Bug 1406303 - Only store 2 levels of bit sizes for the radix tree. r=njn

All levels except the first are using the same size, and in some cases,
even the first uses the same size. Only storing those two different
sizes allows to fix the class size, while not making the code
significantly more complex.

--HG--
extra : rebase_source : 8028c18de2fa84060c5baff7c95cd0a70e7a3c6b

memory/build/mozjemalloc.cpp

@@ -658,7 +658,7 @@ class AddressRadixTree {
   malloc_spinlock_t mLock;
   void** mRoot;
   unsigned mHeight;
-  unsigned mLevel2Bits[1]; // Dynamically sized.
+  unsigned mLevel2Bits[2];
 
 public:
   static AddressRadixTree* Create(unsigned aBits);
@@ -1737,13 +1737,12 @@ AddressRadixTree*
 AddressRadixTree::Create(unsigned aBits)
 {
   AddressRadixTree* ret;
-  unsigned bits_per_level, height, i;
+  unsigned bits_per_level, height;
 
   bits_per_level = AddressRadixTree::kNodeSize2Pow - SIZEOF_PTR_2POW;
   height = (aBits + bits_per_level - 1) / bits_per_level;
 
-  ret = (AddressRadixTree*)base_calloc(1, sizeof(AddressRadixTree) +
-      (sizeof(unsigned) * (height - 1)));
+  ret = (AddressRadixTree*)base_calloc(1, sizeof(AddressRadixTree));
   if (!ret) {
     return nullptr;
   }
@@ -1755,9 +1754,7 @@ AddressRadixTree::Create(unsigned aBits)
   } else {
     ret->mLevel2Bits[0] = bits_per_level;
   }
-  for (i = 1; i < height; i++) {
-    ret->mLevel2Bits[i] = bits_per_level;
-  }
+  ret->mLevel2Bits[1] = bits_per_level;
 
   ret->mRoot = (void**)base_calloc(1 << ret->mLevel2Bits[0], sizeof(void*));
   if (!ret->mRoot) {
@@ -1780,11 +1777,11 @@ AddressRadixTree::GetSlot(void* aKey, bool aCreate)
   for (i = lshift = 0, height = mHeight, node = mRoot;
        i < height - 1;
        i++, lshift += bits, node = child) {
-    bits = mLevel2Bits[i];
+    bits = mLevel2Bits[i ? 1 : 0];
     subkey = (key << lshift) >> ((SIZEOF_PTR << 3) - bits);
     child = (void**) node[subkey];
     if (!child && aCreate) {
-      child = (void**) base_calloc(1 << mLevel2Bits[i + 1], sizeof(void*));
+      child = (void**) base_calloc(1 << mLevel2Bits[1], sizeof(void*));
       if (child) {
         node[subkey] = child;
       }
@@ -1798,7 +1795,7 @@ AddressRadixTree::GetSlot(void* aKey, bool aCreate)
    * node is a leaf, so it contains values rather than node
    * pointers.
    */
-  bits = mLevel2Bits[i];
+  bits = mLevel2Bits[i ? 1 : 0];
   subkey = (key << lshift) >> ((SIZEOF_PTR << 3) - bits);
   return &node[subkey];
 }