Low-hanging fruit optimization in string::__move_assign().

vedantk · vedantk · commit fb36d079e57c · 2018-03-08T21:15:26.000Z
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
diff --git 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
@@ -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
