Improve cast optimizations (#5876)

Simplify the optimization of ref.cast and ref.test in OptimizeInstructions by
moving the loop that examines fallthrough values one at a time out to a shared
function in properties.h. Also simplify ref.cast optimization by analyzing the
cast result in just one place.

In addition to simplifying the code, also make the cast optimizations more
powerful by analyzing the nullability and heap type of the cast value
independently, resulting in a potentially more precise analysis of the cast
behavior. Also improve optimization power by considering fallthrough values when
optimizing the SuccessOnlyIfNonNull case.

1 files changed, 25 insertions, 0 deletions

diff --git a/src/wasm/wasm-type.cpp b/src/wasm/wasm-type.cpp
index 114a83bd6..fd9838b31 100644
--- a/src/wasm/wasm-type.cpp
+++ b/src/wasm/wasm-type.cpp
@@ -1035,6 +1035,31 @@ Type Type::getLeastUpperBound(Type a, Type b) {
   WASM_UNREACHABLE("unexpected type");
 }
 
+Type Type::getGreatestLowerBound(Type a, Type b) {
+  if (a == b) {
+    return a;
+  }
+  if (!a.isRef() || !b.isRef()) {
+    return Type::unreachable;
+  }
+  auto heapA = a.getHeapType();
+  auto heapB = b.getHeapType();
+  if (heapA.getBottom() != heapB.getBottom()) {
+    return Type::unreachable;
+  }
+  auto nullability =
+    (a.isNonNullable() || b.isNonNullable()) ? NonNullable : Nullable;
+  HeapType heapType;
+  if (HeapType::isSubType(heapA, heapB)) {
+    heapType = heapA;
+  } else if (HeapType::isSubType(heapB, heapA)) {
+    heapType = heapB;
+  } else {
+    heapType = heapA.getBottom();
+  }
+  return Type(heapType, nullability);
+}
+
 size_t Type::size() const {
   if (isTuple()) {
     return getTypeInfo(*this)->tuple.size();