Bug 1667804 - Enable stencil-mvp pref. r=arai

Flips the `javascript.options.use_off_thread_parse_global` pref to false.

In this mode, off-thread JS parsing will parse to Stencil and hand that back
to main-thread to instantiate. This removes the `mergeRealms` operation and
more importantly removes restrictions involving atoms-zone GC. This addresses
intermittent stalls that can previously occured where certain incremental GCs
would disable all off-thread parsing for a short while.

Additionally, this switches the JSBC data format from a legacy format to one
based around the Stencil data structures. As a performance consideration, we
currently continue to use the off-thread parse-global for decode operations.
If the GC prevents us from doing this, we simply instantiate the Stencil on
main-thread like for the parse case. Further performance tuning will allow us
to revisit this again in future.

Note that the privileged ScriptPreloader that runs during process startup
still uses the legacy format at this time.

We also change the defaults of the js-shell which requires updating various
help messages.

Differential Revision: https://phabricator.services.mozilla.com/D91743

js/src/shell/js.cpp

@@ -2355,7 +2355,7 @@ static bool Evaluate(JSContext* cx, unsigned argc, Value* vp) {
               JS_ReportErrorASCII(
                   cx,
                   "if both loadBytecode and saveIncrementalBytecode are set "
-                  "and --no-off-thread-parse-global is used, bytecode should "
+                  "and --off-thread-parse-global isn't used, bytecode should "
                   "have been saved with saveIncrementalBytecode");
               return false;
             }
@@ -2367,7 +2367,7 @@ static bool Evaluate(JSContext* cx, unsigned argc, Value* vp) {
               JS_ReportErrorASCII(
                   cx,
                   "if both loadBytecode and saveIncrementalBytecode are set "
-                  "and --no-off-thread-parse-global isn't used, bytecode "
+                  "and --off-thread-parse-global is used, bytecode "
                   "should have been saved with saveBytecode");
               return false;
             }
@@ -5931,7 +5931,7 @@ static bool OffThreadDecodeScript(JSContext* cx, unsigned argc, Value* vp) {
   CompileOptions options(cx);
   options.setIntroductionType("js shell offThreadDecodeScript")
       .setFileAndLine("<string>", 1);
-  // NOTE: If --no-off-thread-parse-global is used, input can be either script
+  // NOTE: If --off-thread-parse-global is not used, input can be either script
   // for saveBytecode, or stencil for saveIncrementalBytecode.
   options.useOffThreadParseGlobal =
       CacheEntry_getKind(cx, cacheEntry) == BytecodeCacheKind::Script;
@@ -8697,10 +8697,10 @@ static const JSFunctionSpecWithHelp shell_functions[] = {
 "      saveIncrementalBytecode: if true, and if the source is a\n"
 "         CacheEntryObject, the bytecode would be incrementally encoded and\n"
 "         saved into the cache entry.\n"
-"         If --no-off-thread-parse-global is used, the encoded bytecode's\n"
+"         If --off-thread-parse-global is not used, the encoded bytecode's\n"
 "         kind is 'stencil'. If not, the encoded bytecode's kind is 'script'\n"
 "         If both loadBytecode and saveIncrementalBytecode are set,\n"
-"         and --no-off-thread-parse-global is used, the input bytecode's\n"
+"         and --off-thread-parse-global is not used, the input bytecode's\n"
 "         kind should be 'stencil'."
 "      assertEqBytecode: if true, and if both loadBytecode and either\n"
 "         saveBytecode or saveIncrementalBytecode is true, then the loaded\n"
@@ -10486,7 +10486,7 @@ static bool SetContextOptions(JSContext* cx, const OptionParser& op) {
                              op.getBoolOption("enable-private-methods");
   enablePrivateClassMethods = op.getBoolOption("enable-private-methods");
   enableTopLevelAwait = op.getBoolOption("enable-top-level-await");
-  useOffThreadParseGlobal = !op.getBoolOption("no-off-thread-parse-global");
+  useOffThreadParseGlobal = op.getBoolOption("off-thread-parse-global");
 
   JS::ContextOptionsRef(cx)
       .setAsmJS(enableAsmJS)
@@ -11475,9 +11475,8 @@ int main(int argc, char** argv, char** envp) {
                         "Enable private class methods") ||
       !op.addBoolOption('\0', "enable-top-level-await",
                         "Enable top-level await") ||
-      !op.addBoolOption('\0', "no-off-thread-parse-global",
-                        "Do not use parseGlobal in off-thread compilation and "
-                        "instead instantiate stencil in main-thread") ||
+      !op.addBoolOption('\0', "off-thread-parse-global",
+                        "Use parseGlobal in all off-thread compilation") ||
       !op.addBoolOption('\0', "enable-large-buffers",
                         "Allow creating ArrayBuffers larger than 2 GB on "
                         "64-bit platforms (experimental!)") ||

js/src/vm/OffThreadScriptCompilation.cpp

@@ -211,7 +211,7 @@ JS_PUBLIC_API void JS::CancelMultiOffThreadScriptsDecoder(
 }
 
 namespace js {
-bool gUseOffThreadParseGlobal = true;
+bool gUseOffThreadParseGlobal = false;
 }  // namespace js
 
 JS_PUBLIC_API void JS::SetUseOffThreadParseGlobal(bool value) {

modules/libpref/init/StaticPrefList.yaml

@@ -5313,7 +5313,7 @@
 # CompilationStencil back to main thread before allocating GC objects.
 - name: javascript.options.off_thread_parse_global
   type: bool
-  value: true
+  value: false
   mirror: once
 
 - name: javascript.options.wasm_optimizingjit