Make all large allocations naturally aligned

This makes any large allocation naturally aligned to its size. This
means all alignment requests can be handled without checks.

README.md

@@ -168,7 +168,7 @@ template<bool committed>
 void* reserve(const size_t* size) noexcept;
 ```
 Only one of these needs to be implemented, depending on whether the underlying
-system can provide strongly (e.g. 16MiB) aligned memory regions.
+system can provide strongly aligned memory regions.
 If the system guarantees only page alignment, implement the second and snmalloc
 will over-allocate and then trim the requested region.
 If the system provides strong alignment, implement the first to return memory

src/mem/largealloc.h

@@ -281,9 +281,6 @@ namespace snmalloc
   template<class MemoryProvider>
   class LargeAlloc
   {
-    void* reserved_start = nullptr;
-    void* reserved_end = nullptr;
-
   public:
     // This will be a zero-size structure if stats are not enabled.
     Stats stats;
@@ -292,38 +289,6 @@ namespace snmalloc
 
     LargeAlloc(MemoryProvider& mp) : memory_provider(mp) {}
 
-    template<AllowReserve allow_reserve>
-    bool reserve_memory(size_t need, size_t add)
-    {
-      assert(reserved_start <= reserved_end);
-
-      /*
-       * Spell this comparison in terms of pointer subtraction like this,
-       * rather than "reserved_start + need < reserved_end" because the
-       * sum might not be representable on CHERI.
-       */
-      if (pointer_diff(reserved_start, reserved_end) < need)
-      {
-        if constexpr (allow_reserve == YesReserve)
-        {
-          stats.segment_create();
-          reserved_start =
-            memory_provider.template reserve<false>(&add, SUPERSLAB_SIZE);
-          reserved_end = pointer_offset(reserved_start, add);
-          reserved_start = pointer_align_up<SUPERSLAB_SIZE>(reserved_start);
-
-          if (add < need)
-            return false;
-        }
-        else
-        {
-          return false;
-        }
-      }
-
-      return true;
-    }
-
     template<ZeroMem zero_mem = NoZero, AllowReserve allow_reserve = YesReserve>
     void* alloc(size_t large_class, size_t size)
     {
@@ -337,23 +302,9 @@ namespace snmalloc
 
       if (p == nullptr)
       {
-        assert(reserved_start <= reserved_end);
-        size_t add;
-
-        if ((rsize + SUPERSLAB_SIZE) < RESERVE_SIZE)
-          add = RESERVE_SIZE;
-        else
-          add = rsize + SUPERSLAB_SIZE;
-
-        if (!reserve_memory<allow_reserve>(rsize, add))
-          return nullptr;
-
-        p = reserved_start;
-        reserved_start = pointer_offset(p, rsize);
-
-        stats.superslab_fresh();
-        // All memory is zeroed since it comes from reserved space.
-        memory_provider.template notify_using<NoZero>(p, size);
+        size_t add = rsize;
+        p = memory_provider.template reserve<false>(&add, rsize);
+        memory_provider.template notify_using<zero_mem>(p, size);
       }
       else
       {

src/mem/sizeclass.h

@@ -179,8 +179,6 @@ namespace snmalloc
   {
     // Client responsible for checking alignment is not zero
     assert(alignment != 0);
-    // Client responsible for checking alignment is not above SUPERSLAB_SIZE
-    assert(alignment <= SUPERSLAB_SIZE);
     // Client responsible for checking alignment is a power of two
     assert(bits::next_pow2(alignment) == alignment);
 

src/override/malloc.cc

@@ -108,8 +108,7 @@ extern "C"
     SNMALLOC_NAME_MANGLE(memalign)(size_t alignment, size_t size)
   {
     if (
-      (alignment == 0) || (alignment == size_t(-1)) ||
-      (alignment > SUPERSLAB_SIZE))
+      (alignment == 0) || (alignment == size_t(-1)))
     {
       errno = EINVAL;
       return nullptr;

src/test/func/malloc/malloc.cc

@@ -108,7 +108,7 @@ int main(int argc, char** argv)
   test_posix_memalign((size_t)-1, 0, EINVAL, true);
   test_posix_memalign(OS_PAGE_SIZE, sizeof(uintptr_t) / 2, EINVAL, true);
 
-  for (size_t align = sizeof(uintptr_t); align <= SUPERSLAB_SIZE; align <<= 1)
+  for (size_t align = sizeof(uintptr_t); align <= SUPERSLAB_SIZE * 8; align <<= 1)
   {
     for (snmalloc::sizeclass_t sc = 0; sc < NUM_SIZECLASSES; sc++)
     {