[wasm] wasm-fuzzer: Add ability to generate test case

Other fuzzers already have this ability. This CL adds it to the fuzzer.
The input has to be valid bytes, otherwise we cannot generate the text
representation.

R=titzer@chromium.org
CC=gdeepti@chromium.org

Change-Id: If1ba8accc707bee3b042e93f4201949f0233c90e
Reviewed-on: https://chromium-review.googlesource.com/1109794
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53926}

test/fuzzer/wasm-fuzzer-common.cc

@@ -144,6 +144,7 @@ struct PrintName {
 std::ostream& operator<<(std::ostream& os, const PrintName& name) {
   return os.write(name.name.start(), name.name.size());
 }
+}  // namespace
 
 void GenerateTestCase(Isolate* isolate, ModuleWireBytes wire_bytes,
                       bool compiles) {
@@ -236,7 +237,6 @@ void GenerateTestCase(Isolate* isolate, ModuleWireBytes wire_bytes,
   }
   os << "})();\n";
 }
-}  // namespace
 
 int WasmExecutionFuzzer::FuzzWasmModule(const uint8_t* data, size_t size,
                                         bool require_valid) {

test/fuzzer/wasm-fuzzer-common.h

@@ -26,6 +26,9 @@ int FuzzWasmSection(SectionCode section, const uint8_t* data, size_t size);
 void InterpretAndExecuteModule(Isolate* isolate,
                                Handle<WasmModuleObject> module_object);
 
+void GenerateTestCase(Isolate* isolate, ModuleWireBytes wire_bytes,
+                      bool compiles);
+
 class WasmExecutionFuzzer {
  public:
   virtual ~WasmExecutionFuzzer() {}

test/fuzzer/wasm.cc

@@ -37,14 +37,20 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   v8::Context::Scope context_scope(support->GetContext());
   v8::TryCatch try_catch(isolate);
   i::wasm::testing::SetupIsolateForWasmModule(i_isolate);
+  i::wasm::ModuleWireBytes wire_bytes(data, data + size);
 
   i::HandleScope scope(i_isolate);
   i::wasm::ErrorThrower thrower(i_isolate, "wasm fuzzer");
-  i::MaybeHandle<i::WasmModuleObject> maybe_object =
-      i_isolate->wasm_engine()->SyncCompile(
-          i_isolate, &thrower, i::wasm::ModuleWireBytes(data, data + size));
   i::Handle<i::WasmModuleObject> module_object;
-  if (maybe_object.ToHandle(&module_object)) {
+  bool compiles = i_isolate->wasm_engine()
+                      ->SyncCompile(i_isolate, &thrower, wire_bytes)
+                      .ToHandle(&module_object);
+
+  if (i::FLAG_wasm_fuzzer_gen_test) {
+    i::wasm::fuzzer::GenerateTestCase(i_isolate, wire_bytes, compiles);
+  }
+
+  if (compiles) {
     i::wasm::fuzzer::InterpretAndExecuteModule(i_isolate, module_object);
   }
   return 0;