Add TailCall optimization pass (--tail-call)

scripts/fuzz_opt.py

@@ -2336,6 +2336,10 @@ def do_run(vm, js, wasm):
                 #
                 # Ignore it, as details of traces differ based on optimizations.
                 continue
+            elif not line:
+                # V8 may print blank lines before stack traces when the top
+                # frame has no script location (e.g. after a return_call to JS).
+                continue
             cleaned.append(line)
         cleaned = '\n'.join(cleaned)
 
@@ -2765,6 +2769,7 @@ def write_commands(commands, filename):
     ("--simplify-locals-notee",),
     ("--simplify-locals-notee-nostructure",),
     ("--ssa",),
+    ("--tail-call",),
     ("--tuple-optimization",),
     ("--type-finalizing",),
     ("--type-refining",),

src/passes/CMakeLists.txt

@@ -137,6 +137,7 @@ set(passes_SOURCES
   StackCheck.cpp
   StripEH.cpp
   SSAify.cpp
+  TailCall.cpp
   TupleOptimization.cpp
   TranslateEH.cpp
   TypeFinalizing.cpp

src/passes/TailCall.cpp

@@ -0,0 +1,324 @@
+/*
+ * Copyright 2026 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.
+ */
+
+//
+// Convert calls in tail position to return calls (tail calls).
+//
+
+#include <unordered_set>
+
+#include "ir/effects.h"
+#include "ir/properties.h"
+#include "ir/utils.h"
+#include "pass.h"
+#include "wasm-traversal.h"
+#include "wasm.h"
+
+namespace wasm {
+
+namespace {
+
+// We are doing a pre-order traversal (i.e. parents before children) rather
+// than the normal post-order traversal because whether an expression is in
+// tail position is propagated down from parents to children. Define our own
+// pre-order traversal task stack, and take the opportunity to pass `isTail`
+// as an extra parameter to each task rather than storing it in a side table.
+template<typename SubType>
+struct PreWalker : public Walker<SubType, Visitor<SubType>> {
+  using TaskFunc = void (*)(SubType*, Expression**, bool);
+
+  struct Task {
+    TaskFunc func;
+    Expression** currp;
+    bool isTail;
+    Task() = default;
+    Task(TaskFunc func, Expression** currp, bool isTail)
+      : func(func), currp(currp), isTail(isTail) {}
+  };
+
+  SmallVector<Task, 10> stack;
+
+  void push(Expression** currp, bool isTail) {
+    assert(*currp);
+    stack.emplace_back(doVisit, currp, isTail);
+  }
+
+  void maybePush(Expression** currp, bool isTail) {
+    if (*currp) {
+      stack.emplace_back(doVisit, currp, isTail);
+    }
+  }
+
+  Task popTask() {
+    auto ret = stack.back();
+    stack.pop_back();
+    return ret;
+  }
+
+  static void doVisit(SubType* self, Expression** currp, bool isTail) {
+    self->visit(*currp, isTail);
+  }
+
+  void walk(Expression*& root) {
+    assert(stack.empty());
+    push(&root, true);
+    while (!stack.empty()) {
+      auto task = popTask();
+      task.func(static_cast<SubType*>(this), task.currp, task.isTail);
+    }
+  }
+
+  void doWalkFunction(Function* func) { walk(func->body); }
+
+  void visitExpression(Expression* curr, bool isTail) {
+    assert(!Properties::isControlFlowStructure(curr) &&
+           "unexpected control flow structure");
+
+#define DELEGATE_ID curr->_id
+#define DELEGATE_START(id) [[maybe_unused]] auto* cast = curr->cast<id>();
+#define DELEGATE_END(id)
+#define DELEGATE_GET_FIELD(id, field) cast->field
+#define DELEGATE_FIELD_CHILD(id, field) push(&cast->field, false);
+#define DELEGATE_FIELD_OPTIONAL_CHILD(id, field) maybePush(&cast->field, false);
+#define DELEGATE_FIELD_INT(id, field)
+#define DELEGATE_FIELD_LITERAL(id, field)
+#define DELEGATE_FIELD_NAME(id, field)
+#define DELEGATE_FIELD_SCOPE_NAME_DEF(id, field)
+#define DELEGATE_FIELD_SCOPE_NAME_USE(id, field)
+#define DELEGATE_FIELD_TYPE(id, field)
+#define DELEGATE_FIELD_HEAPTYPE(id, field)
+#define DELEGATE_FIELD_ADDRESS(id, field)
+
+#include "wasm-delegations-fields.def"
+  }
+
+#define DELEGATE(CLASS_TO_VISIT)                                               \
+  void visit##CLASS_TO_VISIT(CLASS_TO_VISIT* curr, bool isTail) {              \
+    static_cast<SubType*>(this)->visitExpression(curr, isTail);                \
+  }
+
+#include "wasm-delegations.def"
+
+  void visit(Expression* curr, bool isTail) {
+    assert(curr);
+    switch (curr->_id) {
+#define DELEGATE(CLASS_TO_VISIT)                                               \
+  case Expression::Id::CLASS_TO_VISIT##Id:                                     \
+    return static_cast<SubType*>(this)->visit##CLASS_TO_VISIT(                 \
+      static_cast<CLASS_TO_VISIT*>(curr), isTail);
+
+#include "wasm-delegations.def"
+
+      default:
+        WASM_UNREACHABLE("unexpected expression type");
+    }
+  }
+};
+
+struct TailCall : public WalkerPass<PreWalker<TailCall>> {
+  bool isFunctionParallel() override { return true; }
+
+  std::unique_ptr<Pass> create() override {
+    return std::make_unique<TailCall>();
+  }
+
+  // Names of blocks whose exit flows directly out of the function.
+  std::unordered_set<Name> tailBlocks;
+  // Nesting depth of active exception handlers that catch or redirect
+  // exceptions within the current function.
+  size_t ehDepth = 0;
+  // Whether any call in the current function was converted to a return call.
+  bool changed = false;
+
+  void pushEnterTry() { stack.emplace_back(doEnterTryBody, nullptr, false); }
+
+  void pushLeaveTry() { stack.emplace_back(doLeaveTryBody, nullptr, false); }
+
+  static void doEnterTryBody(TailCall* self, Expression**, bool) {
+    ++self->ehDepth;
+  }
+
+  static void doLeaveTryBody(TailCall* self, Expression**, bool) {
+    assert(self->ehDepth > 0);
+    --self->ehDepth;
+  }
+
+  bool hasUnremovableSideEffects(Expression* expr) {
+    return EffectAnalyzer(getPassOptions(), *getModule(), expr)
+      .hasUnremovableSideEffects();
+  }
+
+  bool allTargetsInTailBlocks(Switch* curr) {
+    if (!tailBlocks.contains(curr->default_)) {
+      return false;
+    }
+    for (auto target : curr->targets) {
+      if (!tailBlocks.contains(target)) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  bool isTailTransfer(Break* curr, bool isTail) {
+    if (!tailBlocks.contains(curr->name)) {
+      return false;
+    }
+    // Converting a call in a conditional branch or br_table to a return_call
+    // skips evaluating the condition, so the condition must not have side
+    // effects.
+    return !curr->condition ||
+           (isTail && !hasUnremovableSideEffects(curr->condition));
+  }
+
+  bool isTailTransfer(Switch* curr) {
+    return allTargetsInTailBlocks(curr) &&
+           !hasUnremovableSideEffects(curr->condition);
+  }
+
+  bool isTailTransfer(Expression* curr, bool isTail) {
+    if (curr->is<Return>()) {
+      return true;
+    }
+    if (auto* br = curr->dynCast<Break>()) {
+      return isTailTransfer(br, isTail);
+    }
+    if (auto* sw = curr->dynCast<Switch>()) {
+      return isTailTransfer(sw);
+    }
+    return false;
+  }
+
+  template<typename CallType> void handleCall(CallType* call, bool isTail) {
+    // A call in tail position can have a type incompatible with the function's
+    // return type if it is dead code at the end of a block following an earlier
+    // unreachable instruction.
+    if (call->isReturn || !isTail ||
+        !Type::isSubType(call->type, getFunction()->getResults())) {
+      return;
+    }
+    if (ehDepth > 0 &&
+        ShallowEffectAnalyzer(getPassOptions(), *getModule(), call).throws()) {
+      return;
+    }
+    call->isReturn = true;
+    call->finalize();
+    changed = true;
+  }
+
+  void visitBlock(Block* curr, bool isTail) {
+    if (isTail && curr->name.is()) {
+      tailBlocks.insert(curr->name);
+    }
+    bool nextIsTail = isTail;
+    for (int i = int(curr->list.size()) - 1; i >= 0; --i) {
+      bool itemIsTail = false;
+      if (i == int(curr->list.size()) - 1) {
+        itemIsTail = isTail;
+      } else if (getFunction()->getResults() == Type::none &&
+                 isTailTransfer(curr->list[i + 1], nextIsTail)) {
+        itemIsTail = true;
+      }
+      nextIsTail = itemIsTail;
+      push(&curr->list[i], itemIsTail);
+    }
+  }
+
+  void visitIf(If* curr, bool isTail) {
+    maybePush(&curr->ifFalse, isTail);
+    push(&curr->ifTrue, isTail);
+    push(&curr->condition, false);
+  }
+
+  void visitLoop(Loop* curr, bool isTail) { push(&curr->body, isTail); }
+
+  void visitBreak(Break* curr, bool isTail) {
+    bool valueIsTail = curr->value && isTailTransfer(curr, isTail);
+    maybePush(&curr->condition, false);
+    maybePush(&curr->value, valueIsTail);
+  }
+
+  void visitSwitch(Switch* curr, bool isTail) {
+    bool valueIsTail = curr->value && isTailTransfer(curr);
+    push(&curr->condition, false);
+    maybePush(&curr->value, valueIsTail);
+  }
+
+  void visitReturn(Return* curr, bool isTail) { maybePush(&curr->value, true); }
+
+  void visitTry(Try* curr, bool isTail) {
+    for (int i = int(curr->catchBodies.size()) - 1; i >= 0; --i) {
+      push(&curr->catchBodies[i], isTail);
+    }
+    // A try block that delegates directly to the caller does not catch any
+    // exceptions in this function; exceptions thrown in its body already unwind
+    // the frame to the caller just like a return_call would. All other try
+    // blocks (catch/catch_all or delegating to an outer try) establish a local
+    // handler that would be bypassed by return_call.
+    bool hasLocalHandler =
+      !curr->isDelegate() || curr->delegateTarget != DELEGATE_CALLER_TARGET;
+    if (hasLocalHandler) {
+      pushLeaveTry();
+    }
+    push(&curr->body, isTail);
+    if (hasLocalHandler) {
+      pushEnterTry();
+    }
+  }
+
+  void visitTryTable(TryTable* curr, bool isTail) {
+    bool hasLocalHandler = !curr->catchTags.empty();
+    if (hasLocalHandler) {
+      pushLeaveTry();
+    }
+    push(&curr->body, isTail);
+    if (hasLocalHandler) {
+      pushEnterTry();
+    }
+  }
+
+  void visitCall(Call* curr, bool isTail) {
+    handleCall(curr, isTail);
+    visitExpression(curr, false);
+  }
+
+  void visitCallIndirect(CallIndirect* curr, bool isTail) {
+    handleCall(curr, isTail);
+    visitExpression(curr, false);
+  }
+
+  void visitCallRef(CallRef* curr, bool isTail) {
+    handleCall(curr, isTail);
+    visitExpression(curr, false);
+  }
+
+  void doWalkFunction(Function* func) {
+    if (!getModule()->features.hasTailCall() || func->imported()) {
+      return;
+    }
+    tailBlocks.clear();
+    walk(func->body);
+    if (changed) {
+      ReFinalize().walkFunctionInModule(func, getModule());
+    }
+  }
+};
+
+} // anonymous namespace
+
+Pass* createTailCallPass() { return new TailCall(); }
+
+} // namespace wasm

src/passes/pass.cpp

@@ -555,6 +555,9 @@ void PassRegistry::registerPasses() {
   registerPass("stack-check",
                "enforce limits on llvm's __stack_pointer global",
                createStackCheckPass);
+  registerPass("tail-call",
+               "convert calls in tail position to return calls",
+               createTailCallPass);
   registerPass("strip-debug",
                "strip debug info (including the names section)",
                createStripDebugPass);

src/passes/passes.h

@@ -190,6 +190,7 @@ Pass* createStubUnsupportedJSOpsPass();
 Pass* createSSAifyPass();
 Pass* createSSAifyNoMergePass();
 Pass* createTable64LoweringPass();
+Pass* createTailCallPass();
 Pass* createTranslateToExnrefPass();
 Pass* createTupleOptimizationPass();
 Pass* createTypeGeneralizingPass();

test/lit/help/wasm-metadce.test

@@ -548,6 +548,9 @@
 ;; CHECK-EMPTY:
 ;; CHECK-NEXT:   --table64-lowering                            alias for memory64-lowering
 ;; CHECK-EMPTY:
+;; CHECK-NEXT:   --tail-call                                   convert calls in tail position 
+;; CHECK-NEXT:                                                 to return calls
+;; CHECK-EMPTY:
 ;; CHECK-NEXT:   --trace-calls                                 instrument the build with code 
 ;; CHECK-NEXT:                                                 to intercept specific function 
 ;; CHECK-NEXT:                                                 calls

test/lit/help/wasm-opt.test

@@ -584,6 +584,9 @@
 ;; CHECK-EMPTY:
 ;; CHECK-NEXT:   --table64-lowering                            alias for memory64-lowering
 ;; CHECK-EMPTY:
+;; CHECK-NEXT:   --tail-call                                   convert calls in tail position 
+;; CHECK-NEXT:                                                 to return calls
+;; CHECK-EMPTY:
 ;; CHECK-NEXT:   --trace-calls                                 instrument the build with code 
 ;; CHECK-NEXT:                                                 to intercept specific function 
 ;; CHECK-NEXT:                                                 calls

test/lit/help/wasm2js.test

@@ -512,6 +512,9 @@
 ;; CHECK-EMPTY:
 ;; CHECK-NEXT:   --table64-lowering                            alias for memory64-lowering
 ;; CHECK-EMPTY:
+;; CHECK-NEXT:   --tail-call                                   convert calls in tail position 
+;; CHECK-NEXT:                                                 to return calls
+;; CHECK-EMPTY:
 ;; CHECK-NEXT:   --trace-calls                                 instrument the build with code 
 ;; CHECK-NEXT:                                                 to intercept specific function 
 ;; CHECK-NEXT:                                                 calls