Bug 834108 - avoid O(n^2) chunk-searching in LifoAlloc::ensureUnusedApproximate (r=dvander)

--HG--
extra : rebase_source : d3cb159978d10e6f74984d8556784081b2c940b9

js/src/ds/LifoAlloc.cpp

@@ -140,12 +140,3 @@ LifoAlloc::transferUnusedFrom(LifoAlloc *other)
         other->last = other->latest;
     }
 }
-
-bool
-LifoAlloc::ensureUnusedApproximateSlow(size_t n)
-{
-    // This relies on the behavior that releasing a chunk does not immediately free it.
-    LifoAllocScope scope(this);
-    return !!getOrCreateChunk(n);
-}
-

js/src/ds/LifoAlloc.h

@@ -182,8 +182,6 @@ class LifoAlloc
         last = end;
     }
 
-    bool ensureUnusedApproximateSlow(size_t n);
-
   public:
     explicit LifoAlloc(size_t defaultChunkSize) { reset(defaultChunkSize); }
 
@@ -246,7 +244,12 @@ class LifoAlloc
                 return true;
             chunk = chunk->next();
         }
-        return ensureUnusedApproximateSlow(n);
+        BumpChunk *latestBefore = latest;
+        if (!getOrCreateChunk(n))
+            return false;
+        if (latestBefore)
+            latest = latestBefore;
+        return true;
     }
 
     template <typename T>