From fb36d079e57cf56819bfbc1a1c2c8282accb58d5 Mon Sep 17 00:00:00 2001
From: Vedant Kumar <vsk@apple.com>
Date: Thu, 8 Mar 2018 21:15:26 +0000
Subject: [PATCH] Low-hanging fruit optimization in string::__move_assign().

shrink_to_fit() ends up doing a lot work to get information that we
already know since we just called clear(). This change seems concise
enough to be worth the couple extra lines and my benchmarks show that it
is indeed a pretty decent win. It looks like the same thing is going on
twice in __copy_assign_alloc(), but I didn't want to go overboard since
this is my first contribution to llvm/libc++.

Patch by Timothy VanSlyke!

Differential Revision: https://reviews.llvm.org/D41976

llvm-svn: 327064
---
 libcxx/include/string                              | 27 +++++++++---
 .../string.modifiers/clear_and_shrink_db1.pass.cpp | 48 ++++++++++++++++++++++
 2 files changed, 69 insertions(+), 6 deletions(-)
 create mode 100644 libcxx/test/libcxx/strings/basic.string/string.modifiers/clear_and_shrink_db1.pass.cpp

diff --git a/libcxx/include/string b/libcxx/include/string
index e8ec709..2630799 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1257,6 +1257,8 @@ public:
 
     _LIBCPP_INLINE_VISIBILITY bool __invariants() const;
 
+    _LIBCPP_INLINE_VISIBILITY void __clear_and_shrink();
+    
     _LIBCPP_INLINE_VISIBILITY
     bool __is_long() const _NOEXCEPT
         {return bool(__r_.first().__s.__size_ & __short_mask);}
@@ -1426,16 +1428,14 @@ private:
             {
                 if (!__str.__is_long())
                 {
-                    clear();
-                    shrink_to_fit();
+                    __clear_and_shrink();
                     __alloc() = __str.__alloc();
                 }
                 else
                 {
                     allocator_type __a = __str.__alloc();
                     pointer __p = __alloc_traits::allocate(__a, __str.__get_long_cap());
-                    clear();
-                    shrink_to_fit();
+                    __clear_and_shrink();
                     __alloc() = _VSTD::move(__a);
                     __set_long_pointer(__p);
                     __set_long_cap(__str.__get_long_cap());
@@ -2125,8 +2125,7 @@ basic_string<_CharT, _Traits, _Allocator>::__move_assign(basic_string& __str, tr
     _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value)
 #endif
 {
-    clear();
-    shrink_to_fit();
+    __clear_and_shrink();
     __r_.first() = __str.__r_.first();
     __move_assign_alloc(__str);
     __str.__zero();
@@ -3579,6 +3578,22 @@ basic_string<_CharT, _Traits, _Allocator>::__invariants() const
     return true;
 }
 
+// __clear_and_shrink
+
+template<class _CharT, class _Traits, class _Allocator>
+inline _LIBCPP_INLINE_VISIBILITY
+void 
+basic_string<_CharT, _Traits, _Allocator>::__clear_and_shrink()
+{
+    clear();
+    if(__is_long())
+    {
+        __alloc_traits::deallocate(__alloc(), __get_long_pointer(), capacity() + 1);
+        __set_long_cap(0);
+        __set_short_size(0);
+    }
+} 
+
 // operator==
 
 template<class _CharT, class _Traits, class _Allocator>
diff --git a/libcxx/test/libcxx/strings/basic.string/string.modifiers/clear_and_shrink_db1.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.modifiers/clear_and_shrink_db1.pass.cpp
new file mode 100644
index 0000000..b7e278b
--- /dev/null
+++ b/libcxx/test/libcxx/strings/basic.string/string.modifiers/clear_and_shrink_db1.pass.cpp
@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+
+// Call __clear_and_shrink() and ensure string invariants hold
+
+#if _LIBCPP_DEBUG >= 1
+
+#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
+
+#include <string>
+#include <cassert>
+
+int main()
+{
+    std::string l = "Long string so that allocation definitely, for sure, absolutely happens. Probably.";
+    std::string s = "short";
+
+    assert(l.__invariants());
+    assert(s.__invariants());
+    
+    s.__clear_and_shrink();
+    assert(s.__invariants());
+    assert(s.size() == 0);
+
+    { 
+    std::string::size_type cap = l.capacity();
+    l.__clear_and_shrink();
+    assert(l.__invariants());
+    assert(l.size() == 0);
+    assert(l.capacity() < cap);
+    }
+}
+
+#else
+
+int main()
+{
+}
+
+#endif
-- 
2.7.4