diff --git a/libcxx/docs/UsingLibcxx.rst b/libcxx/docs/UsingLibcxx.rst
--- a/libcxx/docs/UsingLibcxx.rst
+++ b/libcxx/docs/UsingLibcxx.rst
@@ -241,6 +241,13 @@
   warning saying that `std::auto_ptr` is deprecated. If the macro is defined,
   no warning will be emitted. By default, this macro is not defined.
 
+**_LIBCPP_STRING_PRECISE_RESIZE**:
+  This macro enables using precise growth in `std::string::resize/__resize_default_init`,
+  replacing the default behavior of amortized exponential growth. This is a
+  tradeoff of saving memory at the expense of increased CPU usage if there are
+  cases of `resize` being used to grow a particular string repeatedly. Note
+  that `reserve` uses precise growth by default.
+
 C++17 Specific Configuration Macros
 -----------------------------------
 **_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES**:
diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -3276,9 +3276,12 @@
 basic_string<_CharT, _Traits, _Allocator>::resize(size_type __n, value_type __c)
 {
     size_type __sz = size();
-    if (__n > __sz)
+    if (__n > __sz) {
+        #ifdef _LIBCPP_STRING_PRECISE_RESIZE
+        if (__n > capacity()) __shrink_or_extend(__recommend(__n));
+        #endif
         append(__n - __sz, __c);
-    else
+    } else
         __erase_to_end(__n);
 }
 
@@ -3288,6 +3291,9 @@
 {
     size_type __sz = size();
     if (__n > __sz) {
+      #ifdef _LIBCPP_STRING_PRECISE_RESIZE
+      if (__n > capacity()) __shrink_or_extend(__recommend(__n));
+      #endif
        __append_default_init(__n - __sz);
     } else
         __erase_to_end(__n);