[Build] Add support for V8 Lite mode.

Adds a build-time flag to control enabling of V8 Lite mode. Currently
this mode enables optimize-for-size and makes that flag read-only so that
it can't be changed at runtime.

This mode also replaces the --minimal flag which was previously used
to make porting easier.

BUG=v8:8293

Change-Id: I8360b4d55dd15a2a7c18429c94329dc5264dea86
Reviewed-on: https://chromium-review.googlesource.com/c/1276467
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56653}

BUILD.gn

@@ -32,6 +32,11 @@ declare_args() {
   # Sets -DV8_ENABLE_FUTURE.
   v8_enable_future = false
 
+  # Lite mode disables a number of performance optimizations to reduce memory
+  # at the cost of performance.
+  # Sets --DV8_LITE_MODE.
+  v8_enable_lite_mode = false
+
   # Sets -DVERIFY_HEAP.
   v8_enable_verify_heap = ""
 
@@ -314,6 +319,9 @@ config("features") {
   if (v8_enable_future) {
     defines += [ "V8_ENABLE_FUTURE" ]
   }
+  if (v8_enable_lite_mode) {
+    defines += [ "V8_LITE_MODE" ]
+  }
   if (v8_enable_gdbjit) {
     defines += [ "ENABLE_GDB_JIT_INTERFACE" ]
   }

src/code-stubs.cc

@@ -434,7 +434,6 @@ TF_STUB(StoreFastElementStub, CodeStubAssembler) {
 
 // static
 void StoreFastElementStub::GenerateAheadOfTime(Isolate* isolate) {
-  if (FLAG_minimal) return;
   StoreFastElementStub(isolate, false, HOLEY_ELEMENTS, STANDARD_STORE)
       .GetCode();
   StoreFastElementStub(isolate, false, HOLEY_ELEMENTS,

src/flag-definitions.h

@@ -321,17 +321,11 @@ DEFINE_BOOL(feedback_normalization, false,
 DEFINE_BOOL_READONLY(internalize_on_the_fly, true,
                      "internalize string keys for generic keyed ICs on the fly")
 
-// Flags for optimization types.
-DEFINE_BOOL(optimize_for_size, false,
-            "Enables optimizations which favor memory size over execution "
-            "speed")
-
 // Flag for one shot optimiztions.
 DEFINE_BOOL(enable_one_shot_optimization, true,
             "Enable size optimizations for the code that will "
             "only be executed once")
 
-DEFINE_VALUE_IMPLICATION(optimize_for_size, max_semi_space_size, 1)
 
 // Flags for data representation optimizations
 DEFINE_BOOL(unbox_double_arrays, true, "automatically unbox arrays of doubles")
@@ -537,13 +531,6 @@ DEFINE_BOOL(untrusted_code_mitigations, V8_DEFAULT_UNTRUSTED_CODE_MITIGATIONS,
             "Enable mitigations for executing untrusted code")
 #undef V8_DEFAULT_UNTRUSTED_CODE_MITIGATIONS
 
-// Flags to help platform porters
-DEFINE_BOOL(minimal, false,
-            "simplifies execution model to make porting "
-            "easier (e.g. always use Ignition, never optimize)")
-DEFINE_NEG_IMPLICATION(minimal, opt)
-DEFINE_NEG_IMPLICATION(minimal, use_ic)
-
 // Flags for native WebAssembly.
 DEFINE_BOOL(expose_wasm, true, "expose wasm interface to JavaScript")
 DEFINE_BOOL(assume_asmjs_origin, false,
@@ -1162,6 +1149,24 @@ DEFINE_SIZE_T(mock_arraybuffer_allocator_limit, 0,
               "Memory limit for mock ArrayBuffer allocator used to simulate "
               "OOM for testing.")
 
+//
+// Flags only available in non-Lite modes.
+//
+#undef FLAG
+#ifdef V8_LITE_MODE
+#define FLAG FLAG_READONLY
+#define V8_LITE_BOOL true
+#else
+#define FLAG FLAG_FULL
+#define V8_LITE_BOOL false
+#endif
+
+// Favor memory over execution speed.
+DEFINE_BOOL(optimize_for_size, V8_LITE_BOOL,
+            "Enables optimizations which favor memory size over execution "
+            "speed")
+DEFINE_VALUE_IMPLICATION(optimize_for_size, max_semi_space_size, 1)
+
 //
 // GDB JIT integration flags.
 //

test/cctest/heap/test-page-promotion.cc

@@ -15,6 +15,9 @@ namespace v8 {
 namespace internal {
 namespace heap {
 
+// Tests don't work when --optimize-for-size is set.
+#ifndef V8_LITE_MODE
+
 namespace {
 
 v8::Isolate* NewIsolateForPagePromotion(int min_semi_space_size = 8,
@@ -241,6 +244,8 @@ UNINITIALIZED_HEAP_TEST(Regress658718) {
   isolate->Dispose();
 }
 
+#endif  // V8_LITE_MODE
+
 }  // namespace heap
 }  // namespace internal
 }  // namespace v8