Common/MemArena: Add LazyMemoryRegion to represent a zero-initialized memory region whose pages are only allocated on first access.

Source/Core/Common/MemArena.h

@@ -122,4 +122,42 @@ private:
 #endif
 };
 
+// This class represents a single fixed-size memory region where the individual memory pages are
+// only actually allocated on first access. The memory will be zero on first access.
+class LazyMemoryRegion final
+{
+public:
+  LazyMemoryRegion();
+  ~LazyMemoryRegion();
+  LazyMemoryRegion(const LazyMemoryRegion&) = delete;
+  LazyMemoryRegion(LazyMemoryRegion&&) = delete;
+  LazyMemoryRegion& operator=(const LazyMemoryRegion&) = delete;
+  LazyMemoryRegion& operator=(LazyMemoryRegion&&) = delete;
+
+  ///
+  /// Reserve a memory region.
+  ///
+  /// @param size The size of the region.
+  ///
+  /// @return The address the region was mapped at. Returns nullptr on failure.
+  ///
+  void* Create(size_t size);
+
+  ///
+  /// Reset the memory region back to zero, throwing away any mapped pages.
+  /// This can only be called after a successful call to Create().
+  ///
+  void Clear();
+
+  ///
+  /// Release the memory previously reserved with Create(). After this call the pointer that was
+  /// returned by Create() will become invalid.
+  ///
+  void Release();
+
+private:
+  void* m_memory = nullptr;
+  size_t m_size = 0;
+};
+
 }  // namespace Common

Source/Core/Common/MemArenaAndroid.cpp

@@ -19,6 +19,7 @@
 #include <sys/mman.h>
 #include <unistd.h>
 
+#include "Common/Assert.h"
 #include "Common/CommonFuncs.h"
 #include "Common/CommonTypes.h"
 #include "Common/Logging/Log.h"
@@ -142,4 +143,46 @@ void MemArena::UnmapFromMemoryRegion(void* view, size_t size)
   if (retval == MAP_FAILED)
     NOTICE_LOG_FMT(MEMMAP, "mmap failed");
 }
+
+LazyMemoryRegion::LazyMemoryRegion() = default;
+
+LazyMemoryRegion::~LazyMemoryRegion()
+{
+  Release();
+}
+
+void* LazyMemoryRegion::Create(size_t size)
+{
+  ASSERT(!m_memory);
+
+  void* memory = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  if (!memory)
+  {
+    NOTICE_LOG_FMT(MEMMAP, "Memory allocation of {} bytes failed.", size);
+    return nullptr;
+  }
+
+  m_memory = memory;
+  m_size = size;
+
+  return memory;
+}
+
+void LazyMemoryRegion::Clear()
+{
+  ASSERT(m_memory);
+
+  mmap(m_memory, m_size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
+}
+
+void LazyMemoryRegion::Release()
+{
+  if (m_memory)
+  {
+    munmap(m_memory, m_size);
+    m_memory = nullptr;
+    m_size = 0;
+  }
+}
+
 }  // namespace Common

Source/Core/Common/MemArenaUnix.cpp

@@ -16,6 +16,7 @@
 #include <sys/mman.h>
 #include <unistd.h>
 
+#include "Common/Assert.h"
 #include "Common/CommonFuncs.h"
 #include "Common/CommonTypes.h"
 #include "Common/Logging/Log.h"
@@ -108,4 +109,46 @@ void MemArena::UnmapFromMemoryRegion(void* view, size_t size)
   if (retval == MAP_FAILED)
     NOTICE_LOG_FMT(MEMMAP, "mmap failed");
 }
+
+LazyMemoryRegion::LazyMemoryRegion() = default;
+
+LazyMemoryRegion::~LazyMemoryRegion()
+{
+  Release();
+}
+
+void* LazyMemoryRegion::Create(size_t size)
+{
+  ASSERT(!m_memory);
+
+  void* memory = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  if (!memory)
+  {
+    NOTICE_LOG_FMT(MEMMAP, "Memory allocation of {} bytes failed.", size);
+    return nullptr;
+  }
+
+  m_memory = memory;
+  m_size = size;
+
+  return memory;
+}
+
+void LazyMemoryRegion::Clear()
+{
+  ASSERT(m_memory);
+
+  mmap(m_memory, m_size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
+}
+
+void LazyMemoryRegion::Release()
+{
+  if (m_memory)
+  {
+    munmap(m_memory, m_size);
+    m_memory = nullptr;
+    m_size = 0;
+  }
+}
+
 }  // namespace Common

Source/Core/Common/MemArenaWin.cpp

@@ -433,4 +433,47 @@ void MemArena::UnmapFromMemoryRegion(void* view, size_t size)
 
   UnmapViewOfFile(view);
 }
+
+LazyMemoryRegion::LazyMemoryRegion() = default;
+
+LazyMemoryRegion::~LazyMemoryRegion()
+{
+  Release();
+}
+
+void* LazyMemoryRegion::Create(size_t size)
+{
+  ASSERT(!m_memory);
+
+  void* memory = VirtualAlloc(nullptr, size, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
+  if (!memory)
+  {
+    NOTICE_LOG_FMT(MEMMAP, "Memory allocation of {} bytes failed.", size);
+    return nullptr;
+  }
+
+  m_memory = memory;
+  m_size = size;
+
+  return memory;
+}
+
+void LazyMemoryRegion::Clear()
+{
+  ASSERT(m_memory);
+
+  VirtualFree(m_memory, m_size, MEM_DECOMMIT);
+  VirtualAlloc(m_memory, m_size, MEM_COMMIT, PAGE_READWRITE);
+}
+
+void LazyMemoryRegion::Release()
+{
+  if (m_memory)
+  {
+    VirtualFree(m_memory, 0, MEM_RELEASE);
+    m_memory = nullptr;
+    m_size = 0;
+  }
+}
+
 }  // namespace Common