[libcxx-commits] [PATCH] D155486: Optimize basic_string's memory usage

[Martijn Vels via Phabricator via libcxx-commits]

mvels updated this revision to Diff 541104.
mvels added a comment.

Upload grow_by amended changes


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D155486/new/

https://reviews.llvm.org/D155486

Files:
  libcxx/include/string
  libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp


Index: libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp
===================================================================
--- libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp
+++ libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp
@@ -13,12 +13,9 @@
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
-#include <string>
 
-#include "test_macros.h"
-
-// alignment of the string heap buffer is hardcoded to 16
-static const std::size_t alignment = 16;
+// alignment of the string heap buffer is hard coded to 8.
+static const std::size_t alignment = 8;
 
 template <class = int>
 TEST_CONSTEXPR_CXX20 void full_size() {
Index: libcxx/include/string
===================================================================
--- libcxx/include/string
+++ libcxx/include/string
@@ -1872,7 +1872,7 @@
         _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
         size_type __align_it(size_type __s) _NOEXCEPT
             {return (__s + (__a-1)) & ~(__a-1);}
-    enum {__alignment = 16};
+    enum {__alignment = 8};
     static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
     size_type __recommend(size_type __s) _NOEXCEPT
     {
@@ -2318,10 +2318,20 @@
     size_type __ms = max_size();
     if (__delta_cap > __ms - __old_cap - 1)
         __throw_length_error();
-    pointer __old_p = __get_pointer();
-    size_type __cap = __old_cap < __ms / 2 - __alignment ?
-                          __recommend(std::max(__old_cap + __delta_cap, 2 * __old_cap)) :
-                          __ms - 1;
+    pointer __old_p;
+    size_type __cap;
+    if (__is_long()) {
+        __old_p = __get_long_pointer();
+        __cap   = __old_cap < __ms / 2 - __alignment
+                    ? __recommend(std::max(__old_cap + __delta_cap, 2 * __old_cap))
+                    : __ms - 1;
+    } else {
+        // On first allocation SSO -> long, do not apply a minimum doubling
+        // of capacity/ This results in a minimum allocation size of 48 and we
+        // only need to guarantee amortized growth on subsequent growth.
+        __old_p = __get_short_pointer();
+        __cap   = __recommend(__old_cap + __delta_cap);
+    }
     auto __allocation = std::__allocate_at_least(__alloc(), __cap + 1);
     pointer __p = __allocation.ptr;
     __begin_lifetime(__p, __allocation.count);
@@ -2359,10 +2369,18 @@
     size_type __ms = max_size();
     if (__delta_cap > __ms - __old_cap)
         __throw_length_error();
-    pointer __old_p = __get_pointer();
-    size_type __cap = __old_cap < __ms / 2 - __alignment ?
-                          __recommend(std::max(__old_cap + __delta_cap, 2 * __old_cap)) :
-                          __ms - 1;
+    if (__is_long()) {
+        __old_p = __get_long_pointer();
+        __cap   = __old_cap < __ms / 2 - __alignment
+                    ? __recommend(std::max(__old_cap + __delta_cap, 2 * __old_cap))
+                    : __ms - 1;
+    } else {
+        // On first allocation SSO -> long, do not apply a minimum doubling
+        // of capacity/ This results in a minimum allocation size of 48 and we
+        // only need to guarantee amortized growth on subsequent growth.
+        __old_p = __get_short_pointer();
+        __cap   = __recommend(__old_cap + __delta_cap);
+    }
     auto __allocation = std::__allocate_at_least(__alloc(), __cap + 1);
     pointer __p = __allocation.ptr;
     __begin_lifetime(__p, __allocation.count);


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