From ab57c86e3aa232e41d0bbf3ff1856960ae3ec174 Mon Sep 17 00:00:00 2001
From: Matthew Parkinson <mattpark@microsoft.com>
Date: Wed, 16 Jan 2019 17:03:15 +0000
Subject: [PATCH] Addressing CR feedback

---
 src/mem/alloc.h   | 13 ++++++++++---
 src/mem/pagemap.h | 19 ++++++++++++-------
 2 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/src/mem/alloc.h b/src/mem/alloc.h
index 096ad47..ab009e7 100644
--- a/src/mem/alloc.h
+++ b/src/mem/alloc.h
@@ -38,10 +38,17 @@ namespace snmalloc
     PMMediumslab = 2
   };
 
+#ifndef SNMALLOC_MAX_FLATPAGEMAP_SIZE
+// Use flat map is under a single node.
+#  define SNMALLOC_MAX_FLATPAGEMAP_SIZE PAGEMAP_NODE_SIZE
+#endif
+  static constexpr bool USE_FLATPAGEMAP = SNMALLOC_MAX_FLATPAGEMAP_SIZE >=
+    sizeof(FlatPagemap<SUPERSLAB_BITS, uint8_t>);
+
   using SuperslabPagemap = std::conditional_t<
-    bits::is64(),
-    Pagemap<SUPERSLAB_BITS, uint8_t, 0>,
-    FlatPagemap<SUPERSLAB_BITS, uint8_t>>;
+    USE_FLATPAGEMAP,
+    FlatPagemap<SUPERSLAB_BITS, uint8_t>,
+    Pagemap<SUPERSLAB_BITS, uint8_t, 0>>;
 
   HEADER_GLOBAL SuperslabPagemap global_pagemap;
   /**
diff --git a/src/mem/pagemap.h b/src/mem/pagemap.h
index e0e73b1..b902199 100644
--- a/src/mem/pagemap.h
+++ b/src/mem/pagemap.h
@@ -7,13 +7,13 @@
 
 namespace snmalloc
 {
+  static constexpr size_t PAGEMAP_NODE_BITS = 16;
+  static constexpr size_t PAGEMAP_NODE_SIZE = 1ULL << PAGEMAP_NODE_BITS;
+
   template<size_t GRANULARITY_BITS, typename T, T default_content>
   class Pagemap
   {
   private:
-    static constexpr size_t PAGEMAP_BITS = 16;
-    static constexpr size_t PAGEMAP_SIZE = 1 << PAGEMAP_BITS;
-
     static constexpr size_t COVERED_BITS =
       bits::ADDRESS_BITS - GRANULARITY_BITS;
     static constexpr size_t POINTER_BITS =
@@ -22,14 +22,15 @@ namespace snmalloc
       bits::next_pow2_bits_const(sizeof(T));
 
     static_assert(
-      PAGEMAP_BITS - CONTENT_BITS < COVERED_BITS,
+      PAGEMAP_NODE_BITS - CONTENT_BITS < COVERED_BITS,
       "Should use the FlatPageMap as it does not require a tree");
 
-    static constexpr size_t BITS_FOR_LEAF = PAGEMAP_BITS - CONTENT_BITS;
+    static constexpr size_t BITS_FOR_LEAF = PAGEMAP_NODE_BITS - CONTENT_BITS;
     static constexpr size_t ENTRIES_PER_LEAF = 1 << BITS_FOR_LEAF;
     static constexpr size_t LEAF_MASK = ENTRIES_PER_LEAF - 1;
 
-    static constexpr size_t BITS_PER_INDEX_LEVEL = PAGEMAP_BITS - POINTER_BITS;
+    static constexpr size_t BITS_PER_INDEX_LEVEL =
+      PAGEMAP_NODE_BITS - POINTER_BITS;
     static constexpr size_t ENTRIES_PER_INDEX_LEVEL = 1 << BITS_PER_INDEX_LEVEL;
     static constexpr size_t ENTRIES_MASK = ENTRIES_PER_INDEX_LEVEL - 1;
 
@@ -85,7 +86,7 @@ namespace snmalloc
                 value, (PagemapEntry*)LOCKED_ENTRY, std::memory_order_relaxed))
           {
             auto& v = default_memory_provider;
-            value = (PagemapEntry*)v.alloc_chunk(PAGEMAP_SIZE);
+            value = (PagemapEntry*)v.alloc_chunk(PAGEMAP_NODE_SIZE);
             e->store(value, std::memory_order_release);
           }
           else
@@ -229,6 +230,10 @@ namespace snmalloc
     }
   };
 
+  /**
+   * Simple pagemap that for each GRANULARITY_BITS of the address range
+   * stores a T.
+   **/
   template<size_t GRANULARITY_BITS, typename T>
   class FlatPagemap
   {