[EH] Make rethrow's target a try label

I was previously mistaken about `rethrow`'s argument rule and thought
it only counted `catch`'s depth. But it turns out it follows the same
rule with `delegate`'s label: the immediate argument is comptuted in the
same way as branch labels, but it only can target `try` labels
(semantically it targets that `try`'s corresponding `catch`); otherwise
it will be a validation failure. Unlike `delegate`, `rethrow`'s label
denotes not where to rethrow, but which exception to rethrow. For
example,
```wasm
try $l0
catch ($l0)
  try $l1
  catch ($l1)
   rethrow $l0 ;; rethrow the exception caught by 'catch ($l0)'
  end
end
```

Refer to this comment for the more detailed informal semantics:
WebAssembly/exception-handling#146 (comment)

src/binaryen-c.cpp

@@ -1248,8 +1248,9 @@ BinaryenExpressionRef BinaryenThrow(BinaryenModuleRef module,
 }
 
 BinaryenExpressionRef BinaryenRethrow(BinaryenModuleRef module,
-                                      BinaryenIndex depth) {
-  return static_cast<Expression*>(Builder(*(Module*)module).makeRethrow(depth));
+                                      const char* target) {
+  return static_cast<Expression*>(
+    Builder(*(Module*)module).makeRethrow(target));
 }
 
 BinaryenExpressionRef BinaryenI31New(BinaryenModuleRef module,
@@ -2965,15 +2966,15 @@ BinaryenExpressionRef BinaryenThrowRemoveOperandAt(BinaryenExpressionRef expr,
   return static_cast<Throw*>(expression)->operands.removeAt(index);
 }
 // Rethrow
-BinaryenIndex BinaryenRethrowGetDepth(BinaryenExpressionRef expr) {
+const char* BinaryenRethrowGetTarget(BinaryenExpressionRef expr) {
   auto* expression = (Expression*)expr;
   assert(expression->is<Rethrow>());
-  return static_cast<Rethrow*>(expression)->depth;
+  return static_cast<Rethrow*>(expression)->target.c_str();
 }
-void BinaryenRethrowSetDepth(BinaryenExpressionRef expr, BinaryenIndex depth) {
+void BinaryenRethrowSetTarget(BinaryenExpressionRef expr, const char* target) {
   auto* expression = (Expression*)expr;
   assert(expression->is<Rethrow>());
-  static_cast<Rethrow*>(expression)->depth = depth;
+  static_cast<Rethrow*>(expression)->target = target;
 }
 // TupleMake
 BinaryenIndex BinaryenTupleMakeGetNumOperands(BinaryenExpressionRef expr) {

src/binaryen-c.h

@@ -818,7 +818,7 @@ BinaryenThrow(BinaryenModuleRef module,
               BinaryenExpressionRef* operands,
               BinaryenIndex numOperands);
 BINARYEN_API BinaryenExpressionRef BinaryenRethrow(BinaryenModuleRef module,
-                                                   BinaryenIndex depth);
+                                                   const char* target);
 BINARYEN_API BinaryenExpressionRef
 BinaryenTupleMake(BinaryenModuleRef module,
                   BinaryenExpressionRef* operands,
@@ -1829,11 +1829,11 @@ BinaryenThrowRemoveOperandAt(BinaryenExpressionRef expr, BinaryenIndex index);
 
 // Rethrow
 
-// Gets the depth of a `rethrow` expression.
-BINARYEN_API BinaryenIndex BinaryenRethrowGetDepth(BinaryenExpressionRef expr);
-// Sets the exception reference expression of a `rethrow` expression.
-BINARYEN_API void BinaryenRethrowSetDepth(BinaryenExpressionRef expr,
-                                          BinaryenIndex depth);
+// Gets the target catch's corresponding try label of a `rethrow` expression.
+BINARYEN_API const char* BinaryenRethrowGetTarget(BinaryenExpressionRef expr);
+// Sets the target catch's corresponding try label of a `rethrow` expression.
+BINARYEN_API void BinaryenRethrowSetTarget(BinaryenExpressionRef expr,
+                                           const char* target);
 
 // TupleMake
 

src/ir/branch-utils.h

@@ -83,7 +83,7 @@ void operateOnScopeNameUsesAndSentTypes(Expression* expr, T func) {
     } else if (auto* br = expr->dynCast<BrOn>()) {
       func(name, br->getCastType());
     } else {
-      assert(expr->is<Try>()); // delegate
+      assert(expr->is<Try>() || expr->is<Rethrow>()); // delegate or rethrow
     }
   });
 }
@@ -135,14 +135,14 @@ inline bool replacePossibleTarget(Expression* branch, Name from, Name to) {
   return worked;
 }
 
-// Replace all delegate targets within the given AST.
+// Replace all delegate/rethrow targets within the given AST.
 inline void replaceExceptionTargets(Expression* ast, Name from, Name to) {
   struct Replacer
     : public PostWalker<Replacer, UnifiedExpressionVisitor<Replacer>> {
     Name from, to;
     Replacer(Name from, Name to) : from(from), to(to) {}
     void visitExpression(Expression* curr) {
-      if (curr->is<Try>()) {
+      if (curr->is<Try>() || curr->is<Rethrow>()) {
         operateOnScopeNameUses(curr, [&](Name& name) {
           if (name == from) {
             name = to;

src/js/binaryen.js-post.js

@@ -2154,8 +2154,8 @@ function wrapModule(module, self = {}) {
   self['throw'] = function(event_, operands) {
     return preserveStack(() => Module['_BinaryenThrow'](module, strToStack(event_), i32sToStack(operands), operands.length));
   };
-  self['rethrow'] = function(depth) {
-    return Module['_BinaryenRethrow'](module, depth);
+  self['rethrow'] = function(target) {
+    return Module['_BinaryenRethrow'](module, strToStack(target));
   };
 
   self['tuple'] = {
@@ -2916,7 +2916,7 @@ Module['getExpressionInfo'] = function(expr) {
       return {
         'id': id,
         'type': type,
-        'depth': Module['_BinaryenRethrowGetDepth'](expr)
+        'target': UTF8ToString(Module['_BinaryenRethrowGetTarget'](expr))
       };
     case Module['TupleMakeId']:
       return {
@@ -4287,11 +4287,12 @@ Module['Throw'] = makeExpressionWrapper({
 });
 
 Module['Rethrow'] = makeExpressionWrapper({
-  'getDepth'(expr) {
-    return Module['_BinaryenRethrowGetDepth'](expr);
+  'getTarget'(expr) {
+    const target = Module['_BinaryenRethrowGetTarget'](expr);
+    return target ? UTF8ToString(target) : null;
   },
-  'setDepth'(expr, depthExpr) {
-    Module['_BinaryenRethrowSetDepth'](expr, depthExpr);
+  'setTarget'(expr, target) {
+    preserveStack(() => { Module['_BinaryenRethrowSetTarget'](expr, strToStack(target)) });
   }
 });
 

src/passes/Print.cpp

@@ -1778,7 +1778,7 @@ struct PrintExpressionContents
   }
   void visitRethrow(Rethrow* curr) {
     printMedium(o, "rethrow ");
-    o << curr->depth;
+    printName(curr->target, o);
   }
   void visitNop(Nop* curr) { printMinor(o, "nop"); }
   void visitUnreachable(Unreachable* curr) { printMinor(o, "unreachable"); }

src/wasm-builder.h

@@ -708,9 +708,9 @@ class Builder {
     ret->finalize();
     return ret;
   }
-  Rethrow* makeRethrow(Index depth) {
+  Rethrow* makeRethrow(Name target) {
     auto* ret = wasm.allocator.alloc<Rethrow>();
-    ret->depth = depth;
+    ret->target = target;
     ret->finalize();
     return ret;
   }

src/wasm-delegations-fields.h

@@ -532,7 +532,7 @@ switch (DELEGATE_ID) {
   }
   case Expression::Id::RethrowId: {
     DELEGATE_START(Rethrow);
-    DELEGATE_FIELD_INT(Rethrow, depth);
+    DELEGATE_FIELD_SCOPE_NAME_USE(Rethrow, target);
     DELEGATE_END(Rethrow);
     break;
   }

src/wasm-interpreter.h

@@ -2400,7 +2400,8 @@ template<typename GlobalManager, typename SubType> class ModuleInstanceBase {
     : public ExpressionRunner<RuntimeExpressionRunner> {
     ModuleInstanceBase& instance;
     FunctionScope& scope;
-    SmallVector<WasmException, 4> exceptionStack;
+    // Stack of <caught exception, caught catch's try label>
+    SmallVector<std::pair<WasmException, Name>, 4> exceptionStack;
 
   public:
     RuntimeExpressionRunner(ModuleInstanceBase& instance,
@@ -3046,7 +3047,7 @@ template<typename GlobalManager, typename SubType> class ModuleInstanceBase {
         auto processCatchBody = [&](Expression* catchBody) {
           // Push the current exception onto the exceptionStack in case
           // 'rethrow's use it
-          exceptionStack.push_back(e);
+          exceptionStack.push_back(std::make_pair(e, curr->name));
           // We need to pop exceptionStack in either case: when the catch body
           // exits normally or when a new exception is thrown
           Flow ret;
@@ -3074,8 +3075,11 @@ template<typename GlobalManager, typename SubType> class ModuleInstanceBase {
       }
     }
     Flow visitRethrow(Rethrow* curr) {
-      assert(exceptionStack.size() > curr->depth);
-      throwException(exceptionStack[exceptionStack.size() - 1 - curr->depth]);
+      for (int i = exceptionStack.size() - 1; i >= 0; i--) {
+        if (exceptionStack[i].second == curr->target) {
+          throwException(exceptionStack[i].first);
+        }
+      }
       WASM_UNREACHABLE("rethrow");
     }
     Flow visitPop(Pop* curr) {

src/wasm.h

@@ -1335,7 +1335,7 @@ class Rethrow : public SpecificExpression<Expression::RethrowId> {
 public:
   Rethrow(MixedArena& allocator) {}
 
-  Index depth;
+  Name target;
 
   void finalize();
 };

src/wasm/wasm-binary.cpp

@@ -3475,12 +3475,12 @@ Name WasmBinaryBuilder::getExceptionTargetName(int32_t offset) {
   }
   size_t index = breakStack.size() - 1 - offset;
   if (index > breakStack.size()) {
-    throwError("bad delegate index (high)");
+    throwError("bad try index (high)");
   }
-  BYN_TRACE("delegate target " << breakStack[index].name << std::endl);
+  BYN_TRACE("exception target " << breakStack[index].name << std::endl);
   auto& ret = breakStack[index];
-  // if the delegate is in literally unreachable code, then we will not emit it
-  // anyhow, so do not note that the target has delegate to it
+  // if the delegate/rethrow is in literally unreachable code, then we will not
+  // emit it anyhow, so do not note that the target has a reference to it
   if (!willBeIgnored) {
     exceptionTargetNames.insert(ret.name);
   }
@@ -5835,11 +5835,11 @@ void WasmBinaryBuilder::visitTryOrTryInBlock(Expression*& out) {
     curr->delegateTarget = getExceptionTargetName(getU32LEB());
   }
 
-  // For simplicity, we make try's labels only can be targeted by delegates, and
-  // delegates can only target try's labels. (If they target blocks or loops, it
-  // is a validation failure.) Because we create an inner block within each try
-  // and catch body, if any delegate targets those inner blocks, we should make
-  // them target the try's label instead.
+  // For simplicity, we make try's labels only can be targeted by delegates and
+  // rethrows, and delegates/rethrows can only target try's labels. (If they
+  // target blocks or loops, it is a validation failure.) Because we create an
+  // inner block within each try and catch body, if any delegate/rethrow targets
+  // those inner blocks, we should make them target the try's label instead.
   curr->name = getNextLabel();
   if (auto* block = curr->body->dynCast<Block>()) {
     if (block->name.is()) {
@@ -5936,7 +5936,9 @@ void WasmBinaryBuilder::visitThrow(Throw* curr) {
 
 void WasmBinaryBuilder::visitRethrow(Rethrow* curr) {
   BYN_TRACE("zz node: Rethrow\n");
-  curr->depth = getU32LEB();
+  curr->target = getExceptionTargetName(getU32LEB());
+  // This special target is valid only for delegates
+  assert(curr->target != DELEGATE_CALLER_TARGET);
   curr->finalize();
 }
 

src/wasm/wasm-s-parser.cpp

@@ -2067,8 +2067,8 @@ Expression* SExpressionWasmBuilder::makeTry(Element& s) {
     // We create a different name for the wrapping block, because try's name can
     // be used by internal delegates
     block->name = nameMapper.pushLabelName(sName);
-    // For simplicity, try's name canonly be targeted by delegates. Make the
-    // branches target the new wrapping block instead.
+    // For simplicity, try's name can only be targeted by delegates and
+    // rethrows. Make the branches target the new wrapping block instead.
     BranchUtils::replaceBranchTargets(ret, ret->name, block->name);
     block->list.push_back(ret);
     nameMapper.popLabelName(block->name);
@@ -2078,29 +2078,6 @@ Expression* SExpressionWasmBuilder::makeTry(Element& s) {
   return ret;
 }
 
-Expression*
-SExpressionWasmBuilder::makeTryOrCatchBody(Element& s, Type type, bool isTry) {
-  if (isTry && !elementStartsWith(s, "do")) {
-    throw ParseException("invalid try do clause", s.line, s.col);
-  }
-  if (!isTry && !elementStartsWith(s, "catch") &&
-      !elementStartsWith(s, "catch_all")) {
-    throw ParseException("invalid catch clause", s.line, s.col);
-  }
-  if (s.size() == 1) { // (do) / (catch) / (catch_all) without instructions
-    return makeNop();
-  }
-  auto ret = allocator.alloc<Block>();
-  for (size_t i = 1; i < s.size(); i++) {
-    ret->list.push_back(parseExpression(s[i]));
-  }
-  if (ret->list.size() == 1) {
-    return ret->list[0];
-  }
-  ret->finalize(type);
-  return ret;
-}
-
 Expression* SExpressionWasmBuilder::makeThrow(Element& s) {
   auto ret = allocator.alloc<Throw>();
   Index i = 1;
@@ -2118,7 +2095,7 @@ Expression* SExpressionWasmBuilder::makeThrow(Element& s) {
 
 Expression* SExpressionWasmBuilder::makeRethrow(Element& s) {
   auto ret = allocator.alloc<Rethrow>();
-  ret->depth = atoi(s[1]->str().c_str());
+  ret->target = getLabel(*s[1], LabelType::Exception);
   ret->finalize();
   return ret;
 }

src/wasm/wasm-stack.cpp

@@ -1936,7 +1936,7 @@ void BinaryInstWriter::visitThrow(Throw* curr) {
 }
 
 void BinaryInstWriter::visitRethrow(Rethrow* curr) {
-  o << int8_t(BinaryConsts::Rethrow) << U32LEB(curr->depth);
+  o << int8_t(BinaryConsts::Rethrow) << U32LEB(getBreakIndex(curr->target));
 }
 
 void BinaryInstWriter::visitNop(Nop* curr) { o << int8_t(BinaryConsts::Nop); }

src/wasm/wasm-validator.cpp

@@ -236,7 +236,8 @@ struct FunctionValidator : public WalkerPass<PostWalker<FunctionValidator>> {
   };
 
   std::unordered_map<Name, BreakInfo> breakInfos;
-  std::unordered_set<Name> exceptionTargetNames;
+  std::unordered_set<Name> delegateTargetNames;
+  std::unordered_set<Name> rethrowTargetNames;
 
   std::set<Type> returnTypes; // types used in returns
 
@@ -280,7 +281,7 @@ struct FunctionValidator : public WalkerPass<PostWalker<FunctionValidator>> {
   static void visitPreTry(FunctionValidator* self, Expression** currp) {
     auto* curr = (*currp)->cast<Try>();
     if (curr->name.is()) {
-      self->exceptionTargetNames.insert(curr->name);
+      self->delegateTargetNames.insert(curr->name);
     }
   }
 
@@ -300,7 +301,8 @@ struct FunctionValidator : public WalkerPass<PostWalker<FunctionValidator>> {
   static void visitPreCatch(FunctionValidator* self, Expression** currp) {
     auto* curr = (*currp)->cast<Try>();
     if (curr->name.is()) {
-      self->exceptionTargetNames.erase(curr->name);
+      self->delegateTargetNames.erase(curr->name);
+      self->rethrowTargetNames.insert(curr->name);
     }
   }
 
@@ -376,7 +378,8 @@ struct FunctionValidator : public WalkerPass<PostWalker<FunctionValidator>> {
   void visitRefIs(RefIs* curr);
   void visitRefFunc(RefFunc* curr);
   void visitRefEq(RefEq* curr);
-  void noteException(Name name, Expression* curr);
+  void noteDelegate(Name name, Expression* curr);
+  void noteRethrow(Name name, Expression* curr);
   void visitTry(Try* curr);
   void visitThrow(Throw* curr);
   void visitRethrow(Rethrow* curr);
@@ -2073,14 +2076,20 @@ void FunctionValidator::visitRefEq(RefEq* curr) {
     "ref.eq's right argument should be a subtype of eqref");
 }
 
-void FunctionValidator::noteException(Name name, Expression* curr) {
+void FunctionValidator::noteDelegate(Name name, Expression* curr) {
   if (name != DELEGATE_CALLER_TARGET) {
-    shouldBeTrue(exceptionTargetNames.find(name) != exceptionTargetNames.end(),
+    shouldBeTrue(delegateTargetNames.find(name) != delegateTargetNames.end(),
                  curr,
                  "all delegate targets must be valid");
   }
 }
 
+void FunctionValidator::noteRethrow(Name name, Expression* curr) {
+  shouldBeTrue(rethrowTargetNames.find(name) != rethrowTargetNames.end(),
+               curr,
+               "all rethrow targets must be valid");
+}
+
 void FunctionValidator::visitTry(Try* curr) {
   shouldBeTrue(getModule()->features.hasExceptionHandling(),
                curr,
@@ -2122,8 +2131,10 @@ void FunctionValidator::visitTry(Try* curr) {
                "try should have either catches or a delegate");
 
   if (curr->isDelegate()) {
-    noteException(curr->delegateTarget, curr);
+    noteDelegate(curr->delegateTarget, curr);
   }
+
+  rethrowTargetNames.erase(curr->name);
 }
 
 void FunctionValidator::visitThrow(Throw* curr) {
@@ -2167,8 +2178,7 @@ void FunctionValidator::visitRethrow(Rethrow* curr) {
                 Type(Type::unreachable),
                 curr,
                 "rethrow's type must be unreachable");
-  // TODO Validate the depth field. The current LLVM toolchain only generates
-  // depth 0 for C++ support.
+  noteRethrow(curr->target, curr);
 }
 
 void FunctionValidator::visitTupleMake(TupleMake* curr) {
@@ -2533,9 +2543,12 @@ void FunctionValidator::visitFunction(Function* curr) {
 
   shouldBeTrue(
     breakInfos.empty(), curr->body, "all named break targets must exist");
-  shouldBeTrue(exceptionTargetNames.empty(),
+  shouldBeTrue(delegateTargetNames.empty(),
                curr->body,
                "all named delegate targets must exist");
+  shouldBeTrue(rethrowTargetNames.empty(),
+               curr->body,
+               "all named rethrow targets must exist");
   returnTypes.clear();
   labelNames.clear();
   // validate optional local names