[libc++] Refactor some code in monotonic_buffer_resource (#117271)

1. remove unused __default_buffer_alignment
2. two __try_allocate_from_chunk are same, put it together

This patch refactor some code in monotonic_buffer_resource.

1 files changed, 18 insertions, 19 deletions

diff --git a/libcxx/src/memory_resource.cpp b/libcxx/src/memory_resource.cpp
index 0cd575e..e182e5a 100644
--- a/libcxx/src/memory_resource.cpp
+++ b/libcxx/src/memory_resource.cpp
@@ -413,6 +413,8 @@ bool synchronized_pool_resource::do_is_equal(const memory_resource& other) const
 
 // 23.12.6, mem.res.monotonic.buffer
 
+constexpr size_t __default_growth_factor = 2;
+
 static void* align_down(size_t align, size_t size, void*& ptr, size_t& space) {
   if (size > space)
     return nullptr;
@@ -429,23 +431,20 @@ static void* align_down(size_t align, size_t size, void*& ptr, size_t& space) {
   return ptr;
 }
 
-void* monotonic_buffer_resource::__initial_descriptor::__try_allocate_from_chunk(size_t bytes, size_t align) {
-  if (!__cur_)
-    return nullptr;
-  void* new_ptr       = static_cast<void*>(__cur_);
-  size_t new_capacity = (__cur_ - __start_);
-  void* aligned_ptr   = align_down(align, bytes, new_ptr, new_capacity);
-  if (aligned_ptr != nullptr)
-    __cur_ = static_cast<char*>(new_ptr);
-  return aligned_ptr;
-}
-
-void* monotonic_buffer_resource::__chunk_footer::__try_allocate_from_chunk(size_t bytes, size_t align) {
-  void* new_ptr       = static_cast<void*>(__cur_);
-  size_t new_capacity = (__cur_ - __start_);
+template <bool is_initial, typename Chunk>
+void* __try_allocate_from_chunk(Chunk& self, size_t bytes, size_t align) {
+  if constexpr (is_initial) {
+    // only for __initial_descriptor.
+    // if __initial_descriptor.__cur_ equals nullptr, means no available buffer given when ctor.
+    // here we just return nullptr, let the caller do the next handling.
+    if (!self.__cur_)
+      return nullptr;
+  }
+  void* new_ptr       = static_cast<void*>(self.__cur_);
+  size_t new_capacity = (self.__cur_ - self.__start_);
   void* aligned_ptr   = align_down(align, bytes, new_ptr, new_capacity);
   if (aligned_ptr != nullptr)
-    __cur_ = static_cast<char*>(new_ptr);
+    self.__cur_ = static_cast<char*>(new_ptr);
   return aligned_ptr;
 }
 
@@ -462,10 +461,10 @@ void* monotonic_buffer_resource::do_allocate(size_t bytes, size_t align) {
     return roundup(newsize, footer_align) + footer_size;
   };
 
-  if (void* result = __initial_.__try_allocate_from_chunk(bytes, align))
+  if (void* result = __try_allocate_from_chunk<true, __initial_descriptor>(__initial_, bytes, align))
     return result;
   if (__chunks_ != nullptr) {
-    if (void* result = __chunks_->__try_allocate_from_chunk(bytes, align))
+    if (void* result = __try_allocate_from_chunk<false, __chunk_footer>(*__chunks_, bytes, align))
       return result;
   }
 
@@ -478,7 +477,7 @@ void* monotonic_buffer_resource::do_allocate(size_t bytes, size_t align) {
   size_t previous_capacity = previous_allocation_size();
 
   if (aligned_capacity <= previous_capacity) {
-    size_t newsize   = 2 * (previous_capacity - footer_size);
+    size_t newsize   = __default_growth_factor * (previous_capacity - footer_size);
     aligned_capacity = roundup(newsize, footer_align) + footer_size;
   }
 
@@ -491,7 +490,7 @@ void* monotonic_buffer_resource::do_allocate(size_t bytes, size_t align) {
   footer->__align_       = align;
   __chunks_              = footer;
 
-  return __chunks_->__try_allocate_from_chunk(bytes, align);
+  return __try_allocate_from_chunk<false, __chunk_footer>(*__chunks_, bytes, align);
 }
 
 } // namespace pmr