[wasm] Fix throwing and catching exceptions

This reimplements functionality that was present before the decoder
refactoring. It's implemented a bit differently though by generating
the code for re-throwing an uncaught exception earlier (when generating
code for the catch).

R=titzer@chromium.org, kschimpf@chromium.org

Bug: v8:6600
Change-Id: Ie2f11837851c0602ab31506fa63475fc2d0b5047
Reviewed-on: https://chromium-review.googlesource.com/641550
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47687}

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

@@ -1243,7 +1243,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
               break;
             }
 
-            if (c->is_try_catch()) {
+            if (CHECK_ERROR(c->is_try_catch())) {
               OPCODE_ERROR(opcode, "multiple catch blocks not implemented");
               break;
             }

src/wasm/function-body-decoder.cc

@@ -176,18 +176,10 @@ class WasmGraphBuildingInterface
     SetEnv(c->interface_data.end_env);
   }
 
-  void PopControl(Decoder* decoder, const Control& block) {
+  void PopControl(Decoder* decoder, Control& block) {
     if (block.is_onearmed_if()) {
       Goto(decoder, block.interface_data.false_env,
            block.interface_data.end_env);
-    } else if (block.is_try_catch()) {
-      SsaEnv* fallthru_ssa_env = ssa_env_;
-      DCHECK_NOT_NULL(block.interface_data.try_info->catch_env);
-      SetEnv(block.interface_data.try_info->catch_env);
-      BUILD(Rethrow);
-      // TODO(clemensh): Make this work again.
-      // FallThruTo(decoder, &block);
-      SetEnv(fallthru_ssa_env);
     }
   }
 
@@ -400,8 +392,10 @@ class WasmGraphBuildingInterface
 
   void Catch(Decoder* decoder, const ExceptionIndexOperand<true>& operand,
              Control* block) {
-    DCHECK_NOT_NULL(block->interface_data.try_info);
+    DCHECK(block->is_try_catch());
     current_catch_ = block->interface_data.previous_catch;
+    SsaEnv* catch_env = block->interface_data.try_info->catch_env;
+    SetEnv(catch_env);
 
     // Get the exception and see if wanted exception.
     TFNode* exception_as_i32 = BUILD(
@@ -412,12 +406,18 @@ class WasmGraphBuildingInterface
     TFNode* if_true = nullptr;
     TFNode* if_false = nullptr;
     BUILD(BranchNoHint, compare_i32, &if_true, &if_false);
-    SsaEnv* end_env = ssa_env_;
-    SsaEnv* false_env = Split(decoder, end_env);
+    SsaEnv* false_env = Split(decoder, catch_env);
     false_env->control = if_false;
-    SsaEnv* true_env = Steal(decoder->zone(), ssa_env_);
+    SsaEnv* true_env = Steal(decoder->zone(), catch_env);
     true_env->control = if_true;
     block->interface_data.try_info->catch_env = false_env;
+
+    // Generate code to re-throw the exception.
+    DCHECK_NOT_NULL(block->interface_data.try_info->catch_env);
+    SetEnv(false_env);
+    BUILD(Rethrow);
+    FallThruTo(decoder, block);
+
     SetEnv(true_env);
     // TODO(kschimpf): Add code to pop caught exception from isolate.
   }

test/mjsunit/wasm/exceptions.js

@@ -34,8 +34,6 @@ assertFalse(test_throw === 0);
 assertEquals("object", typeof test_throw.exports);
 assertEquals("function", typeof test_throw.exports.throw_if_param_not_zero);
 
-/* TODO(kschimpf) Convert these tests to work for the proposed exceptions.
-
 // Test expected behavior of throws
 assertEquals(1, test_throw.exports.throw_if_param_not_zero(0));
 assertWasmThrows([], function() { test_throw.exports.throw_if_param_not_zero(10) });
@@ -74,6 +72,8 @@ assertEquals("function", typeof test_catch.exports.simple_throw_catch_to_0_1);
 assertEquals(0, test_catch.exports.simple_throw_catch_to_0_1(0));
 assertEquals(1, test_catch.exports.simple_throw_catch_to_0_1(1));
 
+/* TODO(kschimpf) Convert these tests to work for the proposed exceptions.
+
 // The following methods do not attempt to catch the exception they raise.
 var test_throw = (function () {
   var builder = new WasmModuleBuilder();