Enable accessing parent ranges. (#529)

This exposes a feature on Ranges to access ranges higher up the
stack of ranges.  This could be useful for applying operations in the
middle of a pipeline like

   object_range.ancestor<SpecialRange>().init(...);

This allows some initialisation to be added to the middle of pipeline
without breaking the current coding pattern.

It also allows for bypassing some ranges

   object_range.ancestor<LargeObjectsRange>().alloc_chunk(...);

Neither are done in this commit, but both will occur in future commits.

Co-authored-by: Nathaniel Wesley Filardo <nfilardo@microsoft.com>

src/snmalloc/backend_helpers/commitrange.h

@@ -1,13 +1,14 @@
 #pragma once
 #include "../pal/pal.h"
 #include "empty_range.h"
+#include "range_helpers.h"
 
 namespace snmalloc
 {
   template<typename PAL, typename ParentRange = EmptyRange>
-  class CommitRange
+  class CommitRange : public ContainsParent<ParentRange>
   {
-    ParentRange parent{};
+    using ContainsParent<ParentRange>::parent;
 
   public:
     /**

src/snmalloc/backend_helpers/globalrange.h

@@ -10,9 +10,9 @@ namespace snmalloc
    * and protects access with a lock.
    */
   template<typename ParentRange = EmptyRange>
-  class GlobalRange
+  class GlobalRange : public StaticParent<ParentRange>
   {
-    SNMALLOC_REQUIRE_CONSTINIT static inline ParentRange parent{};
+    using StaticParent<ParentRange>::parent;
 
     /**
      * This is infrequently used code, a spin lock simplifies the code

src/snmalloc/backend_helpers/largebuddyrange.h

@@ -189,9 +189,9 @@ namespace snmalloc
     SNMALLOC_CONCEPT(ConceptBuddyRangeMeta) Pagemap,
     size_t MIN_REFILL_SIZE_BITS = 0,
     typename ParentRange = EmptyRange>
-  class LargeBuddyRange
+  class LargeBuddyRange : public ContainsParent<ParentRange>
   {
-    ParentRange parent{};
+    using ContainsParent<ParentRange>::parent;
 
     /**
      * Maximum size of a refill

src/snmalloc/backend_helpers/logrange.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "empty_range.h"
+#include "range_helpers.h"
 
 namespace snmalloc
 {
@@ -11,9 +12,9 @@ namespace snmalloc
    * ParentRange is what the range is logging calls to.
    */
   template<size_t RangeName, typename ParentRange = EmptyRange>
-  class LogRange
+  class LogRange : public ContainsParent<ParentRange>
   {
-    ParentRange parent{};
+    using ContainsParent<ParentRange>::parent;
 
   public:
     /**

src/snmalloc/backend_helpers/pagemapregisterrange.h

@@ -2,6 +2,7 @@
 
 #include "../pal/pal.h"
 #include "empty_range.h"
+#include "range_helpers.h"
 
 namespace snmalloc
 {
@@ -9,9 +10,9 @@ namespace snmalloc
     SNMALLOC_CONCEPT(ConceptBackendMetaRange) Pagemap,
     bool CanConsolidate = true,
     typename ParentRange = EmptyRange>
-  class PagemapRegisterRange
+  class PagemapRegisterRange : public ContainsParent<ParentRange>
   {
-    ParentRange state{};
+    using ContainsParent<ParentRange>::parent;
 
   public:
     /**
@@ -28,7 +29,7 @@ namespace snmalloc
 
     capptr::Chunk<void> alloc_range(size_t size)
     {
-      auto base = state.alloc_range(size);
+      auto base = parent.alloc_range(size);
 
       if (base != nullptr)
         Pagemap::register_range(address_cast(base), size);

src/snmalloc/backend_helpers/range_helpers.h

@@ -69,4 +69,65 @@ namespace snmalloc
    */
   template<typename... Args>
   using Pipe = typename PipeImpl<Args...>::result;
+
+  /**
+   * Helper class for allowing a range to be navigated to find an
+   * ancestor of a specific type. The parent is an instance field.
+   */
+  template<typename Parent>
+  class ContainsParent
+  {
+  protected:
+    Parent parent{};
+
+  public:
+    /**
+     * Returns the outermost Ancestor with the correct type.
+     *
+     * Fails to compile if no such ancestor exists.
+     */
+    template<typename Anc>
+    Anc* ancestor()
+    {
+      if constexpr (std::is_same_v<Anc, Parent>)
+      {
+        return &parent;
+      }
+      else
+      {
+        return parent.template ancestor<Anc>();
+      }
+    }
+  };
+
+  /**
+   * Helper class for allowing a range to be navigated to find an
+   * ancestor of a specific type. The parent is a static field.
+   */
+  template<typename Parent>
+  class StaticParent
+  {
+  protected:
+    SNMALLOC_REQUIRE_CONSTINIT inline static Parent parent{};
+
+  public:
+    /**
+     * Returns the outermost Ancestor with the correct type.
+     *
+     * Fails to compile if no such ancestor exists.
+     */
+    template<typename Anc>
+    Anc* ancestor()
+    {
+      if constexpr (std::is_same_v<Anc, Parent>)
+      {
+        return &parent;
+      }
+      else
+      {
+        return parent.template ancestor<Anc>();
+      }
+    }
+  };
+
 } // namespace snmalloc

src/snmalloc/backend_helpers/smallbuddyrange.h

@@ -144,9 +144,9 @@ namespace snmalloc
   };
 
   template<typename ParentRange = EmptyRange>
-  class SmallBuddyRange
+  class SmallBuddyRange : public ContainsParent<ParentRange>
   {
-    ParentRange parent{};
+    using ContainsParent<ParentRange>::parent;
 
     static constexpr size_t MIN_BITS =
       bits::next_pow2_bits_const(sizeof(FreeChunk));

src/snmalloc/backend_helpers/statsrange.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "empty_range.h"
+#include "range_helpers.h"
 
 #include <atomic>
 
@@ -10,9 +11,9 @@ namespace snmalloc
    * Used to measure memory usage.
    */
   template<typename ParentRange = EmptyRange>
-  class StatsRange
+  class StatsRange : public ContainsParent<ParentRange>
   {
-    ParentRange parent{};
+    using ContainsParent<ParentRange>::parent;
 
     static inline std::atomic<size_t> current_usage{};
     static inline std::atomic<size_t> peak_usage{};

src/snmalloc/backend_helpers/subrange.h

@@ -10,9 +10,9 @@ namespace snmalloc
    * the end of the large allocation.
    */
   template<typename PAL, size_t RATIO_BITS, typename ParentRange = EmptyRange>
-  class SubRange
+  class SubRange : public ContainsParent<ParentRange>
   {
-    ParentRange parent{};
+    using ContainsParent<ParentRange>::parent;
 
   public:
     /**