From d2a48afe09dd5b22079c748a97ebaebaf69a19a7 Mon Sep 17 00:00:00 2001
From: Alon Zakai <azakai@google.com>
Date: Fri, 12 Jul 2024 16:15:53 -0700
Subject: Monomorphize dropped functions (#6734)

We now consider a drop to be part of the call context: If we see

(drop
  (call $foo)
)

(func $foo (result i32)
  (i32.const 42)
)

Then we'd monomorphize to this:

(call $foo_1)  ;; call the specialized function instead

(func $foo_1   ;; the specialized function returns nothing
  (drop        ;; the drop was moved into here
    (i32.const 42)
  )
)

With the drop now in the called function, we may be able to optimize out unused work.

Refactor a bit of code out of DAE that we can reuse here, into a new return-utils.h.
---
 src/ir/return-utils.cpp | 99 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)
 create mode 100644 src/ir/return-utils.cpp

(limited to 'src/ir/return-utils.cpp')

diff --git a/src/ir/return-utils.cpp b/src/ir/return-utils.cpp
new file mode 100644
index 000000000..20b3a194b
--- /dev/null
+++ b/src/ir/return-utils.cpp
@@ -0,0 +1,99 @@
+/*
+ * Copyright 2024 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.
+ */
+
+#include "ir/return-utils.h"
+#include "ir/module-utils.h"
+#include "wasm-builder.h"
+#include "wasm-traversal.h"
+#include "wasm.h"
+
+namespace wasm::ReturnUtils {
+
+namespace {
+
+struct ReturnValueRemover : public PostWalker<ReturnValueRemover> {
+  void visitReturn(Return* curr) {
+    auto* value = curr->value;
+    assert(value);
+    curr->value = nullptr;
+    Builder builder(*getModule());
+    replaceCurrent(builder.makeSequence(builder.makeDrop(value), curr));
+  }
+
+  void visitCall(Call* curr) { handleReturnCall(curr); }
+  void visitCallIndirect(CallIndirect* curr) { handleReturnCall(curr); }
+  void visitCallRef(CallRef* curr) { handleReturnCall(curr); }
+
+  template<typename T> void handleReturnCall(T* curr) {
+    if (curr->isReturn) {
+      Fatal() << "Cannot remove return_calls in ReturnValueRemover";
+    }
+  }
+
+  void visitFunction(Function* curr) {
+    if (curr->body->type.isConcrete()) {
+      curr->body = Builder(*getModule()).makeDrop(curr->body);
+    }
+  }
+};
+
+} // anonymous namespace
+
+void removeReturns(Function* func, Module& wasm) {
+  ReturnValueRemover().walkFunctionInModule(func, &wasm);
+}
+
+std::unordered_map<Function*, bool> findReturnCallers(Module& wasm) {
+  ModuleUtils::ParallelFunctionAnalysis<bool> analysis(
+    wasm, [&](Function* func, bool& hasReturnCall) {
+      if (func->imported()) {
+        return;
+      }
+
+      struct Finder : PostWalker<Finder> {
+        bool hasReturnCall = false;
+
+        void visitCall(Call* curr) {
+          if (curr->isReturn) {
+            hasReturnCall = true;
+          }
+        }
+        void visitCallIndirect(CallIndirect* curr) {
+          if (curr->isReturn) {
+            hasReturnCall = true;
+          }
+        }
+        void visitCallRef(CallRef* curr) {
+          if (curr->isReturn) {
+            hasReturnCall = true;
+          }
+        }
+      } finder;
+
+      finder.walk(func->body);
+      hasReturnCall = finder.hasReturnCall;
+    });
+
+  // Convert to an unordered map for fast lookups. TODO: Avoid a copy here.
+  std::unordered_map<Function*, bool> ret;
+  ret.reserve(analysis.map.size());
+  for (auto& [k, v] : analysis.map) {
+    ret[k] = v;
+  }
+  return ret;
+}
+
+} // namespace wasm::ReturnUtils
-- 
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