Add an Architecture Abstraction Layer.

Currently, we support one architecture, but this provides a layer for
adding other architectures without adding more nested `#ifdef`s.

Fixes #42

src/aal/aal.h

@@ -0,0 +1,86 @@
+#pragma once
+#include "ds/defines.h"
+
+#if defined(__i386__) || defined(_M_IX86) || defined(_X86_) || \
+  defined(__amd64__) || defined(__x86_64__) || defined(_M_X64) || \
+  defined(_M_AMD64)
+#  define PLATFORM_IS_X86
+#endif
+
+namespace snmalloc
+{
+  /**
+   * Architecture Abstraction Layer. Includes default implementations of some
+   * functions using compiler builtins.  Falls back to the definitions in the
+   * platform's AAL if the builtin does not exist.
+   */
+  template<class Arch>
+  struct AAL_Generic : Arch
+  {
+    /**
+     * Prefetch a specific address.
+     *
+     * If the compiler provides a portable prefetch builtin, use it directly,
+     * otherwise delegate to the architecture-specific layer.  This allows new
+     * architectures to avoid needing to implement a custom `prefetch` method
+     * if they are used only with a compiler that provides the builtin.
+     */
+    static inline void prefetch(void* ptr)
+    {
+#if __has_builtin(__builtin_prefetch) && !defined(SNMALLOC_NO_AAL_BUILTINS)
+      __builtin_prefetch(ptr);
+#else
+      Arch::prefetch(ptr);
+#endif
+    }
+
+    /**
+     * Return an architecture-specific cycle counter.
+     *
+     * If the compiler provides a portable prefetch builtin, use it directly,
+     * otherwise delegate to the architecture-specific layer.  This allows new
+     * architectures to avoid needing to implement a custom `tick` method
+     * if they are used only with a compiler that provides the builtin.
+     */
+    static inline uint64_t tick()
+    {
+#if __has_builtin(__builtin_readcyclecounter) && \
+  !defined(SNMALLOC_NO_AAL_BUILTINS)
+      return __builtin_readcyclecounter();
+#else
+      return Arch::tick();
+#endif
+    }
+  };
+
+}
+
+#ifdef PLATFORM_IS_X86
+#  include "aal_x86.h"
+#endif
+
+namespace snmalloc
+{
+  using AAL = AAL_Generic<AAL_Arch>;
+}
+
+#if defined(_MSC_VER) && defined(SNMALLOC_VA_BITS_32)
+#  include <intsafe.h>
+#endif
+
+#ifdef __POINTER_WIDTH__
+#  if ((__POINTER_WIDTH__ == 64) && !defined(SNMALLOC_VA_BITS_64)) || \
+    ((__POINTER_WIDTH__ == 32) && !defined(SNMALLOC_VA_BITS_32))
+#    error Compiler and PAL define inconsistent bit widths
+#  endif
+#endif
+
+#if defined(SNMALLOC_VA_BITS_32) && defined(SNMALLOC_VA_BITS_64)
+#  error Only one of SNMALLOC_VA_BITS_64 and SNMALLOC_VA_BITS_32 may be defined!
+#endif
+
+#ifdef SNMALLOC_VA_BITS_32
+static_assert(sizeof(size_t) == 4);
+#elif defined(SNMALLOC_VA_BITS_64)
+static_assert(sizeof(size_t) == 8);
+#endif