From 0128c5a9eeee0d3fc0deb9129dd20eb79338c8f4 Mon Sep 17 00:00:00 2001
From: Koen Kooi <koen.kooi@linaro.org>
Date: Mon, 2 Mar 2015 19:08:59 +0800
Subject: [PATCH 4/5] mozbug746112-no-decommit-on-large-pages

---
Upstream-status: Pending

 js/src/gc/Heap.h | 15 ++++++++++-----
 js/src/jsgc.cpp  | 15 ++++++++++++---
 2 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/js/src/gc/Heap.h b/js/src/gc/Heap.h
index b8f8c78..1cfd269 100644
--- a/js/src/gc/Heap.h
+++ b/js/src/gc/Heap.h
@@ -103,26 +103,31 @@ struct Cell
 };
 
 /*
- * Page size is 4096 by default, except for SPARC, where it is 8192.
+ * Page size must be static to support our arena pointer optimizations, so we
+ * are forced to support each platform with non-4096 pages as a special case.
+ * Note: The freelist supports a maximum arena shift of 15.
  * Note: Do not use JS_CPU_SPARC here, this header is used outside JS.
  * Bug 692267: Move page size definition to gc/Memory.h and include it
  *             directly once jsgc.h is no longer an installed header.
  */
 #if defined(SOLARIS) && (defined(__sparc) || defined(__sparcv9))
 const size_t PageShift = 13;
+const size_t ArenaShift = PageShift;
+#elif defined(__powerpc__)
+const size_t PageShift = 16;
+const size_t ArenaShift = 12;
 #else
 const size_t PageShift = 12;
+const size_t ArenaShift = PageShift;
 #endif
 const size_t PageSize = size_t(1) << PageShift;
+const size_t ArenaSize = size_t(1) << ArenaShift;
+const size_t ArenaMask = ArenaSize - 1;
 
 const size_t ChunkShift = 20;
 const size_t ChunkSize = size_t(1) << ChunkShift;
 const size_t ChunkMask = ChunkSize - 1;
 
-const size_t ArenaShift = PageShift;
-const size_t ArenaSize = PageSize;
-const size_t ArenaMask = ArenaSize - 1;
-
 /*
  * This is the maximum number of arenas we allow in the FreeCommitted state
  * before we trigger a GC_SHRINK to release free arenas to the OS.
diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index b3caf05..a258d2d 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -251,6 +251,13 @@ static const int BackgroundPhaseLength[] = {
     sizeof(BackgroundPhaseStrings) / sizeof(AllocKind)
 };
 
+/* Unused memory decommiting requires the arena size match the page size. */
+static bool
+DecommitEnabled()
+{
+    return PageSize == ArenaSize;
+}
+
 #ifdef DEBUG
 void
 ArenaHeader::checkSynchronizedWithFreeList() const
@@ -742,7 +749,8 @@ Chunk::fetchNextDecommittedArena()
     decommittedArenas.unset(offset);
 
     Arena *arena = &arenas[offset];
-    MarkPagesInUse(arena, ArenaSize);
+    if (DecommitEnabled())
+        MarkPagesInUse(arena, ArenaSize);
     arena->aheader.setAsNotAllocated();
 
     return &arena->aheader;
@@ -2731,7 +2739,7 @@ DecommitArenasFromAvailableList(JSRuntime *rt, Chunk **availableListHeadp)
                 chunk->removeFromAvailableList();
 
             size_t arenaIndex = Chunk::arenaIndex(aheader->arenaAddress());
-            bool ok;
+            bool ok = true;
             {
                 /*
                  * If the main thread waits for the decommit to finish, skip
@@ -2741,7 +2749,8 @@ DecommitArenasFromAvailableList(JSRuntime *rt, Chunk **availableListHeadp)
                 Maybe<AutoUnlockGC> maybeUnlock;
                 if (!rt->isHeapBusy())
                     maybeUnlock.construct(rt);
-                ok = MarkPagesUnused(aheader->getArena(), ArenaSize);
+                if (DecommitEnabled())
+                    ok = MarkPagesUnused(aheader->getArena(), ArenaSize);
             }
 
             if (ok) {
-- 
1.9.3