Bug 961288 - Add an object metadata callback for tracking object allocation sites. r=ejpbruel

js/src/builtin/TestingFunctions.cpp

@@ -870,6 +870,20 @@ SaveStack(JSContext *cx, unsigned argc, jsval *vp)
     return true;
 }
 
+static bool
+EnableTrackAllocations(JSContext *cx, unsigned argc, jsval *vp)
+{
+    SetObjectMetadataCallback(cx, SavedStacksMetadataCallback);
+    return true;
+}
+
+static bool
+DisableTrackAllocations(JSContext *cx, unsigned argc, jsval *vp)
+{
+    SetObjectMetadataCallback(cx, nullptr);
+    return true;
+}
+
 #if defined(DEBUG) || defined(JS_OOM_BREAKPOINT)
 static bool
 OOMAfterAllocations(JSContext *cx, unsigned argc, jsval *vp)
@@ -1631,6 +1645,16 @@ static const JSFunctionSpecWithHelp TestingFunctions[] = {
 "saveStack()",
 "  Capture a stack.\n"),
 
+    JS_FN_HELP("enableTrackAllocations", EnableTrackAllocations, 0, 0,
+"enableTrackAllocations()",
+"  Start capturing the JS stack at every allocation. Note that this sets an "
+"  object metadata callback that will override any other object metadata "
+"  callback that may be set."),
+
+    JS_FN_HELP("disableTrackAllocations", DisableTrackAllocations, 0, 0,
+"disableTrackAllocations()",
+"  Stop capturing the JS stack at every allocation."),
+
 #if defined(DEBUG) || defined(JS_OOM_BREAKPOINT)
     JS_FN_HELP("oomAfterAllocations", OOMAfterAllocations, 1, 0,
 "oomAfterAllocations(count)",

js/src/jit-test/tests/basic/track-allocation-sites.js

@@ -0,0 +1,24 @@
+// Test that we can track allocation sites.
+
+enableTrackAllocations();
+
+const tests = [
+  { name: "object literal",  object: {},                    line: Error().lineNumber },
+  { name: "array literal",   object: [],                    line: Error().lineNumber },
+  { name: "regexp literal",  object: /(two|2)\s*problems/,  line: Error().lineNumber },
+  { name: "new constructor", object: new function Ctor(){}, line: Error().lineNumber },
+  { name: "new Object",      object: new Object(),          line: Error().lineNumber },
+  { name: "new Array",       object: new Array(),           line: Error().lineNumber },
+  { name: "new Date",        object: new Date(),            line: Error().lineNumber }
+];
+
+disableTrackAllocations();
+
+for (let { name, object, line } of tests) {
+  print("Entering test: " + name);
+
+  let allocationSite = getObjectMetadata(object);
+  print(allocationSite);
+
+  assertEq(allocationSite.line, line);
+}

js/src/vm/SavedStacks.cpp

@@ -509,4 +509,14 @@ SavedStacks::createFrameFromLookup(JSContext *cx, SavedFrame::Lookup &lookup)
     return &f;
 }
 
+bool
+SavedStacksMetadataCallback(JSContext *cx, JSObject **pmetadata)
+{
+    Rooted<SavedFrame *> frame(cx);
+    if (!cx->compartment()->savedStacks().saveCurrentStack(cx, &frame))
+        return false;
+    *pmetadata = frame;
+    return true;
+}
+
 } /* namespace js */

js/src/vm/SavedStacks.h

@@ -139,6 +139,8 @@ class SavedStacks {
     SavedFrame *createFrameFromLookup(JSContext *cx, SavedFrame::Lookup &lookup);
 };
 
+bool SavedStacksMetadataCallback(JSContext *cx, JSObject **pmetadata);
+
 } /* namespace js */
 
 #endif /* vm_SavedStacks_h */