Fully implement unions.

c++/src/capnproto/benchmark/capnproto-eval.c++

@@ -35,7 +35,6 @@ int32_t makeExpression(Expression::Builder exp, uint depth) {
   uint32_t left, right;
 
   if (fastRand(8) < depth) {
-    exp.setLeftIsValue(true);
     left = fastRand(128) + 1;
     exp.setLeftValue(left);
   } else {
@@ -43,7 +42,6 @@ int32_t makeExpression(Expression::Builder exp, uint depth) {
   }
 
   if (fastRand(8) < depth) {
-    exp.setRightIsValue(true);
     right = fastRand(128) + 1;
     exp.setRightValue(right);
   } else {
@@ -66,18 +64,24 @@ int32_t makeExpression(Expression::Builder exp, uint depth) {
 }
 
 int32_t evaluateExpression(Expression::Reader exp) {
-  int32_t left, right;
+  int32_t left = 0, right = 0;
 
-  if (exp.getLeftIsValue()) {
+  switch (exp.whichLeft()) {
+    case Expression::Left::LEFT_VALUE:
       left = exp.getLeftValue();
-  } else {
+      break;
+    case Expression::Left::LEFT_EXPRESSION:
       left = evaluateExpression(exp.getLeftExpression());
+      break;
   }
 
-  if (exp.getRightIsValue()) {
+  switch (exp.whichRight()) {
+    case Expression::Right::RIGHT_VALUE:
       right = exp.getRightValue();
-  } else {
+      break;
+    case Expression::Right::RIGHT_EXPRESSION:
       right = evaluateExpression(exp.getRightExpression());
+      break;
   }
 
   switch (exp.getOp()) {

c++/src/capnproto/benchmark/eval.capnp

@@ -32,15 +32,13 @@ enum Operation {
 struct Expression {
   op@0: Operation;
 
-  # TODO:  Use unions once fully-implemented.
+  union left @1;
+  leftValue@2 in left: Int32;
+  leftExpression@3 in left: Expression;
 
-  leftIsValue@1: Bool;
-  leftValue@2: Int32;
-  leftExpression@3: Expression;
-
-  rightIsValue@4: Bool;
-  rightValue@5: Int32;
-  rightExpression@6: Expression;
+  union right @4;
+  rightValue@5 in right: Int32;
+  rightExpression@6 in right: Expression;
 }
 
 struct EvaluationResult {

c++/src/capnproto/encoding-test.c++

@@ -107,6 +107,151 @@ TEST(Encoding, DefaultsFromEmptyMessage) {
   checkTestMessage(readMessageTrusted<TestDefaults>(emptyMessage.words));
 }
 
+#ifdef NDEBUG
+#define EXPECT_DEBUG_ANY_THROW(EXP)
+#else
+#define EXPECT_DEBUG_ANY_THROW EXPECT_ANY_THROW
+#endif
+
+TEST(Encoding, Unions) {
+  MallocMessageBuilder builder;
+  TestUnion::Builder root = builder.getRoot<TestUnion>();
+
+  EXPECT_EQ(TestUnion::Union0::U0F0S0, root.whichUnion0());
+  EXPECT_EQ(Void::VOID, root.getU0f0s0());
+  EXPECT_DEBUG_ANY_THROW(root.getU0f0s1());
+
+  root.setU0f0s1(true);
+  EXPECT_EQ(TestUnion::Union0::U0F0S1, root.whichUnion0());
+  EXPECT_TRUE(root.getU0f0s1());
+  EXPECT_DEBUG_ANY_THROW(root.getU0f0s0());
+
+  root.setU0f0s8(123);
+  EXPECT_EQ(TestUnion::Union0::U0F0S8, root.whichUnion0());
+  EXPECT_EQ(123, root.getU0f0s8());
+  EXPECT_DEBUG_ANY_THROW(root.getU0f0s1());
+}
+
+struct UnionState {
+  uint discriminants[4];
+  int dataOffset;
+
+  UnionState(std::initializer_list<uint> discriminants, int dataOffset)
+      : dataOffset(dataOffset) {
+    memcpy(this->discriminants, discriminants.begin(), sizeof(discriminants));
+  }
+
+  bool operator==(const UnionState& other) const {
+    for (uint i = 0; i < 4; i++) {
+      if (discriminants[i] != other.discriminants[i]) {
+        return false;
+      }
+    }
+
+    return dataOffset == other.dataOffset;
+  }
+};
+
+std::ostream& operator<<(std::ostream& os, const UnionState& us) {
+  os << "UnionState({";
+
+  for (uint i = 0; i < 4; i++) {
+    if (i > 0) os << ", ";
+    os << us.discriminants[i];
+  }
+
+  return os << "}, " << us.dataOffset << ")";
+}
+
+template <typename T> T one() { return static_cast<T>(1); }
+template <> Text::Reader one() { return "1"; }
+template <> Void one() { return Void::VOID; }
+
+template <typename T>
+UnionState initUnion(void (TestUnion::Builder::*setter)(T value)) {
+  // Use the given setter to initialize the given union field and then return a struct indicating
+  // the location of the data that was written as well as the values of the four union
+  // discriminants.
+
+  MallocMessageBuilder builder;
+  (builder.getRoot<TestUnion>().*setter)(one<T>());
+  ArrayPtr<const word> segment = builder.getSegmentsForOutput()[0];
+
+  CAPNPROTO_ASSERT(segment.size() > 2, "bug");
+
+  // Find the offset of the first set bit after the union discriminants.
+  int offset = 0;
+  for (const uint8_t* p = reinterpret_cast<const uint8_t*>(segment.begin() + 2);
+       p < reinterpret_cast<const uint8_t*>(segment.end()); p++) {
+    if (*p != 0) {
+      uint8_t bits = *p;
+      while ((bits & 1) == 0) {
+        ++offset;
+        bits >>= 1;
+      }
+      goto found;
+    }
+    offset += 8;
+  }
+  offset = -1;
+
+found:
+  const uint8_t* discriminants = reinterpret_cast<const uint8_t*>(segment.begin() + 1);
+  return UnionState({discriminants[0], discriminants[2], discriminants[4], discriminants[6]},
+                    offset);
+}
+
+TEST(Encoding, UnionLayout) {
+  EXPECT_EQ(UnionState({ 0,0,0,0},  -1), initUnion(&TestUnion::Builder::setU0f0s0));
+  EXPECT_EQ(UnionState({ 1,0,0,0},   0), initUnion(&TestUnion::Builder::setU0f0s1));
+  EXPECT_EQ(UnionState({ 2,0,0,0},   0), initUnion(&TestUnion::Builder::setU0f0s8));
+  EXPECT_EQ(UnionState({ 3,0,0,0},   0), initUnion(&TestUnion::Builder::setU0f0s16));
+  EXPECT_EQ(UnionState({ 4,0,0,0},   0), initUnion(&TestUnion::Builder::setU0f0s32));
+  EXPECT_EQ(UnionState({ 5,0,0,0},   0), initUnion(&TestUnion::Builder::setU0f0s64));
+  EXPECT_EQ(UnionState({ 6,0,0,0}, 448), initUnion(&TestUnion::Builder::setU0f0sp));
+
+  EXPECT_EQ(UnionState({ 7,0,0,0},  -1), initUnion(&TestUnion::Builder::setU0f1s0));
+  EXPECT_EQ(UnionState({ 8,0,0,0},   0), initUnion(&TestUnion::Builder::setU0f1s1));
+  EXPECT_EQ(UnionState({ 9,0,0,0},   0), initUnion(&TestUnion::Builder::setU0f1s8));
+  EXPECT_EQ(UnionState({10,0,0,0},   0), initUnion(&TestUnion::Builder::setU0f1s16));
+  EXPECT_EQ(UnionState({11,0,0,0},   0), initUnion(&TestUnion::Builder::setU0f1s32));
+  EXPECT_EQ(UnionState({12,0,0,0},   0), initUnion(&TestUnion::Builder::setU0f1s64));
+  EXPECT_EQ(UnionState({13,0,0,0}, 448), initUnion(&TestUnion::Builder::setU0f1sp));
+
+  EXPECT_EQ(UnionState({0, 0,0,0},  -1), initUnion(&TestUnion::Builder::setU1f0s0));
+  EXPECT_EQ(UnionState({0, 1,0,0},  65), initUnion(&TestUnion::Builder::setU1f0s1));
+  EXPECT_EQ(UnionState({0, 2,0,0},  65), initUnion(&TestUnion::Builder::setU1f1s1));
+  EXPECT_EQ(UnionState({0, 3,0,0},  72), initUnion(&TestUnion::Builder::setU1f0s8));
+  EXPECT_EQ(UnionState({0, 4,0,0},  72), initUnion(&TestUnion::Builder::setU1f1s8));
+  EXPECT_EQ(UnionState({0, 5,0,0},  80), initUnion(&TestUnion::Builder::setU1f0s16));
+  EXPECT_EQ(UnionState({0, 6,0,0},  80), initUnion(&TestUnion::Builder::setU1f1s16));
+  EXPECT_EQ(UnionState({0, 7,0,0},  96), initUnion(&TestUnion::Builder::setU1f0s32));
+  EXPECT_EQ(UnionState({0, 8,0,0},  96), initUnion(&TestUnion::Builder::setU1f1s32));
+  EXPECT_EQ(UnionState({0, 9,0,0}, 128), initUnion(&TestUnion::Builder::setU1f0s64));
+  EXPECT_EQ(UnionState({0,10,0,0}, 128), initUnion(&TestUnion::Builder::setU1f1s64));
+  EXPECT_EQ(UnionState({0,11,0,0}, 512), initUnion(&TestUnion::Builder::setU1f0sp));
+  EXPECT_EQ(UnionState({0,12,0,0}, 512), initUnion(&TestUnion::Builder::setU1f1sp));
+
+  EXPECT_EQ(UnionState({0,13,0,0},  -1), initUnion(&TestUnion::Builder::setU1f2s0));
+  EXPECT_EQ(UnionState({0,14,0,0}, 128), initUnion(&TestUnion::Builder::setU1f2s1));
+  EXPECT_EQ(UnionState({0,15,0,0}, 128), initUnion(&TestUnion::Builder::setU1f2s8));
+  EXPECT_EQ(UnionState({0,16,0,0}, 128), initUnion(&TestUnion::Builder::setU1f2s16));
+  EXPECT_EQ(UnionState({0,17,0,0}, 128), initUnion(&TestUnion::Builder::setU1f2s32));
+  EXPECT_EQ(UnionState({0,18,0,0}, 128), initUnion(&TestUnion::Builder::setU1f2s64));
+  EXPECT_EQ(UnionState({0,19,0,0}, 512), initUnion(&TestUnion::Builder::setU1f2sp));
+
+  EXPECT_EQ(UnionState({0,0,0,0}, 192), initUnion(&TestUnion::Builder::setU2f0s1));
+  EXPECT_EQ(UnionState({0,0,0,0}, 193), initUnion(&TestUnion::Builder::setU3f0s1));
+  EXPECT_EQ(UnionState({0,0,1,0}, 200), initUnion(&TestUnion::Builder::setU2f0s8));
+  EXPECT_EQ(UnionState({0,0,0,1}, 208), initUnion(&TestUnion::Builder::setU3f0s8));
+  EXPECT_EQ(UnionState({0,0,2,0}, 224), initUnion(&TestUnion::Builder::setU2f0s16));
+  EXPECT_EQ(UnionState({0,0,0,2}, 240), initUnion(&TestUnion::Builder::setU3f0s16));
+  EXPECT_EQ(UnionState({0,0,3,0}, 256), initUnion(&TestUnion::Builder::setU2f0s32));
+  EXPECT_EQ(UnionState({0,0,0,3}, 288), initUnion(&TestUnion::Builder::setU3f0s32));
+  EXPECT_EQ(UnionState({0,0,4,0}, 320), initUnion(&TestUnion::Builder::setU2f0s64));
+  EXPECT_EQ(UnionState({0,0,0,4}, 384), initUnion(&TestUnion::Builder::setU3f0s64));
+}
+
 }  // namespace
 }  // namespace internal
 }  // namespace capnproto

c++/src/capnproto/message-test.c++

@@ -30,6 +30,7 @@ namespace {
 
 TEST(Message, MallocBuilderWithFirstSegment) {
   word scratch[16];
+  memset(scratch, 0, sizeof(scratch));
   MallocMessageBuilder builder(arrayPtr(scratch, 16), AllocationStrategy::FIXED_SIZE);
 
   ArrayPtr<word> segment = builder.allocateSegment(1);

c++/src/capnproto/test.capnp

@@ -151,3 +151,78 @@ struct TestDefaults {
   enumList      @32 : List(TestEnum) = [foo, garply];
   interfaceList @33 : List(Void);  # TODO
 }
+
+struct TestUnion {
+  union union0 @0;
+  union union1 @1;
+  union union2 @2;
+  union union3 @3;
+
+  # Pack union 0 under ideal conditions: there is no unused padding space prior to it.
+  u0f0s0  @4 in union0: Void;
+  u0f0s1  @5 in union0: Bool;
+  u0f0s8  @6 in union0: Int8;
+  u0f0s16 @7 in union0: Int16;
+  u0f0s32 @8 in union0: Int32;
+  u0f0s64 @9 in union0: Int64;
+  u0f0sp  @10 in union0: Text;
+
+  # Pack more stuff into union1 -- should go in same space.
+  u0f1s0  @11 in union0: Void;
+  u0f1s1  @12 in union0: Bool;
+  u0f1s8  @13 in union0: Int8;
+  u0f1s16 @14 in union0: Int16;
+  u0f1s32 @15 in union0: Int32;
+  u0f1s64 @16 in union0: Int64;
+  u0f1sp  @17 in union0: Text;
+
+  # Pack one bit in order to make pathological situation for union1.
+  bit0 @18: Bool;
+
+  # Pack pathologically bad case.  Each field takes up new space.
+  u1f0s0  @19 in union1: Void;
+  u1f0s1  @20 in union1: Bool;
+  u1f1s1  @21 in union1: Bool;
+  u1f0s8  @22 in union1: Int8;
+  u1f1s8  @23 in union1: Int8;
+  u1f0s16 @24 in union1: Int16;
+  u1f1s16 @25 in union1: Int16;
+  u1f0s32 @26 in union1: Int32;
+  u1f1s32 @27 in union1: Int32;
+  u1f0s64 @28 in union1: Int64;
+  u1f1s64 @29 in union1: Int64;
+  u1f0sp  @30 in union1: Text;
+  u1f1sp  @31 in union1: Text;
+
+  # Pack more stuff into union1 -- should go into same space as u1f0s64.
+  u1f2s0  @32 in union1: Void;
+  u1f2s1  @33 in union1: Bool;
+  u1f2s8  @34 in union1: Int8;
+  u1f2s16 @35 in union1: Int16;
+  u1f2s32 @36 in union1: Int32;
+  u1f2s64 @37 in union1: Int64;
+  u1f2sp  @38 in union1: Text;
+
+  # Fill in the rest of that bitfield from earlier.
+  bit2 @39: Bool;
+  bit3 @40: Bool;
+  bit4 @41: Bool;
+  bit5 @42: Bool;
+  bit6 @43: Bool;
+  bit7 @44: Bool;
+  byte0 @49: UInt8;
+
+  # Interleave two unions to be really annoying.
+  # Also declare in reverse order to make sure union discriminant values are sorted by field number
+  # and not by declaration order.
+  u3f0s64 @55 in union3: Int64;
+  u2f0s64 @54 in union2: Int64;
+  u3f0s32 @53 in union3: Int32;
+  u2f0s32 @52 in union2: Int32;
+  u3f0s16 @51 in union3: Int16;
+  u2f0s16 @50 in union2: Int16;
+  u3f0s8 @48 in union3: Int8;
+  u2f0s8 @47 in union2: Int8;
+  u3f0s1 @46 in union3: Bool;
+  u2f0s1 @45 in union2: Bool;
+}

compiler/src/Compiler.hs

@@ -366,44 +366,68 @@ packValue Size1 s@(PackingState { packingHole1 = h1 }) =
     (h1, s { packingHole1 = if mod (h1 + 1) 8 == 0 then 0 else h1 + 1 })
 packValue Size0 s = (0, s)
 
-initialUnionPackingState = UnionPackingState Nothing Nothing Nothing
+initialUnionPackingState = UnionPackingState Nothing Nothing
 
 packUnionizedValue :: FieldSize             -- Size of field to pack.
-                   -> Bool                  -- Whether the field is retroactively unionized.
                    -> UnionPackingState     -- Current layout of the union
                    -> PackingState          -- Current layout of the struct.
                    -> (Integer, UnionPackingState, PackingState)
-packUnionizedValue (SizeInlineComposite _ _) _ _ _ = error "Can't put inline composite into union."
-packUnionizedValue Size0 _ u s = (0, u, s)
+packUnionizedValue (SizeInlineComposite _ _) _ _ = error "Can't put inline composite into union."
+packUnionizedValue Size0 u s = (0, u, s)
 
 -- Pack reference when we already have a reference slot allocated.
-packUnionizedValue SizeReference _ u@(UnionPackingState _ (Just offset) _) s = (offset, u, s)
+packUnionizedValue SizeReference u@(UnionPackingState _ (Just offset)) s = (offset, u, s)
 
 -- Pack reference when we don't have a reference slot.
-packUnionizedValue SizeReference _ (UnionPackingState d Nothing retro) s = (offset, u2, s2) where
+packUnionizedValue SizeReference (UnionPackingState d Nothing) s = (offset, u2, s2) where
     (offset, s2) = packValue SizeReference s
-    u2 = UnionPackingState d (Just offset) retro
+    u2 = UnionPackingState d (Just offset)
 
--- Pack data that fits into the retro slot.
-packUnionizedValue size _ u@(UnionPackingState _ _ (Just (offset, retroSize))) s
-    | sizeInBits retroSize >= sizeInBits size =
-        (offset * div (sizeInBits retroSize) (sizeInBits size), u, s)
-
--- Pack data when a data word has been allocated.
-packUnionizedValue size _ u@(UnionPackingState (Just offset) _ _) s =
-    (offset * div 64 (sizeInBits size), u, s)
-
--- Pack retroactive data when no data word has been allocated.
-packUnionizedValue size True (UnionPackingState Nothing r Nothing) s =
-    (offset, u2, s2) where
+-- Pack data.
+packUnionizedValue size (UnionPackingState d r) s =
+    case packUnionizedData (fromMaybe (0, Size0) d) s size of
+        Just (offset, slotOffset, slotSize, s2) ->
+            (offset, UnionPackingState (Just (slotOffset, slotSize)) r, s2)
+        Nothing -> let
             (offset, s2) = packValue size s
-        u2 = UnionPackingState Nothing r (Just (offset, size))
-
--- Pack non-retroactive data when no data word has been allocated.
-packUnionizedValue size _ (UnionPackingState Nothing r retro) s =
-    (offset * div 64 (sizeInBits size), u2, s2) where
-        (offset, s2) = packValue Size64 s
-        u2 = UnionPackingState (Just offset) r retro
+            in (offset, UnionPackingState (Just (offset, size)) r, s2)
+
+packUnionizedData :: (Integer, FieldSize)        -- existing slot to expand
+                  -> PackingState                -- existing packing state
+                  -> FieldSize                   -- desired field size
+                  -> Maybe (Integer,       -- Offset of the new field (in multiples of field size).
+                            Integer,       -- New offset of the slot (in multiples of slot size).
+                            FieldSize,     -- New size of the slot.
+                            PackingState)  -- New struct packing state.
+
+-- Don't try to allocate space for voids.
+packUnionizedData (slotOffset, slotSize) state Size0 = Just (0, slotOffset, slotSize, state)
+
+-- If slot is bigger than desired size, no expansion is needed.
+packUnionizedData (slotOffset, slotSize) state desiredSize
+    | sizeInBits slotSize >= sizeInBits desiredSize =
+    Just (div (sizeInBits slotSize) (sizeInBits desiredSize) * slotOffset,
+          slotOffset, slotSize, state)
+
+-- If slot is a bit, and it is the first bit in its byte, and the bit hole immediately follows
+-- expand it to a byte.
+packUnionizedData (slotOffset, Size1) p@(PackingState { packingHole1 = hole }) desiredSize
+    | mod slotOffset 8 == 0 && hole == slotOffset + 1 =
+        packUnionizedData (div slotOffset 8, Size8) (p { packingHole1 = 0 }) desiredSize
+
+-- If slot is size N, and the next N bits are padding, expand.
+packUnionizedData (slotOffset, Size8) p@(PackingState { packingHole8 = hole }) desiredSize
+    | hole == slotOffset + 1 =
+        packUnionizedData (div slotOffset 2, Size16) (p { packingHole8 = 0 }) desiredSize
+packUnionizedData (slotOffset, Size16) p@(PackingState { packingHole16 = hole }) desiredSize
+    | hole == slotOffset + 1 =
+        packUnionizedData (div slotOffset 2, Size32) (p { packingHole16 = 0 }) desiredSize
+packUnionizedData (slotOffset, Size32) p@(PackingState { packingHole32 = hole }) desiredSize
+    | hole == slotOffset + 1 =
+        packUnionizedData (div slotOffset 2, Size64) (p { packingHole32 = 0 }) desiredSize
+
+-- Otherwise, we fail.
+packUnionizedData _ _ _ = Nothing
 
 -- Determine the offset for the given field, and update the packing states to include the field.
 packField :: FieldDesc -> PackingState -> Map.Map Integer UnionPackingState
@@ -416,9 +440,8 @@ packField fieldDesc state unionState =
         Just unionDesc -> let
             n = unionNumber unionDesc
             oldUnionPacking = fromMaybe initialUnionPackingState (Map.lookup n unionState)
-            isRetro = fieldNumber fieldDesc < unionNumber unionDesc
             (offset, newUnionPacking, newState) =
-                packUnionizedValue (fieldSize $ fieldType fieldDesc) isRetro oldUnionPacking state
+                packUnionizedValue (fieldSize $ fieldType fieldDesc) oldUnionPacking state
             newUnionState = Map.insert n newUnionPacking unionState
             in (offset, newState, newUnionState)
 
@@ -427,7 +450,7 @@ packField fieldDesc state unionState =
 packUnion :: UnionDesc -> PackingState -> Map.Map Integer UnionPackingState
           -> (Integer, PackingState, Map.Map Integer UnionPackingState)
 packUnion _ state unionState = (offset, newState, unionState) where
-    (offset, newState) = packValue Size8 state
+    (offset, newState) = packValue Size16 state
 
 packFields :: [FieldDesc] -> [UnionDesc]
     -> (PackingState, Map.Map Integer UnionPackingState, Map.Map Integer (Integer, PackingState))
@@ -533,7 +556,7 @@ compileDecl scope (StructDecl (Located _ name) decls) =
         return (let
             fields = [d | DescField d <- members]
             unions = [d | DescUnion d <- members]
-            (packing, unionPackingMap, fieldPackingMap) = packFields fields unions
+            (packing, _, fieldPackingMap) = packFields fields unions
             in DescStruct StructDesc
             { structName = name
             , structParent = scope
@@ -549,26 +572,23 @@ compileDecl scope (StructDecl (Located _ name) decls) =
             , structMemberMap = memberMap
             , structStatements = statements
             , structFieldPackingMap = fieldPackingMap
-            , structUnionPackingMap = unionPackingMap
             })))
 
 compileDecl (DescStruct parent) (UnionDecl (Located _ name) (Located numPos number) decls) =
     CompiledMemberStatus name (feedback (\desc -> do
         (_, _, options, statements) <- compileChildDecls desc decls
-        let fields = [f | f <- structFields parent, fieldInUnion name f]
+        let compareFieldNumbers a b = compare (fieldNumber a) (fieldNumber b)
+            fields = List.sortBy compareFieldNumbers
+                [f | f <- structFields parent, fieldInUnion name f]
         requireNoMoreThanOneFieldNumberLessThan name numPos number fields
         return (let
             (tagOffset, tagPacking) = structFieldPackingMap parent ! number
-            unionPacking = structUnionPackingMap parent ! number
             in DescUnion UnionDesc
             { unionName = name
             , unionParent = parent
             , unionNumber = number
             , unionTagOffset = tagOffset
             , unionTagPacking = tagPacking
-            , unionDataOffset = unionPackDataOffset unionPacking
-            , unionReferenceOffset = unionPackReferenceOffset unionPacking
-            , unionRetroactiveSlot = unionPackRetroactiveSlot unionPacking
             , unionFields = fields
             , unionOptions = options
             , unionStatements = statements

compiler/src/CxxGenerator.hs

@@ -176,9 +176,11 @@ defaultBytesContext parent t bytes = mkStrContext context where
         _ -> error "defaultBlobSize used on non-blob."
     context s = parent s
 
+descDecl desc = head $ lines $ descToCode "" desc
+
 fieldContext parent desc = mkStrContext context where
     context "fieldName" = MuVariable $ fieldName desc
-    context "fieldDecl" = MuVariable $ descToCode "" (DescField desc)
+    context "fieldDecl" = MuVariable $ descDecl $ DescField desc
     context "fieldTitleCase" = MuVariable $ toTitleCase $ fieldName desc
     context "fieldUpperCase" = MuVariable $ toUpperCaseWithUnderscores $ fieldName desc
     context "fieldIsPrimitive" = MuBool $ isPrimitive $ fieldType desc
@@ -202,11 +204,24 @@ fieldContext parent desc = mkStrContext context where
         MuVariable $ cxxFieldSizeString $ elementSize $ elementType $ fieldType desc
     context "fieldElementType" =
         MuVariable $ cxxTypeString $ elementType $ fieldType desc
+    context "fieldUnion" = case fieldUnion desc of
+        Just u -> MuList [unionContext context u]
+        Nothing -> muNull
+    context s = parent s
+
+unionContext parent desc = mkStrContext context where
+    context "unionName" = MuVariable $ unionName desc
+    context "unionDecl" = MuVariable $ descDecl $ DescUnion desc
+    context "unionTitleCase" = MuVariable $ toTitleCase $ unionName desc
+    context "unionTagOffset" = MuVariable $ unionTagOffset desc
+    context "unionFields" = MuList $ map (fieldContext context) $ unionFields desc
+    context "unionHasRetro" = MuBool $ unionHasRetro desc
     context s = parent s
 
 structContext parent desc = mkStrContext context where
     context "structName" = MuVariable $ structName desc
     context "structFields" = MuList $ map (fieldContext context) $ structFields desc
+    context "structUnions" = MuList $ map (unionContext context) $ structUnions desc
     context "structDataSize" = MuVariable $ packingDataSize $ structPacking desc
     context "structReferenceCount" = MuVariable $ packingReferenceCount $ structPacking desc
     context "structChildren" = MuList []  -- TODO

compiler/src/Semantics.hs

@@ -162,9 +162,8 @@ packingSize PackingState { packingDataSize = ds, packingReferenceCount = rc } =
 --   this is the piece that had been allocated to that field, and is now retroactively part of the
 --   union.
 data UnionPackingState = UnionPackingState
-    { unionPackDataOffset :: Maybe Integer
+    { unionPackDataOffset :: Maybe (Integer, FieldSize)
     , unionPackReferenceOffset :: Maybe Integer
-    , unionPackRetroactiveSlot :: Maybe (Integer, FieldSize)
     }
 
 data FieldSize = Size0 | Size1 | Size8 | Size16 | Size32 | Size64 | SizeReference
@@ -310,11 +309,10 @@ data StructDesc = StructDesc
     , structMemberMap :: MemberMap
     , structStatements :: [CompiledStatement]
 
-    -- Don't use these directly, use the members of FieldDesc and UnionDesc.
-    -- These fields are exposed here only because I was too lazy to create a way to pass them on
+    -- Don't use this directly, use the members of FieldDesc and UnionDesc.
+    -- This field is exposed here only because I was too lazy to create a way to pass it on
     -- the side when compiling members of a struct.
     , structFieldPackingMap :: Map.Map Integer (Integer, PackingState)
-    , structUnionPackingMap :: Map.Map Integer UnionPackingState
     }
 
 data UnionDesc = UnionDesc
@@ -323,14 +321,15 @@ data UnionDesc = UnionDesc
     , unionNumber :: Integer
     , unionTagOffset :: Integer
     , unionTagPacking :: PackingState
-    , unionDataOffset :: Maybe Integer
-    , unionReferenceOffset :: Maybe Integer
-    , unionRetroactiveSlot :: Maybe (Integer, FieldSize)
-    , unionFields :: [FieldDesc]
+    , unionFields :: [FieldDesc]  -- ordered by field number
     , unionOptions :: OptionMap
     , unionStatements :: [CompiledStatement]
     }
 
+unionHasRetro desc = case unionFields desc of
+    [] -> False
+    f:_ -> fieldNumber f < unionNumber desc
+
 data FieldDesc = FieldDesc
     { fieldName :: String
     , fieldParent :: StructDesc
@@ -422,7 +421,7 @@ descToCode indent (DescField desc) = printf "%s%s@%d%s: %s%s;  # %s\n" indent
 --    (maybeBlockCode indent $ fieldStatements desc)
 descToCode indent (DescUnion desc) = printf "%sunion %s@%d;  # [%d, %d)\n" indent
     (unionName desc) (unionNumber desc)
-    (unionTagOffset desc * 8) (unionTagOffset desc * 8 + 8)
+    (unionTagOffset desc * 16) (unionTagOffset desc * 16 + 16)
 --    (maybeBlockCode indent $ unionStatements desc)
 descToCode indent (DescInterface desc) = printf "%sinterface %s%s" indent
     (interfaceName desc)

doc/encoding.md

@@ -161,7 +161,8 @@ When unions are present, add the following logic:
             If an earlier member of the union is in the same section as
                     this field and it combined with any following padding
                     is at least as large as the new field {
-                Give the new field the same offset, so they overlap.
+                Give the new field the same offset and the highest-numbered
+                    such previous field, so they overlap.
             } else {
                 Assign a new offset to this field as if it were not a union
                     member at all.  (See no-union logic, above.)