[EH] Make rethrow's target a try label (#3568)

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 `delegate`'s label: the immediate argument follows the same rule as
when computing 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:
https://github.com/WebAssembly/exception-handling/issues/146#issuecomment-777714491

---

This also reverts some of `delegateTarget` -> `exceptionTarget` changes
done in #3562 in the validator. Label validation rules apply differently
for `delegate` and `rethrow` for try-catch. For example, this is valid:
```wasm
try $l0
  try
  delegate $l0
catch ($l0)
end
```
But this is NOT valid:
```wasm
try $l0
catch ($l0)
  try
  delegate $l0
end
```
So `try`'s label should be used within try-catch range (not catch-end
range) for `delegate`s.

But for the `rethrow` the rule is different. For example, this is valid:
```wasm
try $l0
catch ($l0)
  rethrow $l0
end
```
But this is NOT valid:
```wasm
try $l0
  rethrow $l0
catch ($l0)
end
```
So the `try`'s label should be used within catch-end range instead.

1 files changed, 8 insertions, 7 deletions

diff --git a/src/js/binaryen.js-post.js b/src/js/binaryen.js-post.js
index 63be4d446..7c53298ad 100644
--- a/src/js/binaryen.js-post.js
+++ b/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)) });
   }
 });