From d27001da519a7a65b920d3b6da88df2b365d767f Mon Sep 17 00:00:00 2001
From: Gregor Wagner <anygregor@gmail.com>
Date: Thu, 10 Mar 2011 15:27:21 -0800
Subject: [PATCH] Bug 638199 - Shrink ArenaHeader (r=wmccloskey)

---
 js/src/jsgc.cpp | 21 +++++++++++++--------
 js/src/jsgc.h   |  8 +++++---
 2 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index 9c019c91e7c6..b76f7be04b8e 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -170,8 +170,6 @@ Arena<T>::init(JSCompartment *compartment, unsigned thingKind)
     aheader.compartment = compartment;
     aheader.thingKind = thingKind;
     aheader.freeList = &t.things[0].cell;
-    aheader.thingSize = sizeof(T);
-    aheader.isUsed = true;
     JS_ASSERT(sizeof(T) == sizeof(ThingOrCell<T>));
     ThingOrCell<T> *thing = &t.things[0];
     ThingOrCell<T> *last = &t.things[JS_ARRAY_LENGTH(t.things) - 1];
@@ -181,6 +179,8 @@ Arena<T>::init(JSCompartment *compartment, unsigned thingKind)
     }
     last->cell.link = NULL;
 #ifdef DEBUG
+    aheader.thingSize = sizeof(T);
+    aheader.isUsed = true;
     aheader.hasFreeThings = true;
 #endif
 }
@@ -211,16 +211,15 @@ template<typename T>
 inline ConservativeGCTest
 Arena<T>::mark(T *thing, JSTracer *trc)
 {
-    JS_ASSERT(sizeof(T) == aheader.thingSize);
-
     T *alignedThing = getAlignedThing(thing);
 
     if (alignedThing > &t.things[ThingsPerArena-1].t || alignedThing < &t.things[0].t)
         return CGCT_NOTARENA;
 
-    if (!aheader.isUsed || inFreeList(alignedThing))
+    if (!aheader.compartment || inFreeList(alignedThing))
         return CGCT_NOTLIVE;
 
+    JS_ASSERT(sizeof(T) == aheader.thingSize);
     JS_SET_TRACING_NAME(trc, "machine stack");
     Mark(trc, alignedThing);
 
@@ -290,11 +289,17 @@ Chunk::init(JSRuntime *rt)
     Arena<FreeCell> *last = &arenas[JS_ARRAY_LENGTH(arenas) - 1];
     while (arena < last) {
         arena->header()->next = arena + 1;
+        arena->header()->compartment = NULL;
+#ifdef DEBUG
         arena->header()->isUsed = false;
+#endif
         ++arena;
     }
     last->header()->next = NULL;
+    last->header()->compartment = NULL;
+#ifdef DEBUG
     last->header()->isUsed = false;
+#endif
     info.numFree = ArenasPerChunk;
 }
 
@@ -353,7 +358,10 @@ Chunk::releaseArena(Arena<T> *arena)
     rt->gcBytes -= sizeof(Arena<T>);
     comp->gcBytes -= sizeof(Arena<T>);
     info.emptyArenaLists.insert((Arena<Cell> *)arena);
+#ifdef DEBUG
     arena->header()->isUsed = false;
+#endif
+    arena->header()->compartment = NULL;
     ++info.numFree;
     if (unused())
         info.age = 0;
@@ -632,9 +640,6 @@ MarkIfGCThingWord(JSTracer *trc, jsuword w, uint32 &thingKind)
 
     ArenaHeader *aheader = cell->arena()->header();
 
-    if (!aheader->isUsed)
-        return CGCT_FREEARENA;
-
     ConservativeGCTest test;
     thingKind = aheader->thingKind;
 
diff --git a/js/src/jsgc.h b/js/src/jsgc.h
index bb666e617e55..774b36837a9e 100644
--- a/js/src/jsgc.h
+++ b/js/src/jsgc.h
@@ -113,9 +113,9 @@ struct ArenaHeader {
     Arena<FreeCell> *next;
     FreeCell        *freeList;
     unsigned        thingKind;
-    bool            isUsed;
-    size_t          thingSize;
 #ifdef DEBUG
+    size_t          thingSize;
+    bool            isUsed;
     bool            hasFreeThings;
 #endif
 };
@@ -304,7 +304,9 @@ EmptyArenaLists::getNext(JSCompartment *comp, unsigned thingKind) {
     if (arena) {
         JS_ASSERT(arena->header()->isUsed == false);
         JS_ASSERT(arena->header()->thingSize == sizeof(T));
+#ifdef DEBUG
         arena->header()->isUsed = true;
+#endif
         arena->header()->thingKind = thingKind;
         arena->header()->compartment = comp;
         return arena;
@@ -433,7 +435,7 @@ Arena<T>::getAlignedThing(void *thing)
 {
     jsuword start = reinterpret_cast<jsuword>(&t.things[0]);
     jsuword offset = reinterpret_cast<jsuword>(thing) - start;
-    offset -= offset % aheader.thingSize;
+    offset -= offset % sizeof(T);
     return reinterpret_cast<T *>(start + offset);
 }