Natural alignment for USE_MALLOC (#248)

* Add concept of natural alignment to tests.

snmalloc naturally aligns blocks very heavily, so that
the largest power-of-two in the rounded size is the alignment.
This checks that in the test, and provides a method for
finding the natural alignment of a block.

* Improve USE_MALLOC to provide alignment

snmalloc provides a lot of alginment guarantees. This ensures that when
we pass through to the system allocator we still get those alignment
guarantees.

The commit also fixes the tests to work with USE_MALLOC, and builds a
set of unit tests for ctest to check behaviour.

src/mem/alloc.h

@@ -10,6 +10,7 @@
 #include "../test/histogram.h"
 #include "allocstats.h"
 #include "chunkmap.h"
+#include "external_alloc.h"
 #include "largealloc.h"
 #include "mediumslab.h"
 #include "pooled.h"
@@ -125,14 +126,18 @@ namespace snmalloc
     SNMALLOC_FAST_PATH ALLOCATOR void* alloc()
     {
       static_assert(size != 0, "Size must not be zero.");
-#ifdef USE_MALLOC
+#ifdef SNMALLOC_PASS_THROUGH
       static_assert(
         allow_reserve == YesReserve,
         "When passing to malloc, cannot require NoResereve");
-      if constexpr (zero_mem == NoZero)
-        return malloc(size);
-      else
-        return calloc(1, size);
+      // snmalloc guarantees a lot of alignment, so we can depend on this
+      // make pass through call aligned_alloc with the alignment snmalloc
+      // would guarantee.
+      void* result = external_alloc::aligned_alloc(
+        natural_alignment(size), round_size(size));
+      if constexpr (zero_mem == YesZero)
+        memset(result, 0, size);
+      return result;
 #else
       constexpr sizeclass_t sizeclass = size_to_sizeclass_const(size);
 
@@ -162,14 +167,18 @@ namespace snmalloc
     template<ZeroMem zero_mem = NoZero, AllowReserve allow_reserve = YesReserve>
     SNMALLOC_FAST_PATH ALLOCATOR void* alloc(size_t size)
     {
-#ifdef USE_MALLOC
+#ifdef SNMALLOC_PASS_THROUGH
       static_assert(
         allow_reserve == YesReserve,
         "When passing to malloc, cannot require NoResereve");
-      if constexpr (zero_mem == NoZero)
-        return malloc(size);
-      else
-        return calloc(1, size);
+      // snmalloc guarantees a lot of alignment, so we can depend on this
+      // make pass through call aligned_alloc with the alignment snmalloc
+      // would guarantee.
+      void* result = external_alloc::aligned_alloc(
+        natural_alignment(size), round_size(size));
+      if constexpr (zero_mem == YesZero)
+        memset(result, 0, size);
+      return result;
 #else
       // Perform the - 1 on size, so that zero wraps around and ends up on
       // slow path.
@@ -241,9 +250,9 @@ namespace snmalloc
     template<size_t size>
     void dealloc(void* p)
     {
-#ifdef USE_MALLOC
+#ifdef SNMALLOC_PASS_THROUGH
       UNUSED(size);
-      return free(p);
+      return external_alloc::free(p);
 #else
       check_size(p, size);
       constexpr sizeclass_t sizeclass = size_to_sizeclass_const(size);
@@ -281,9 +290,9 @@ namespace snmalloc
      */
     SNMALLOC_FAST_PATH void dealloc(void* p, size_t size)
     {
-#ifdef USE_MALLOC
+#ifdef SNMALLOC_PASS_THROUGH
       UNUSED(size);
-      return free(p);
+      return external_alloc::free(p);
 #else
       SNMALLOC_ASSERT(p != nullptr);
       check_size(p, size);
@@ -327,8 +336,8 @@ namespace snmalloc
      */
     SNMALLOC_FAST_PATH void dealloc(void* p)
     {
-#ifdef USE_MALLOC
-      return free(p);
+#ifdef SNMALLOC_PASS_THROUGH
+      return external_alloc::free(p);
 #else
 
       uint8_t size = chunkmap().get(address_cast(p));
@@ -397,7 +406,7 @@ namespace snmalloc
     template<Boundary location = Start>
     static void* external_pointer(void* p)
     {
-#ifdef USE_MALLOC
+#ifdef SNMALLOC_PASS_THROUGH
       error("Unsupported");
       UNUSED(p);
 #else
@@ -463,6 +472,9 @@ namespace snmalloc
   public:
     SNMALLOC_FAST_PATH static size_t alloc_size(const void* p)
     {
+#ifdef SNMALLOC_PASS_THROUGH
+      return external_alloc::malloc_usable_size(const_cast<void*>(p));
+#else
       // This must be called on an external pointer.
       size_t size = ChunkMap::get(address_cast(p));
 
@@ -492,6 +504,7 @@ namespace snmalloc
       }
 
       return alloc_size_error();
+#endif
     }
 
     size_t get_id()