Add basic exception handling support (#2282)

This adds basic support for exception handling instructions, according
to the spec:
https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md

This PR includes support for:
- Binary reading/writing
- Wast reading/writing
- Stack IR
- Validation
- binaryen.js + C API
- Few IR routines: branch-utils, type-updating, etc
- Few passes: just enough to make `wasm-opt -O` pass
- Tests

This PR does not include support for many optimization passes, fuzzer,
or interpreter. They will be follow-up PRs.

Try-catch construct is modeled in Binaryen IR in a similar manner to
that of if-else: each of try body and catch body will contain a block,
which can be omitted if there is only a single instruction. This block
will not be emitted in wast or binary, as in if-else. As in if-else,
`class Try` contains two expressions each for try body and catch body,
and `catch` is not modeled as an instruction. `exnref` value pushed by
`catch` is get by `pop` instruction.

`br_on_exn` is special: it returns different types of values when taken
and not taken. We make `exnref`, the type `br_on_exn` pushes if not
taken, as `br_on_exn`'s type.

2 files changed, 27 insertions, 0 deletions

diff --git a/test/unit/input/exception_handling_target_feature.wasm b/test/unit/input/exception_handling_target_feature.wasm
new file mode 100755
index 000000000..487aaf679
--- /dev/null
+++ b/test/unit/input/exception_handling_target_feature.wasm
diff --git a/test/unit/test_features.py b/test/unit/test_features.py
index 1836aa08a..3fd362aed 100644
--- a/test/unit/test_features.py
+++ b/test/unit/test_features.py
@@ -145,6 +145,27 @@ class FeatureValidationTest(BinaryenTestCase):
     '''
     self.check_tail_call(module, 'return_call_indirect requires tail calls to be enabled')
 
+  def test_exnref_local(self):
+    module = '''
+    (module
+     (func $foo
+      (local exnref)
+     )
+    )
+    '''
+    self.check_exception_handling(module, 'all used types should be allowed')
+
+  def test_event(self):
+    module = '''
+    (module
+     (event $e (attr 0) (param i32))
+     (func $foo
+      (throw $e (i32.const 0))
+     )
+    )
+    '''
+    self.check_exception_handling(module, 'Module has events')
+
 
 class TargetFeaturesSectionTest(BinaryenTestCase):
   def test_atomics(self):
@@ -190,6 +211,12 @@ class TargetFeaturesSectionTest(BinaryenTestCase):
     self.check_features(filename, ['tail-call'])
     self.assertIn('return_call', self.disassemble(filename))
 
+  def test_exception_handling(self):
+    filename = 'exception_handling_target_feature.wasm'
+    self.roundtrip(filename)
+    self.check_features(filename, ['exception-handling'])
+    self.assertIn('throw', self.disassemble(filename))
+
   def test_incompatible_features(self):
     path = self.input_path('signext_target_feature.wasm')
     p = run_process(