Bug 1716043 - Support Wasm try block with no other clauses r=rhunt

The proposal spec for exception handling was recently changed
to allow a `try` block with no `catch` or `delegate` clauses:

  WebAssembly/exception-handling#157

Differential Revision: https://phabricator.services.mozilla.com/D118088

js/src/jit-test/tests/wasm/exceptions/instructions.js

@@ -1,5 +1,44 @@
 // Tests for Wasm exception proposal instructions.
 
+// Test try blocks with no handlers.
+assertEq(
+  wasmEvalText(
+    `(module
+       (func (export "f") (result i32)
+         try (result i32) (i32.const 0) end))`
+  ).exports.f(),
+  0
+);
+
+assertEq(
+  wasmEvalText(
+    `(module
+       (func (export "f") (result i32)
+         try (result i32) (i32.const 0) (br 0) (i32.const 1) end))`
+  ).exports.f(),
+  0
+);
+
+assertEq(
+  wasmEvalText(
+    `(module
+       (type (func))
+       (event $exn (type 0))
+       (func (export "f") (result i32)
+         try (result i32)
+           try (result i32)
+             (throw $exn)
+             (i32.const 1)
+           end
+           drop
+           (i32.const 2)
+         catch $exn
+           (i32.const 0)
+         end))`
+  ).exports.f(),
+  0
+);
+
 // Test trivial try-catch with empty bodies.
 assertEq(
   wasmEvalText(

js/src/jit-test/tests/wasm/exceptions/throw-to-js.js

@@ -13,6 +13,17 @@ function assertWasmThrowsExn(thunk) {
   assertEq(thrown, true, "missing exception");
 }
 
+// Test that handler-less trys don't catch anything.
+assertWasmThrowsExn(() =>
+  wasmEvalText(
+    `(module
+       (type (func (param)))
+       (event $exn (type 0))
+       (func (export "f")
+         try (throw $exn) end))`
+  ).exports.f()
+);
+
 // Test throwing simple empty exceptions to JS.
 assertWasmThrowsExn(() =>
   wasmEvalText(

js/src/jit-test/tests/wasm/exceptions/validation.js

@@ -75,31 +75,6 @@ function testValidateDecode() {
     /expected event index/
   );
 
-  // Try blocks must have a second branch after the main body.
-  wasmInvalid(
-    moduleWithSections([
-      sigSection([emptyType]),
-      declSection([0]),
-      eventSection([{ type: 0 }]),
-      bodySection([
-        funcBody({
-          locals: [],
-          body: [
-            TryCode,
-            I32Code,
-            I32ConstCode,
-            0x01,
-            // Missing instruction here.
-            EndCode,
-            DropCode,
-            ReturnCode,
-          ],
-        }),
-      ]),
-    ]),
-    /try without catch or catch_all not allowed/
-  );
-
   // Rethrow must have a depth argument.
   wasmInvalid(
     moduleWithSections([
@@ -244,6 +219,27 @@ function testValidateTryCatch() {
   wasmValid(valid1);
   wasmInvalid(invalid1, /unused values not explicitly dropped/);
   wasmValid(valid2);
+
+  // Test handler-less try blocks.
+  wasmValidateText(
+    `(module (func try end))`
+  );
+
+  wasmValidateText(
+    `(module (func (result i32) try (result i32) (i32.const 1) end))`
+  );
+
+  wasmValidateText(
+    `(module
+       (func (result i32)
+         try (result i32) (i32.const 1) (br 0) end))`
+  );
+
+  wasmFailValidateText(
+    `(module
+       (func try (result i32) end))`,
+    /popping value from empty stack/
+  );
 }
 
 function testValidateCatch() {

js/src/wasm/WasmBaselineCompile.cpp

@@ -10262,9 +10262,21 @@ bool BaseCompiler::emitEnd() {
       }
       break;
 #ifdef ENABLE_WASM_EXCEPTIONS
-    case LabelKind::Try:
-      MOZ_CRASH("Try-catch block cannot end without catch.");
+    case LabelKind::Try: {
+      // The beginning of a `try` block introduces this try note, but
+      // we need to remove it in case we have no handlers in this block
+      // as it can never be the target of an exception.
+      WasmTryNoteVector& tryNotes = masm.tryNotes();
+      // This will shift the indices of any try notes in between the `try`
+      // instruction and this `end`. This is fine as long as try notes are
+      // only accessed through ControlItems, as the shifted try notes will
+      // correspond only to items that are popped by this point.
+      tryNotes.erase(&tryNotes[controlItem().tryNoteIndex]);
+      if (!endBlock(type)) {
+        return false;
+      }
       break;
+    }
     case LabelKind::Catch:
     case LabelKind::CatchAll:
       if (!endTryCatch(type)) {

js/src/wasm/WasmOpIter.h

@@ -1277,12 +1277,6 @@ inline bool OpIter<Policy>::readEnd(LabelKind* kind, ResultType* type,
 
   Control& block = controlStack_.back();
 
-#ifdef ENABLE_WASM_EXCEPTIONS
-  if (block.kind() == LabelKind::Try) {
-    return fail("try without catch or catch_all not allowed");
-  }
-#endif
-
   if (!checkStackAtEndOfBlock(type, results)) {
     return false;
   }