[Wasm GC] OptimizeInstructions: Optimize ref.eq on equal inputs with a tee (#4749)

(ref.eq
  (local.tee $x (..))
  (local.get $x)
)

That will definitely return 1. Before this PR the side effects of tee stopped us
from optimizing.

2 files changed, 72 insertions, 3 deletions

diff --git a/src/ir/drop.h b/src/ir/drop.h
new file mode 100644
index 000000000..9ed5276ff
--- /dev/null
+++ b/src/ir/drop.h
@@ -0,0 +1,62 @@
+/*
+ * Copyright 2022 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_ir_drop_h
+#define wasm_ir_drop_h
+
+#include "ir/effects.h"
+#include "ir/iteration.h"
+#include "wasm-builder.h"
+#include "wasm.h"
+
+namespace wasm {
+
+// Given an expression, returns a new expression that drops the given
+// expression's children that cannot be removed outright due to their side
+// effects. This is useful if we know the node is not needed but may need to
+// keep the children around; this utility will automatically remove any children
+// we do not actually need to keep, based on their effects.
+//
+// The caller must also pass in a last item to append to the output (which is
+// typically what the original expression is replaced with).
+Expression* getDroppedChildrenAndAppend(Expression* curr,
+                                        Module& wasm,
+                                        const PassOptions& options,
+                                        Expression* last) {
+  Builder builder(wasm);
+  std::vector<Expression*> contents;
+  for (auto* child : ChildIterator(curr)) {
+    if (!EffectAnalyzer(options, wasm, child).hasUnremovableSideEffects()) {
+      continue;
+    }
+    if (child->type.isConcrete()) {
+      contents.push_back(builder.makeDrop(child));
+    } else {
+      // The child is unreachable, or none (none is possible as a child of a
+      // block or loop, etc.); in both cases we do not need a drop.
+      contents.push_back(child);
+    }
+  }
+  contents.push_back(last);
+  if (contents.size() == 1) {
+    return contents[0];
+  }
+  return builder.makeBlock(contents);
+}
+
+} // namespace wasm
+
+#endif // wasm_ir_drop_h
diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index f2ca4e0be..400ed6af0 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -25,6 +25,7 @@
 #include <ir/abstract.h>
 #include <ir/bits.h>
 #include <ir/cost.h>
+#include <ir/drop.h>
 #include <ir/effects.h>
 #include <ir/eh-utils.h>
 #include <ir/find_all.h>
@@ -1377,9 +1378,15 @@ struct OptimizeInstructions
     skipCast(curr->right, Type::eqref);
 
     // Identical references compare equal.
-    if (areConsecutiveInputsEqualAndRemovable(curr->left, curr->right)) {
-      replaceCurrent(
-        Builder(*getModule()).makeConst(Literal::makeOne(Type::i32)));
+    // (Technically we do not need to check if the inputs are also foldable into
+    // a single one, but we do not have utility code to handle non-foldable
+    // cases yet; the foldable case we do handle is the common one of the first
+    // child being a tee and the second a get of that tee. TODO)
+    if (areConsecutiveInputsEqualAndFoldable(curr->left, curr->right)) {
+      auto* result =
+        Builder(*getModule()).makeConst(Literal::makeOne(Type::i32));
+      replaceCurrent(getDroppedChildrenAndAppend(
+        curr, *getModule(), getPassOptions(), result));
       return;
     }