[Liftoff] Ensure that asm.js code is compiled with Turbofan

Liftoff currently does not support all asm.js code, and tier-up does
not work with lazy compilation. Hence, disable Liftoff for asm.js in
general. We will look at this later, when we have better tier-up.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I00d9e27d861067b22c0738ade7070538ee8c919c
Reviewed-on: https://chromium-review.googlesource.com/1047245
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53158}

src/wasm/function-compiler.cc

@@ -53,6 +53,10 @@ WasmCompilationUnit::WasmCompilationUnit(Isolate* isolate, ModuleEnv* env,
       mode_(mode) {
   DCHECK_GE(index, env->module->num_imported_functions);
   DCHECK_LT(index, env->module->functions.size());
+  // Always disable Liftoff for asm.js, for two reasons:
+  //    1) asm-specific opcodes are not implemented, and
+  //    2) tier-up does not work with lazy compilation.
+  if (env->module->is_asm_js()) mode = CompilationMode::kTurbofan;
   SwitchMode(mode);
 }
 

src/wasm/module-compiler.cc

@@ -3398,8 +3398,10 @@ CompilationState::CompilationState(internal::Isolate* isolate, ModuleEnv& env)
     : isolate_(isolate),
       module_env_(env),
       max_memory_(GetMaxUsableMemorySize(isolate) / 2),
-      compile_mode_(FLAG_wasm_tier_up ? CompileMode::kTiering
-                                      : CompileMode::kRegular),
+      // TODO(clemensh): Fix fuzzers such that {env.module} is always non-null.
+      compile_mode_(FLAG_wasm_tier_up && (!env.module || env.module->is_wasm())
+                        ? CompileMode::kTiering
+                        : CompileMode::kRegular),
       wire_bytes_(ModuleWireBytes(nullptr, nullptr)),
       max_background_tasks_(std::max(
           1, std::min(FLAG_wasm_num_compilation_tasks,