Add a pipe operation to ranges (#527)

The ranges are naturally put together with pipes. This
commit does some template magic to make the code more
readable.  There should be now functional changes with
this change.

src/snmalloc/backend/backend.h

@@ -98,11 +98,12 @@ namespace snmalloc
 #endif
 
     // Set up source of memory
-    using P = PalRange<PAL>;
     using Base = std::conditional_t<
       fixed_range,
       EmptyRange,
-      PagemapRegisterRange<Pagemap, P, CONSOLIDATE_PAL_ALLOCS>>;
+      Pipe<
+        PalRange<Pal>,
+        PagemapRegisterRange<Pagemap, CONSOLIDATE_PAL_ALLOCS>>>;
 
     static constexpr size_t MinBaseSizeBits()
     {
@@ -117,50 +118,56 @@ namespace snmalloc
     }
 
     // Global range of memory
-    using GlobalR = GlobalRange<LogRange<
-      2,
-      LargeBuddyRange<Base, 24, bits::BITS - 1, Pagemap, MinBaseSizeBits()>>>;
+    using GlobalR = Pipe<
+      Base,
+      LargeBuddyRange<24, bits::BITS - 1, Pagemap, MinBaseSizeBits()>,
+      LogRange<2>,
+      GlobalRange<>>;
 
 #ifdef SNMALLOC_META_PROTECTED
     // Introduce two global ranges, so we don't mix Object and Meta
-    using CentralObjectRange = GlobalRange<LogRange<
-      3,
-      LargeBuddyRange<
-        GlobalR,
-        24,
-        bits::BITS - 1,
-        Pagemap,
-        MinBaseSizeBits()>>>;
-    using CentralMetaRange = GlobalRange<LogRange<
-      4,
-      LargeBuddyRange<
-        SubRange<GlobalR, PAL, 6>, // Use SubRange to introduce guard pages.
-        24,
-        bits::BITS - 1,
-        Pagemap,
-        MinBaseSizeBits()>>>;
+    using CentralObjectRange = Pipe<
+      GlobalR,
+      LargeBuddyRange<24, bits::BITS - 1, Pagemap, MinBaseSizeBits()>,
+      LogRange<3>,
+      GlobalRange<>>;
+
+    using CentralMetaRange = Pipe<
+      GlobalR,
+      SubRange<PAL, 6>, // Use SubRange to introduce guard pages.
+      LargeBuddyRange<24, bits::BITS - 1, Pagemap, MinBaseSizeBits()>,
+      LogRange<4>,
+      GlobalRange<>>;
 
     // Source for object allocations
-    using StatsObject = StatsRange<CommitRange<CentralObjectRange, PAL>>;
+    using StatsObject =
+      Pipe<CentralObjectRange, CommitRange<PAL>, StatsRange<>>;
+
     using ObjectRange =
-      LogRange<5, LargeBuddyRange<StatsObject, 21, 21, Pagemap>>;
+      Pipe<StatsObject, LargeBuddyRange<21, 21, Pagemap>, LogRange<5>>;
 
-    using StatsMeta = StatsRange<CommitRange<CentralMetaRange, PAL>>;
+    using StatsMeta = Pipe<CentralMetaRange, CommitRange<PAL>, StatsRange<>>;
+
+    using MetaRange = Pipe<
+      StatsMeta,
+      LargeBuddyRange<21 - 6, bits::BITS - 1, Pagemap>,
+      SmallBuddyRange<>>;
 
-    using MetaRange = SmallBuddyRange<
-      LargeBuddyRange<StatsMeta, 21 - 6, bits::BITS - 1, Pagemap>>;
     // Create global range that can service small meta-data requests.
     // Don't want to add this to the CentralMetaRange to move Commit outside the
     // lock on the common case.
-    using GlobalMetaRange = GlobalRange<SmallBuddyRange<StatsMeta>>;
+    using GlobalMetaRange = Pipe<StatsMeta, SmallBuddyRange<>, GlobalRange<>>;
     using Stats = StatsCombiner<StatsObject, StatsMeta>;
 #else
     // Source for object allocations and metadata
     // No separation between the two
-    using Stats = StatsRange<GlobalR>;
-    using ObjectRange = SmallBuddyRange<
-      LargeBuddyRange<CommitRange<Stats, PAL>, 21, 21, Pagemap>>;
-    using GlobalMetaRange = GlobalRange<ObjectRange>;
+    using Stats = Pipe<GlobalR, StatsRange<>>;
+    using ObjectRange = Pipe<
+      Stats,
+      CommitRange<PAL>,
+      LargeBuddyRange<21, 21, Pagemap>,
+      SmallBuddyRange<>>;
+    using GlobalMetaRange = Pipe<ObjectRange, GlobalRange<>>;
 #endif
 
     struct LocalState

src/snmalloc/backend_helpers/commitrange.h

@@ -1,15 +1,21 @@
 #pragma once
-
 #include "../pal/pal.h"
+#include "empty_range.h"
 
 namespace snmalloc
 {
-  template<typename ParentRange, typename PAL>
+  template<typename PAL, typename ParentRange = EmptyRange>
   class CommitRange
   {
     ParentRange parent{};
 
   public:
+    /**
+     * We use a nested Apply type to enable a Pipe operation.
+     */
+    template<typename ParentRange2>
+    using Apply = CommitRange<PAL, ParentRange2>;
+
     static constexpr bool Aligned = ParentRange::Aligned;
 
     static constexpr bool ConcurrencySafe = ParentRange::ConcurrencySafe;

src/snmalloc/backend_helpers/globalrange.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "../ds/ds.h"
+#include "empty_range.h"
 
 namespace snmalloc
 {
@@ -8,7 +9,7 @@ namespace snmalloc
    * Makes the supplied ParentRange into a global variable,
    * and protects access with a lock.
    */
-  template<typename ParentRange>
+  template<typename ParentRange = EmptyRange>
   class GlobalRange
   {
     SNMALLOC_REQUIRE_CONSTINIT static inline ParentRange parent{};
@@ -20,6 +21,12 @@ namespace snmalloc
     SNMALLOC_REQUIRE_CONSTINIT static inline FlagWord spin_lock{};
 
   public:
+    /**
+     * We use a nested Apply type to enable a Pipe operation.
+     */
+    template<typename ParentRange2>
+    using Apply = GlobalRange<ParentRange2>;
+
     static constexpr bool Aligned = ParentRange::Aligned;
 
     static constexpr bool ConcurrencySafe = true;

src/snmalloc/backend_helpers/largebuddyrange.h

@@ -3,6 +3,7 @@
 #include "../ds/ds.h"
 #include "../mem/mem.h"
 #include "buddy.h"
+#include "empty_range.h"
 #include "range_helpers.h"
 
 #include <string>
@@ -183,11 +184,11 @@ namespace snmalloc
    * for
    */
   template<
-    typename ParentRange,
     size_t REFILL_SIZE_BITS,
     size_t MAX_SIZE_BITS,
     SNMALLOC_CONCEPT(ConceptBuddyRangeMeta) Pagemap,
-    size_t MIN_REFILL_SIZE_BITS = 0>
+    size_t MIN_REFILL_SIZE_BITS = 0,
+    typename ParentRange = EmptyRange>
   class LargeBuddyRange
   {
     ParentRange parent{};
@@ -327,6 +328,17 @@ namespace snmalloc
     }
 
   public:
+    /**
+     * We use a nested Apply type to enable a Pipe operation.
+     */
+    template<typename ParentRange2>
+    using Apply = LargeBuddyRange<
+      REFILL_SIZE_BITS,
+      MAX_SIZE_BITS,
+      Pagemap,
+      MIN_REFILL_SIZE_BITS,
+      ParentRange2>;
+
     static constexpr bool Aligned = true;
 
     static constexpr bool ConcurrencySafe = false;

src/snmalloc/backend_helpers/logrange.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include "empty_range.h"
+
 namespace snmalloc
 {
   /**
@@ -8,12 +10,18 @@ namespace snmalloc
    *
    * ParentRange is what the range is logging calls to.
    */
-  template<size_t RangeName, typename ParentRange>
+  template<size_t RangeName, typename ParentRange = EmptyRange>
   class LogRange
   {
     ParentRange parent{};
 
   public:
+    /**
+     * We use a nested Apply type to enable a Pipe operation.
+     */
+    template<typename ParentRange2>
+    using Apply = LogRange<RangeName, ParentRange2>;
+
     static constexpr bool Aligned = ParentRange::Aligned;
 
     static constexpr bool ConcurrencySafe = ParentRange::ConcurrencySafe;

src/snmalloc/backend_helpers/pagemapregisterrange.h

@@ -1,18 +1,25 @@
 #pragma once
 
 #include "../pal/pal.h"
+#include "empty_range.h"
 
 namespace snmalloc
 {
   template<
     SNMALLOC_CONCEPT(ConceptBackendMetaRange) Pagemap,
-    typename ParentRange,
-    bool CanConsolidate = true>
+    bool CanConsolidate = true,
+    typename ParentRange = EmptyRange>
   class PagemapRegisterRange
   {
     ParentRange state{};
 
   public:
+    /**
+     * We use a nested Apply type to enable a Pipe operation.
+     */
+    template<typename ParentRange2>
+    using Apply = PagemapRegisterRange<Pagemap, CanConsolidate, ParentRange2>;
+
     constexpr PagemapRegisterRange() = default;
 
     static constexpr bool Aligned = ParentRange::Aligned;

src/snmalloc/backend_helpers/range_helpers.h

@@ -34,4 +34,39 @@ namespace snmalloc
       length -= align;
     }
   }
+
+  /**
+   * Forward definition to allow multiple template specialisations.
+   *
+   * This struct is used to recursively compose ranges.
+   */
+  template<typename... Args>
+  struct PipeImpl;
+
+  /**
+   * Base case of one range that needs nothing.
+   */
+  template<typename Only>
+  struct PipeImpl<Only>
+  {
+    using result = Only;
+  };
+
+  /**
+   * Recursive case of applying a base range as an argument to the
+   * next, and then using that as the new base range.
+   */
+  template<typename First, typename Fun, typename... Rest>
+  struct PipeImpl<First, Fun, Rest...>
+  {
+  public:
+    using result =
+      typename PipeImpl<typename Fun::template Apply<First>, Rest...>::result;
+  };
+
+  /**
+   * Nice type so the caller doesn't need to call result explicitly.
+   */
+  template<typename... Args>
+  using Pipe = typename PipeImpl<Args...>::result;
 } // namespace snmalloc

src/snmalloc/backend_helpers/smallbuddyrange.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "../pal/pal.h"
+#include "empty_range.h"
 #include "range_helpers.h"
 
 namespace snmalloc
@@ -142,7 +143,7 @@ namespace snmalloc
     }
   };
 
-  template<typename ParentRange>
+  template<typename ParentRange = EmptyRange>
   class SmallBuddyRange
   {
     ParentRange parent{};
@@ -179,6 +180,12 @@ namespace snmalloc
     }
 
   public:
+    /**
+     * We use a nested Apply type to enable a Pipe operation.
+     */
+    template<typename ParentRange2>
+    using Apply = SmallBuddyRange<ParentRange2>;
+
     static constexpr bool Aligned = true;
     static_assert(ParentRange::Aligned, "ParentRange must be aligned");
 

src/snmalloc/backend_helpers/statsrange.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include "empty_range.h"
+
 #include <atomic>
 
 namespace snmalloc
@@ -7,7 +9,7 @@ namespace snmalloc
   /**
    * Used to measure memory usage.
    */
-  template<typename ParentRange>
+  template<typename ParentRange = EmptyRange>
   class StatsRange
   {
     ParentRange parent{};
@@ -16,6 +18,12 @@ namespace snmalloc
     static inline std::atomic<size_t> peak_usage{};
 
   public:
+    /**
+     * We use a nested Apply type to enable a Pipe operation.
+     */
+    template<typename ParentRange2>
+    using Apply = StatsRange<ParentRange2>;
+
     static constexpr bool Aligned = ParentRange::Aligned;
 
     static constexpr bool ConcurrencySafe = ParentRange::ConcurrencySafe;

src/snmalloc/backend_helpers/subrange.h

@@ -1,5 +1,6 @@
 #pragma once
 #include "../mem/mem.h"
+#include "empty_range.h"
 
 namespace snmalloc
 {
@@ -8,12 +9,18 @@ namespace snmalloc
    * 2^RATIO_BITS.  Will not return a the block at the start or
    * the end of the large allocation.
    */
-  template<typename ParentRange, typename PAL, size_t RATIO_BITS>
+  template<typename PAL, size_t RATIO_BITS, typename ParentRange = EmptyRange>
   class SubRange
   {
     ParentRange parent{};
 
   public:
+    /**
+     * We use a nested Apply type to enable a Pipe operation.
+     */
+    template<typename ParentRange2>
+    using Apply = SubRange<PAL, RATIO_BITS, ParentRange2>;
+
     constexpr SubRange() = default;
 
     static constexpr bool Aligned = ParentRange::Aligned;