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.

1 files changed, 123 insertions, 0 deletions

diff --git a/test/spec/exception-handling.wast b/test/spec/exception-handling.wast
new file mode 100644
index 000000000..d81bd433c
--- /dev/null
+++ b/test/spec/exception-handling.wast
@@ -0,0 +1,123 @@
+(module
+  (event $e0 (attr 0) (param i32))
+  (event $e1 (attr 0) (param i32 f32))
+
+  (func $exnref_test (param $0 exnref) (result exnref)
+    (local.get $0)
+  )
+
+  (func $eh_test (local $exn exnref)
+    (try
+      (throw $e0 (i32.const 0))
+      (catch
+        ;; Multi-value is not available yet, so block can't take a value from
+        ;; stack. So this uses locals for now.
+        (local.set $exn (exnref.pop))
+        (drop
+          (block $l0 (result i32)
+            (rethrow
+              (br_on_exn $l0 $e0 (local.get $exn))
+            )
+          )
+        )
+      )
+    )
+  )
+)
+
+(assert_invalid
+  (module
+    (func $f0
+      (try
+        (nop)
+        (catch (i32.const 0))
+      )
+    )
+  )
+  "try's body type must match catch's body type"
+)
+
+(assert_invalid
+  (module
+    (func $f0
+      (try
+        (i32.const 0)
+        (catch (i32.const 0))
+      )
+    )
+  )
+   "try's type does not match try body's type"
+)
+
+(assert_invalid
+  (module
+    (event $e0 (attr 0) (param i32))
+    (func $f0
+      (throw $e0 (f32.const 0))
+    )
+  )
+  "event param types must match"
+)
+
+(assert_invalid
+  (module
+    (event $e0 (attr 0) (param i32 f32))
+    (func $f0
+      (throw $e0 (f32.const 0))
+    )
+  )
+  "event's param numbers must match"
+)
+
+(assert_invalid
+  (module
+    (func $f0
+      (rethrow (i32.const 0))
+    )
+  )
+  "rethrow's argument must be exnref type"
+)
+
+(assert_invalid
+  (module
+    (event $e0 (attr 0) (param i32))
+    (func $f0 (result i32)
+      (block $l0 (result i32)
+        (drop
+          (br_on_exn $l0 $e0 (i32.const 0))
+        )
+        (i32.const 0)
+      )
+    )
+  )
+  "br_on_exn's argument must be unreachable or exnref type"
+)
+
+(assert_invalid
+  (module
+    (event $e0 (attr 0) (param i32))
+    (func $f0 (result i32) (local $0 exnref)
+      (block $l0 (result i32)
+        (i32.eqz
+          (br_on_exn $l0 $e0 (local.get $0))
+        )
+      )
+    )
+  )
+  "i32.eqz input must be i32"
+)
+
+(assert_invalid
+  (module
+    (event $e0 (attr 0) (param i32))
+    (func $f0 (result f32) (local $0 exnref)
+      (block $l0 (result f32)
+        (drop
+          (br_on_exn $l0 $e0 (local.get $0))
+        )
+        (f32.const 0)
+      )
+    )
+  )
+  "block+breaks must have right type if breaks return a value"
+)