[nojit] Add build- and runtime flags

This adds flags for the upcoming JIT-less V8.

Build-time: v8_enable_jitless_mode in gn.args
Runtime: --jitless command-line flag

The build-time flag is expected to be removed as the implementation
matures.

Bug: v8:7777
Change-Id: Ieea404a14c7859a66e18175d1740d73182db9b1b
Reviewed-on: https://chromium-review.googlesource.com/c/1335559
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57533}

BUILD.gn

@@ -87,6 +87,11 @@ declare_args() {
   v8_enable_embedded_builtins =
       v8_use_snapshot && !is_aix && (!is_win || is_clang)
 
+  # Build-time flag for enabling nojit mode.
+  # TODO(v8:7777): Remove the build-time flag once the --jitless runtime flag
+  # does everything we need.
+  v8_enable_jitless_mode = false
+
   # Enable code-generation-time checking of types in the CodeStubAssembler.
   v8_enable_verify_csa = false
 
@@ -217,10 +222,13 @@ if (v8_check_microtasks_scopes_consistency == "") {
 
 assert(!v8_enable_embedded_builtins || v8_use_snapshot,
        "Embedded builtins only work with snapshots")
-assert(
-    v8_current_cpu != "x86" || !v8_enable_embedded_builtins ||
-        !v8_untrusted_code_mitigations,
-    "Embedded builtins on ia32 and untrusted code mitigations are incompatible")
+assert(v8_current_cpu != "x86" || !v8_untrusted_code_mitigations,
+       "Untrusted code mitigations are unsupported on ia32")
+
+assert(!v8_enable_jitless_mode || v8_enable_embedded_builtins,
+       "JIT-less mode requires embedded builtins")
+assert(!v8_enable_jitless_mode || v8_use_snapshot,
+       "JIT-less mode requires a snapshot build")
 
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"
@@ -405,6 +413,9 @@ config("features") {
       "V8_EMBEDDED_BYTECODE_HANDLERS",
     ]
   }
+  if (v8_enable_jitless_mode) {
+    defines += [ "V8_JITLESS_MODE" ]
+  }
   if (v8_use_multi_snapshots) {
     defines += [ "V8_MULTI_SNAPSHOTS" ]
   }
@@ -1172,6 +1183,7 @@ action("v8_dump_build_config") {
     "v8_target_cpu=\"$v8_target_cpu\"",
     "v8_use_snapshot=$v8_use_snapshot",
     "v8_enable_embedded_builtins=$v8_enable_embedded_builtins",
+    "v8_enable_jitless_mode=$v8_enable_jitless_mode",
     "v8_enable_verify_csa=$v8_enable_verify_csa",
     "v8_enable_lite_mode=$v8_enable_lite_mode",
   ]

src/flag-definitions.h

@@ -1076,6 +1076,14 @@ DEFINE_BOOL(serialization_statistics, false,
 DEFINE_UINT(serialization_chunk_size, 4096,
             "Custom size for serialization chunks")
 
+// JIT-less V8. Design doc: goo.gl/kRnhVe
+#ifdef V8_JITLESS_MODE
+DEFINE_BOOL(jitless, false, "Disable runtime allocation of executable memory.")
+#else
+DEFINE_BOOL_READONLY(jitless, false,
+                     "Disable runtime allocation of executable memory.")
+#endif
+
 // Regexp
 DEFINE_BOOL(regexp_optimization, true, "generate optimized regexp code")
 DEFINE_BOOL(regexp_mode_modifiers, false, "enable inline flags in regexp.")