Index: mozilla/js/src/gc/Heap.h
===================================================================
--- mozilla.orig/js/src/gc/Heap.h
+++ mozilla/js/src/gc/Heap.h
@@ -110,19 +110,22 @@ struct Cell
  */
 #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.
Index: mozilla/js/src/gc/Memory.cpp
===================================================================
--- mozilla.orig/js/src/gc/Memory.cpp
+++ mozilla/js/src/gc/Memory.cpp
@@ -15,6 +15,15 @@
 namespace js {
 namespace gc {
 
+/* Unused memory decommiting requires the arena size match the page size. */
+extern const size_t PageSize;
+extern const size_t ArenaSize;
+static bool
+DecommitEnabled()
+{
+    return PageSize == ArenaSize;
+}
+
 #if defined(XP_WIN)
 #include "jswin.h"
 #include <psapi.h>
@@ -83,6 +92,9 @@ UnmapPages(void *p, size_t size)
 bool
 MarkPagesUnused(void *p, size_t size)
 {
+    if (!DecommitEnabled())
+        return false;
+
     JS_ASSERT(uintptr_t(p) % PageSize == 0);
     LPVOID p2 = VirtualAlloc(p, size, MEM_RESET, PAGE_READWRITE);
     return p2 == p;
@@ -352,6 +364,9 @@ UnmapPages(void *p, size_t size)
 bool
 MarkPagesUnused(void *p, size_t size)
 {
+    if (!DecommitEnabled())
+        return false;
+
     JS_ASSERT(uintptr_t(p) % PageSize == 0);
     int result = madvise(p, size, MADV_DONTNEED);
     return result != -1;