[wasm][eh] Make try block handler optional

A try block may have no handler. Relevant links:
https://github.com/WebAssembly/exception-handling/issues/131
https://github.com/WebAssembly/exception-handling/pull/157

R=clemensb@chromium.org

Bug: v8:8091
Change-Id: Ia429762714dd87ed6b043f95c3496c65aaa8495f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2949100
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75100}

src/wasm/baseline/liftoff-compiler.cc

@@ -1287,9 +1287,8 @@ class LiftoffCompiler {
     }
     DCHECK(!c->is_try_catchall());
     if (c->is_try_catch()) {
-      // Drop the implicit exception ref.
-      DCHECK_EQ(c->label_state.stack_height() + 1,
-                __ cache_state()->stack_height());
+      // Drop the implicit exception ref if any. There may be none if this is a
+      // catch-less try block.
       __ MergeStackWith(c->label_state, c->br_merge()->arity,
                         LiftoffAssembler::kForwardJump);
     } else {

src/wasm/function-body-decoder-impl.h

@@ -2718,6 +2718,11 @@ class WasmFullDecoder : public WasmDecoder<validate> {
   DECODE(End) {
     DCHECK(!control_.empty());
     Control* c = &control_.back();
+    if (c->is_incomplete_try()) {
+      // Catch-less try, fall through to the implicit catch-all.
+      c->kind = kControlTryCatch;
+      current_catch_ = c->previous_catch;  // Pop try scope.
+    }
     if (c->is_try_catch()) {
       // Emulate catch-all + re-throw.
       FallThrough();
@@ -2730,10 +2735,6 @@ class WasmFullDecoder : public WasmDecoder<validate> {
       PopControl();
       return 1;
     }
-    if (!VALIDATE(!c->is_incomplete_try())) {
-      this->DecodeError("missing catch or catch-all in try");
-      return 0;
-    }
     if (c->is_onearmed_if()) {
       if (!VALIDATE(TypeCheckOneArmedIf(c))) return 0;
     }

test/mjsunit/wasm/exceptions.js

@@ -1112,3 +1112,22 @@ d8.file.execute("test/mjsunit/wasm/exceptions-utils.js");
   let instance = builder.instantiate();
   assertEquals(42, instance.exports.throw_with_local());
 })();
+
+(function TestCatchlessTry() {
+  print(arguments.callee.name);
+  let builder = new WasmModuleBuilder();
+  let except = builder.addException(kSig_v_v);
+  builder.addFunction('catchless_try', kSig_v_i)
+    .addBody([
+        kExprTry, kWasmVoid,
+          kExprLocalGet, 0,
+          kExprIf, kWasmVoid,
+            kExprThrow, except,
+          kExprEnd,
+        kExprEnd,
+    ]).exportFunc();
+
+  let instance = builder.instantiate();
+  assertDoesNotThrow(() => instance.exports.catchless_try(0));
+  assertWasmThrows(instance, except, [], () => instance.exports.catchless_try(1));
+})();

test/unittests/wasm/function-body-decoder-unittest.cc

@@ -2956,14 +2956,13 @@ TEST_F(FunctionBodyDecoderTest, TryCatch) {
   ExpectValidates(sigs.v_v(), {WASM_TRY_OP, kExprCatch, ex, kExprEnd});
   ExpectValidates(sigs.v_v(),
                   {WASM_TRY_OP, kExprCatch, ex, kExprCatchAll, kExprEnd});
+  ExpectValidates(sigs.v_v(), {WASM_TRY_OP, kExprEnd}, kAppendEnd);
   ExpectFailure(sigs.v_v(),
                 {WASM_TRY_OP, kExprCatchAll, kExprCatch, ex, kExprEnd},
                 kAppendEnd, "catch after catch-all for try");
   ExpectFailure(sigs.v_v(),
                 {WASM_TRY_OP, kExprCatchAll, kExprCatchAll, kExprEnd},
                 kAppendEnd, "catch-all already present for try");
-  ExpectFailure(sigs.v_v(), {WASM_TRY_OP, kExprEnd}, kAppendEnd,
-                "missing catch or catch-all in try");
   ExpectFailure(sigs.v_v(), {kExprCatch, ex, kExprEnd}, kAppendEnd,
                 "catch does not match a try");
 }