[wasm-gc] Implement br_on_* type relaxation

Additional arguments are allowed along with the reference, but the
targeted branch must have at least one output (corresponding to the
cast reference).

Bug: v8:7748
Change-Id: I17383165e4bae1cada1676c6282437e1fa71905d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2685161Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72649}

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

@@ -4198,23 +4198,26 @@ class WasmFullDecoder : public WasmDecoder<validate> {
           return 0;
         }
         Control* c = control_at(branch_depth.depth);
-        ValueType type_on_branch =
-            rtt.type.is_bottom()
-                ? kWasmBottom
-                : ValueType::Ref(rtt.type.ref_index(), kNonNullable);
-
-        TypeCheckBranchResult check_result = TypeCheckBrOn(c, type_on_branch);
+        if (c->br_merge()->arity == 0) {
+          this->DecodeError(
+              "br_on_cast must target a branch of arity at least 1");
+          return 0;
+        }
+        // We temporarily push this value to the stack for TypeCheckBranchResult
+        // and for MergeValuesInto in the interface.
+        Value* result_on_branch =
+            Push(rtt.type.is_bottom()
+                     ? kWasmBottom
+                     : ValueType::Ref(rtt.type.ref_index(), kNonNullable));
+        TypeCheckBranchResult check_result = TypeCheckBranch(c, true);
         if (V8_LIKELY(check_result == kReachableBranch)) {
-          // We temporarily push this value for the interface to merge it into
-          // the branch merge.
-          Value* result_on_branch = Push(type_on_branch);
           CALL_INTERFACE(BrOnCast, obj, rtt, result_on_branch,
                          branch_depth.depth);
           c->br_merge()->reached = true;
-          Pop(0);  // Drop {result_on_branch}, restore original value.
         } else if (check_result == kInvalidStack) {
           return 0;
         }
+        Pop(0);  // Drop {result_on_branch}, restore original value.
         Value* result_on_fallthrough = Push(obj.type);
         *result_on_fallthrough = obj;
         return opcode_length + branch_depth.length;
@@ -4264,14 +4267,16 @@ class WasmFullDecoder : public WasmDecoder<validate> {
             opcode == kExprBrOnFunc
                 ? HeapType::kFunc
                 : opcode == kExprBrOnData ? HeapType::kData : HeapType::kI31;
-
-        ValueType type_on_branch = ValueType::Ref(heap_type, kNonNullable);
-
-        TypeCheckBranchResult check_result = TypeCheckBrOn(c, type_on_branch);
+        if (c->br_merge()->arity == 0) {
+          this->DecodeError("%s must target a branch of arity at least 1",
+                            SafeOpcodeNameAt(this->pc_));
+          return 0;
+        }
+        // We temporarily push this value to the stack for TypeCheckBranchResult
+        // and for MergeValuesInto in the interface.
+        Value* result_on_branch = Push(ValueType::Ref(heap_type, kNonNullable));
+        TypeCheckBranchResult check_result = TypeCheckBranch(c, true);
         if (V8_LIKELY(check_result == kReachableBranch)) {
-          // We temporarily push this value for the interface to merge it into
-          // the branch merge.
-          Value* result_on_branch = Push(type_on_branch);
           if (opcode == kExprBrOnFunc) {
             CALL_INTERFACE(BrOnFunc, obj, result_on_branch, branch_depth.depth);
           } else if (opcode == kExprBrOnData) {
@@ -4280,10 +4285,10 @@ class WasmFullDecoder : public WasmDecoder<validate> {
             CALL_INTERFACE(BrOnI31, obj, result_on_branch, branch_depth.depth);
           }
           c->br_merge()->reached = true;
-          Pop(0);  // Drop {result_on_branch}, restore original value.
         } else if (check_result == kInvalidStack) {
           return 0;
         }
+        Pop(0);  // Drop {result_on_branch}, restore original value.
         Value* result_on_fallthrough = Push(obj.type);
         *result_on_fallthrough = obj;
         return opcode_length + branch_depth.length;
@@ -4699,25 +4704,6 @@ class WasmFullDecoder : public WasmDecoder<validate> {
                : kInvalidStack;
   }
 
-  TypeCheckBranchResult TypeCheckBrOn(Control* c, ValueType type_on_branch) {
-    if (V8_LIKELY(control_.back().reachable())) {
-      // We only do type-checking here. This is only needed during validation.
-      if (!VALIDATE(c->br_merge()->arity == 1 &&
-                    (type_on_branch == kWasmBottom ||
-                     IsSubtypeOf(type_on_branch, (*c->br_merge())[0].type,
-                                 this->module_)))) {
-        this->DecodeError("%s must target a branch of a supertype of type %s",
-                          SafeOpcodeNameAt(this->pc_),
-                          type_on_branch.name().c_str());
-        return kInvalidStack;
-      }
-      return kReachableBranch;
-    }
-
-    return TypeCheckUnreachableMerge(*c->br_merge(), false) ? kUnreachableBranch
-                                                            : kInvalidStack;
-  }
-
   bool TypeCheckReturn() {
     int num_returns = static_cast<int>(this->sig_->return_count());
     // No type checking is needed if there are no returns.