[PATCH] D153668: [Support][Allocator] Add C++17 memory_resource adaptor for LLVM allocators

[Jeremy Furtek via Phabricator via llvm-commits]

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C++17 added support for polymorphic memory resources to the standard library:

https://isocpp.org/files/papers/N3916.pdf

Prior to polymorphic memory resources, the allocator was part of the compile-
time type for C++ containers. (As an example, `std::vector` instances with
different allocators were distinct C++ types.) With polymorphic memory
resources, the `memory_resource` abstract base class provides an interface that
different allocators can implement, allowing containers with different
allocators to have the same type. Runtime allocator dispatch occurs via a
virtual function call.

This diff adds a `memory_resource` adaptor for LLVM allocators (and in
particular, the `BumpPtrAllocator`). This is an opt-in modification: no existing
uses of the ubiquitous BumpPtrAllocator are affected.

With the new adaptor, LLVM code that uses the `BumpPtrAllocator` can make use of
the standard C++ library containers with PMR support (i.e. `std::pmr::vector<>`,
`std::pmr::list<>`, `std::pmr::string`, ...), with container allocations using
the LLVM `BumpPtrAllocator`.

Future MLIR diffs may use this feature. Those are being kept separate from this
diff to facilitate review.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D153668

Files:
  llvm/include/llvm/Support/Allocator.h
  llvm/unittests/Support/AllocatorTest.cpp


Index: llvm/unittests/Support/AllocatorTest.cpp
===================================================================
--- llvm/unittests/Support/AllocatorTest.cpp
+++ llvm/unittests/Support/AllocatorTest.cpp
@@ -9,6 +9,7 @@
 #include "llvm/Support/Allocator.h"
 #include "gtest/gtest.h"
 #include <cstdlib>
+#include <numeric>
 
 using namespace llvm;
 
@@ -258,4 +259,24 @@
   EXPECT_GT(MockSlabAllocator::GetLastSlabSize(), 4096u);
 }
 
+// Test an adaptor to allow using LLVM allocators with C++ standard library
+// containers.
+TEST(AllocatorTest, MemoryResource) {
+  // Declare a BumpPtrAllocator instance with an associated PMR memory resource.
+  // (This is similar to using a C++17 monotonic_buffer_resource, but instead
+  // uses the LLVM BumpPtrAllocator.)
+  using Allocator = AllocatorWithMemoryResource<BumpPtrAllocator>;
+  Allocator Alloc;
+  std::pmr::vector<size_t> vec(&Alloc);
+  // Populate the vector with increasing values
+  vec.resize(64);
+  std::iota(vec.begin(), vec.end(), 0);
+  for (size_t i = 0; i < vec.size(); ++i) {
+    // Verify the vector values
+    EXPECT_EQ(vec.at(i), i);
+    // Verify that the vector elements are part of the provide allocator
+    EXPECT_TRUE(Alloc.identifyObject(&(vec[i])).has_value());
+  }
+}
+
 }  // anonymous namespace
Index: llvm/include/llvm/Support/Allocator.h
===================================================================
--- llvm/include/llvm/Support/Allocator.h
+++ llvm/include/llvm/Support/Allocator.h
@@ -27,6 +27,7 @@
 #include <cstddef>
 #include <cstdint>
 #include <iterator>
+#include <memory_resource>
 #include <optional>
 #include <utility>
 
@@ -432,6 +433,44 @@
   T *Allocate(size_t num = 1) { return Allocator.Allocate<T>(num); }
 };
 
+// This class inherits from the C++17 polymorphic memory resource abstract base
+// class to adapt LLVM allocators for use with C++ standard libraries that have
+// pmr support (e.g. std::pmr::vector<>, std::pmr::list<>, ...). This allows
+// containers with different PMR allocators to interact, at the cost of a
+// virtual function call.
+template <class TAlloc>
+class AllocMemoryResource : public std::pmr::memory_resource {
+public:
+  AllocMemoryResource(TAlloc *alloc) noexcept : Alloc(alloc) {}
+
+protected:
+  void *do_allocate(size_t Bytes, size_t AlignTo) override {
+    return Alloc->Allocate(Bytes, AlignTo);
+  };
+  void do_deallocate(void *p, size_t Bytes, size_t AlignTo) override {
+    Alloc->Deallocate(p, Bytes, AlignTo);
+  };
+  bool
+  do_is_equal(const std::pmr::memory_resource &other) const noexcept override {
+    return (static_cast<const std::pmr::memory_resource *>(this) == &other);
+  }
+
+private:
+  TAlloc *Alloc;
+};
+
+// This class inherits from a template allocator class, as well as the
+// AllocMemoryResource class, allowing LLVM allocators to opt-in to supporting
+// both the LLVM Allocator (Allocate()/Deallocate()) interface as well as the
+// C++17 polymorphic memory resource interface.
+template <class TAlloc>
+class AllocatorWithMemoryResource : public TAlloc,
+                                    public AllocMemoryResource<TAlloc> {
+public:
+  AllocatorWithMemoryResource()
+      : AllocMemoryResource<TAlloc>(static_cast<TAlloc *>(this)) {}
+};
+
 } // end namespace llvm
 
 template <typename AllocatorT, size_t SlabSize, size_t SizeThreshold,


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