[libc++] Update the status for lwg-3143

Current implementation use the larger one either requested bytes or growth
factor multiply previous buffer size.

This patch update the lwg 3143 issue latest status.

Author: love1angel

libcxx/docs/Status/Cxx23Issues.csv

@@ -1,7 +1,7 @@
 "Issue #","Issue Name","Meeting","Status","First released version","Notes"
 "`LWG2839 <https://wg21.link/LWG2839>`__","Self-move-assignment of library types, again","2020-11 (Virtual)","|Nothing To Do|","",""
 "`LWG3117 <https://wg21.link/LWG3117>`__","Missing ``packaged_task`` deduction guides","2020-11 (Virtual)","|Complete|","16",""
-"`LWG3143 <https://wg21.link/LWG3143>`__","``monotonic_buffer_resource`` growth policy is unclear","2020-11 (Virtual)","","",""
+"`LWG3143 <https://wg21.link/LWG3143>`__","``monotonic_buffer_resource`` growth policy is unclear","2020-11 (Virtual)","|Complete|","16",""
 "`LWG3195 <https://wg21.link/LWG3195>`__","What is the stored pointer value of an empty ``weak_ptr``?","2020-11 (Virtual)","|Nothing To Do|","",""
 "`LWG3211 <https://wg21.link/LWG3211>`__","``std::tuple<>`` should be trivially constructible","2020-11 (Virtual)","|Complete|","9",""
 "`LWG3236 <https://wg21.link/LWG3236>`__","Random access iterator requirements lack limiting relational operators domain to comparing those from the same range","2020-11 (Virtual)","|Nothing To Do|","",""

libcxx/include/__memory_resource/monotonic_buffer_resource.h

@@ -27,8 +27,8 @@ namespace pmr {
 // [mem.res.monotonic.buffer]
 
 class _LIBCPP_AVAILABILITY_PMR _LIBCPP_EXPORTED_FROM_ABI monotonic_buffer_resource : public memory_resource {
-  static const size_t __default_buffer_capacity  = 1024;
-  static const size_t __default_buffer_alignment = 16;
+  static constexpr size_t __default_buffer_capacity  = 1024;
+  static constexpr size_t __default_growth_factor = 2;
 
   struct __chunk_footer {
     __chunk_footer* __next_;

libcxx/src/memory_resource.cpp

@@ -478,7 +478,7 @@ 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;
   }
 

libcxx/test/std/utilities/utility/mem.res/mem.res.monotonic.buffer/mem.res.monotonic.buffer.mem/allocate_in_geometric_progression.pass.cpp renamed to libcxx/test/std/utilities/utility/mem.res/mem.res.monotonic.buffer/mem.res.monotonic.buffer.mem/allocate_growth_capacity.cpp

@@ -20,10 +20,10 @@
 #include "count_new.h"
 #include "test_macros.h"
 
-void test_geometric_progression() {
+// LWG 3143: https://cplusplus.github.io/LWG/issue3143
+void test_growth_capacity() {
   // mem.res.monotonic.buffer 1.3
-  // Each additional buffer is larger than the previous one, following a
-  // geometric progression.
+  // Each additional buffer is larger than the previous one
 
   globalMemCounter.reset();
   std::pmr::monotonic_buffer_resource mono1(100, std::pmr::new_delete_resource());
@@ -35,23 +35,23 @@ void test_geometric_progression() {
   assert(ret != nullptr);
   assert(globalMemCounter.checkNewCalledEq(1));
   assert(globalMemCounter.last_new_size >= next_buffer_size);
-  next_buffer_size = globalMemCounter.last_new_size + 1;
+  next_buffer_size = globalMemCounter.last_new_size;
 
   int new_called = 1;
   while (new_called < 5) {
     ret = r1.allocate(10, 1);
     if (globalMemCounter.new_called > new_called) {
       assert(globalMemCounter.new_called == new_called + 1);
-      assert(globalMemCounter.last_new_size >= next_buffer_size);
-      next_buffer_size = globalMemCounter.last_new_size + 1;
+      next_buffer_size = next_buffer_size * 2 - 4 * sizeof(void*);
+      assert(globalMemCounter.last_new_size == next_buffer_size);
       new_called += 1;
     }
   }
 }
 
 int main(int, char**) {
 #if TEST_SUPPORTS_LIBRARY_INTERNAL_ALLOCATIONS && !defined(DISABLE_NEW_COUNT)
-  test_geometric_progression();
+  test_growth_capacity();
 #endif
 
   return 0;