[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, 1 insertions, 1 deletions

diff --git a/src/wasm/wasm-stack.cpp b/src/wasm/wasm-stack.cpp
index d36c2386c..b6c0392ca 100644
--- a/src/wasm/wasm-stack.cpp
+++ b/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); }