Bug 1248726 - Simplify PCLocationMap even further; r=fitzgen

--HG--
extra : rebase_source : 67a0d2a65524b4caae980f86efab0302582e68dc

js/src/vm/SavedStacks.cpp

@@ -999,10 +999,8 @@ SavedFrame::toStringMethod(JSContext* cx, unsigned argc, Value* vp)
 bool
 SavedStacks::init()
 {
-    if (!pcLocationMap.init())
-        return false;
-
-    return frames.init();
+    return frames.init() &&
+           pcLocationMap.init();
 }
 
 bool
@@ -1049,13 +1047,7 @@ SavedStacks::sweep()
 void
 SavedStacks::trace(JSTracer* trc)
 {
-    if (pcLocationMap.initialized()) {
-        // Mark each of the source strings in our pc to location cache.
-        for (PCLocationMap::Enum e(pcLocationMap); !e.empty(); e.popFront()) {
-            LocationValue& loc = e.front().value();
-            TraceEdge(trc, &loc.source, "SavedStacks::PCLocationMap's memoized script source name");
-        }
-    }
+    pcLocationMap.trace(trc);
 }
 
 uint32_t

js/src/vm/SavedStacks.h

@@ -223,6 +223,7 @@ class SavedStacks {
         RelocatablePtrScript script;
         jsbytecode* pc;
 
+        void trace(JSTracer* trc) { /* PCKey is weak. */ }
         bool needsSweep() { return IsAboutToBeFinalized(&script); }
     };
 
@@ -239,8 +240,12 @@ class SavedStacks {
         }
 
         bool needsSweep() {
+            // LocationValue is always held strongly, but in a weak map.
+            // Assert that it has been marked already, but allow it to be
+            // ejected from the map when the key dies.
             MOZ_ASSERT(source);
-            return IsAboutToBeFinalized(&source);
+            MOZ_ASSERT(!IsAboutToBeFinalized(&source));
+            return true;
         }
 
         RelocatablePtrAtom source;
@@ -282,6 +287,13 @@ class SavedStacks {
         }
     };
 
+    // We eagerly Atomize the script source stored in LocationValue because
+    // asm.js does not always have a JSScript and the source might not be
+    // available when we need it later. However, since the JSScript does not
+    // actually hold this atom, we have to trace it strongly to keep it alive.
+    // Thus, it takes two GC passes to fully clean up this table: the first GC
+    // removes the dead script; the second will clear out the source atom since
+    // it is no longer held by the table.
     using PCLocationMap = GCHashMap<PCKey, LocationValue, PCLocationHasher, SystemAllocPolicy>;
     PCLocationMap pcLocationMap;