategy.emitValueInjection(IGM, builder, ei->getElement(), data, out);
     return replaceUnalignedIntegerValues(IGM, std::move(out));
   } else if (auto *ILI = dyn_cast<IntegerLiteralInst>(operand)) {
     return emitConstantInt(IGM, ILI);

lib/IRGen/GenEnum.cpp

@@ -425,6 +425,8 @@ namespace {
         getLoadableSingleton()->reexplode(params, out);
     }
 
+    bool emitPayloadDirectlyIntoConstant() const override { return true; }
+
     void destructiveProjectDataForLoad(IRGenFunction &IGF,
                                        SILType T,
                                        Address enumAddr) const override {

lib/IRGen/GenEnum.h

@@ -334,6 +334,11 @@ class EnumImplStrategy {
                                   Explosion &params,
                                   Explosion &out) const = 0;
 
+  /// Return true for single-case (singleton) enums.
+  /// The enum doesn't need any tag bits and the payload of the enum case can
+  /// (and needs!) to be emitted as-is into a constant.
+  virtual bool emitPayloadDirectlyIntoConstant() const { return false; }
+
   /// Return an i1 value that indicates whether the specified loadable enum
   /// value holds the specified case.  This is a light-weight form of a switch.
   virtual llvm::Value *emitValueCaseTest(IRGenFunction &IGF,

test/SILOptimizer/static_enums.swift

@@ -218,6 +218,12 @@ func printFunctionEnum() {
   }
 }
 
+enum SingleCaseEnum {
+  case a(b: Bool, i: Int)
+
+  static var x = Self.a(b:true, i: 42)
+}
+
 @main
 struct Main {
   static func main() {
@@ -293,6 +299,8 @@ struct Main {
     print("optSalmon:", optSalmon as Any)
     // CHECK-OUTPUT: test.C: 27
     printFunctionEnum()
+    // CHECK-OUTPUT: SingleCaseEnum: a(b: true, i: 42)
+    print("SingleCaseEnum:", SingleCaseEnum.x)
   }
 }
 

validation-test/SILOptimizer/static_enums_fuzzing.swift

@@ -73,6 +73,15 @@ var typeDefinitions: String {
       case X, Y, Z
     }
 
+    public enum SPSCE<T> {
+      case A(T)
+    }
+
+    public enum SPE<T> {
+      case A(T)
+      case B
+    }
+
     public enum MPE<T, V> {
       case A(T)
       case B(V)
@@ -285,6 +294,72 @@ struct Enum : Value {
   }
 }
 
+struct SinglePayloadSingleCaseEnum : Value {
+  let payload: any Value
+  
+  init(generator: inout RandomGenerator, depth: Int) {
+    self.payload = generator.createValue(depth: depth)
+  }
+  
+  func getType() -> String {
+    "SPSCE<\(payload.getType())>"
+  }
+
+  func getInitValue() -> String  {
+    return "SPSCE.A(\(payload.getInitValue()))"
+  }
+
+  func getExpectedOutput(topLevel: Bool) -> String  {
+    let prefix = topLevel ? "" : "\(getRuntimeTypeName(topLevel: topLevel))."
+    return "\(prefix)A(\(payload.getExpectedOutput(topLevel: false)))"
+  }
+
+  func getRuntimeTypeName(topLevel: Bool) -> String {
+    let prefix = topLevel ? "" : "test."
+    return "\(prefix)SPSCE<\(payload.getRuntimeTypeName(topLevel: topLevel))>"
+  }
+
+  var containsEnum: Bool { true }
+}
+
+struct SinglePayloadEnum : Value {
+  let payload: any Value
+  let caseIdx: Int
+  
+  init(generator: inout RandomGenerator, depth: Int) {
+    self.caseIdx = Int.random(in: 0..<2, using: &generator)
+    self.payload = generator.createValue(depth: depth)
+  }
+  
+  func getType() -> String {
+    "SPE<\(payload.getType())>"
+  }
+
+  func getInitValue() -> String  {
+    switch caseIdx {
+      case 0: return "SPE.A(\(payload.getInitValue()))"
+      case 1: return "SPE.B"
+      default: fatalError()
+    }
+  }
+
+  func getExpectedOutput(topLevel: Bool) -> String  {
+    let prefix = topLevel ? "" : "\(getRuntimeTypeName(topLevel: topLevel))."
+    switch caseIdx {
+      case 0: return "\(prefix)A(\(payload.getExpectedOutput(topLevel: false)))"
+      case 1: return "\(prefix)B"
+      default: fatalError()
+    }
+  }
+
+  func getRuntimeTypeName(topLevel: Bool) -> String {
+    let prefix = topLevel ? "" : "test."
+    return "\(prefix)SPE<\(payload.getRuntimeTypeName(topLevel: topLevel))>"
+  }
+
+  var containsEnum: Bool { true }
+}
+
 struct MultiPayloadEnum : Value {
   let payloadA: any Value
   let payloadB: any Value
@@ -407,6 +482,8 @@ struct RandomGenerator : RandomNumberGenerator {
   private static let allValueTypes: [any Value.Type] = allTerminalTypes + [
     OptionalValue.self,
     Struct.self,
+    SinglePayloadSingleCaseEnum.self,
+    SinglePayloadEnum.self,
     MultiPayloadEnum.self
   ]