Bug 990353 - Add a pref for system source discarding. r=bent

b2g/app/b2g.js

@@ -601,6 +601,13 @@ pref("dom.forms.color", false);
 // Turns on gralloc-based direct texturing for Gonk
 pref("gfx.gralloc.enabled", false);
 
+// This preference instructs the JS engine to discard the
+// source of any privileged JS after compilation. This saves
+// memory, but makes things like Function.prototype.toSource()
+// fail.
+// XXXbholley - we flip this to true in the last patch.
+pref("javascript.options.discardSystemSource", false);
+
 // XXXX REMOVE FOR PRODUCTION. Turns on GC and CC logging
 pref("javascript.options.mem.log", false);
 

js/xpconnect/src/XPCJSRuntime.cpp

@@ -40,6 +40,7 @@
 #include "mozilla/dom/BindingUtils.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/WindowBinding.h"
+#include "mozilla/Atomics.h"
 #include "mozilla/Attributes.h"
 #include "AccessCheck.h"
 #include "nsGlobalWindow.h"
@@ -84,11 +85,10 @@ const char* const XPCJSRuntime::mStrings[] = {
 
 /***************************************************************************/
 
-struct CX_AND_XPCRT_Data
-{
-    JSContext* cx;
-    XPCJSRuntime* rt;
-};
+static mozilla::Atomic<bool> sDiscardSystemSource(false);
+
+bool
+xpc::ShouldDiscardSystemSource() { return sDiscardSystemSource; }
 
 static void * const UNMARK_ONLY = nullptr;
 static void * const UNMARK_AND_SWEEP = (void *)1;
@@ -1560,6 +1560,8 @@ ReloadPrefsCallback(const char *pref, void *data)
                                                  "baselinejit.unsafe_eager_compilation");
     bool useIonEager = Preferences::GetBool(JS_OPTIONS_DOT_STR "ion.unsafe_eager_compilation");
 
+    sDiscardSystemSource = Preferences::GetBool(JS_OPTIONS_DOT_STR "discardSystemSource");
+
     JS::RuntimeOptionsRef(rt).setBaseline(useBaseline)
                              .setIon(useIon)
                            .  setAsmJS(useAsmJS);

js/xpconnect/src/xpcpublic.h

@@ -478,6 +478,11 @@ RecordAdoptedNode(JSCompartment *c);
 void
 RecordDonatedNode(JSCompartment *c);
 
+// This function may be used off-main-thread, in which case it is benignly
+// racey.
+bool
+ShouldDiscardSystemSource();
+
 } // namespace xpc
 
 namespace mozilla {

modules/libpref/src/init/all.js

@@ -785,6 +785,11 @@ pref("javascript.options.ion",              true);
 pref("javascript.options.asmjs",            true);
 pref("javascript.options.parallel_parsing", true);
 pref("javascript.options.ion.parallel_compilation", true);
+// This preference instructs the JS engine to discard the
+// source of any privileged JS after compilation. This saves
+// memory, but makes things like Function.prototype.toSource()
+// fail.
+pref("javascript.options.discardSystemSource", false);
 // This preference limits the memory usage of javascript.
 // If you want to change these values for your device,
 // please find Bug 417052 comment 17 and Bug 456721