From 8dd493542b20841bd0b1bb42d407fe42499bcba8 Mon Sep 17 00:00:00 2001
From: Mike Hommey <mh+mozilla@glandium.org>
Date: Fri, 6 Oct 2017 15:24:07 +0900
Subject: [PATCH] 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 | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/memory/build/mozjemalloc.cpp b/memory/build/mozjemalloc.cpp
index 66fa86a3c3925..907c87682be37 100644
--- a/memory/build/mozjemalloc.cpp
+++ b/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];
 }