Use fewer bits for BasicHeapType (#7404)

Now that we aren't supporting exact reference types, we no longer need
to leave bit 2 free for use by the Type representation. Shift the basic
HeapType representations down to start at bit 2 instead of bit 3.

Author: tlively

src/wasm-type.h

@@ -95,13 +95,13 @@ class HeapType {
   // should also be passed by value.
   uintptr_t id;
 
-  static constexpr int TypeBits = 3;
+  static constexpr int TypeBits = 2;
   static constexpr int UsedBits = TypeBits + 1;
   static constexpr int SharedMask = 1 << TypeBits;
 
 public:
-  // Bits 0-2 are used by the Type representation, so need to be left free.
-  // Bit 3 determines whether the basic heap type is shared (1) or unshared (0).
+  // Bits 0-1 are used by the Type representation, so need to be left free.
+  // Bit 2 determines whether the basic heap type is shared (1) or unshared (0).
   enum BasicHeapType : uint32_t {
     ext = 1 << UsedBits,
     func = 2 << UsedBits,
@@ -278,7 +278,7 @@ class Type {
   // bit 0 set. When that bit is masked off, they are pointers to the underlying
   // vectors of types. Otherwise, the type is a reference type, and is
   // represented as a heap type with bit 1 set iff the reference type is
-  // nullable and bit 2 set iff the reference type is exact.
+  // nullable.
   //
   // Since `Type` is really just a single integer, it should be passed by value.
   // This is a uintptr_t rather than a TypeID (uint64_t) to save memory on
@@ -287,7 +287,6 @@ class Type {
 
   static constexpr int TupleMask = 1 << 0;
   static constexpr int NullMask = 1 << 1;
-  static constexpr int ExactMask = 1 << 2;
 
 public:
   enum BasicType : uint32_t {
@@ -320,8 +319,7 @@ class Type {
   // Signature, Struct or Array via implicit conversion to HeapType.
   Type(HeapType heapType, Nullability nullable)
     : Type(heapType.getID() | (nullable == Nullable ? NullMask : 0)) {
-    assert(heapType.isBasic() ||
-           !(heapType.getID() & (TupleMask | NullMask | ExactMask)));
+    assert(heapType.isBasic() || !(heapType.getID() & (TupleMask | NullMask)));
   }
 
   // Predicates
@@ -372,7 +370,7 @@ class Type {
   bool isNonNullable() const { return isRef() && !(id & NullMask); }
   HeapType getHeapType() const {
     assert(isRef());
-    return HeapType(id & ~(NullMask | ExactMask));
+    return HeapType(id & ~NullMask);
   }
 
   bool isFunction() const { return isRef() && getHeapType().isFunction(); }

test/example/c-api-kitchen-sink.txt

@@ -20,17 +20,17 @@ BinaryenTypeAuto: -1
 BinaryenPackedTypeNotPacked: 0
 BinaryenPackedTypeInt8: 1
 BinaryenPackedTypeInt16: 2
-BinaryenHeapTypeExt: 16
-BinaryenHeapTypeFunc: 32
-BinaryenHeapTypeAny: 64
-BinaryenHeapTypeEq: 80
-BinaryenHeapTypeI31: 96
-BinaryenHeapTypeStruct: 112
-BinaryenHeapTypeArray: 128
-BinaryenHeapTypeString: 160
-BinaryenHeapTypeNone: 176
-BinaryenHeapTypeNoext: 192
-BinaryenHeapTypeNofunc: 208
+BinaryenHeapTypeExt: 8
+BinaryenHeapTypeFunc: 16
+BinaryenHeapTypeAny: 32
+BinaryenHeapTypeEq: 40
+BinaryenHeapTypeI31: 48
+BinaryenHeapTypeStruct: 56
+BinaryenHeapTypeArray: 64
+BinaryenHeapTypeString: 80
+BinaryenHeapTypeNone: 88
+BinaryenHeapTypeNoext: 96
+BinaryenHeapTypeNofunc: 104
 BinaryenFeatureMVP: 0
 BinaryenFeatureAtomics: 1
 BinaryenFeatureBulkMemory: 16