From 4f241823a281ef7c00a64a0c1d23eeccd6b39879 Mon Sep 17 00:00:00 2001
From: Eric Fiselier <eric@efcs.ca>
Date: Fri, 28 Aug 2015 03:02:37 +0000
Subject: [PATCH] [libcxx] Optimize away unneeded length calculation in
 basic_string::compare(const char*)

Summary:
This patch optimizes basic_string::compare to use strcmp when the default char_traits has been given.
See PR19900 for more information. https://llvm.org/bugs/show_bug.cgi?id=19900

Reviewers: mclow.lists

Subscribers: bkramer, cfe-commits

Differential Revision: http://reviews.llvm.org/D12355

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@246266 91177308-0d34-0410-b5e6-96231b3b80d8
---
 include/string | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/string b/include/string
index 372522ce..504f9e1a 100644
--- a/include/string
+++ b/include/string
@@ -3795,7 +3795,11 @@ bool
 operator==(const _CharT* __lhs,
            const basic_string<_CharT, _Traits, _Allocator>& __rhs) _NOEXCEPT
 {
-    return __rhs.compare(__lhs) == 0;
+    typedef basic_string<_CharT, _Traits, _Allocator> _String;
+    _LIBCPP_ASSERT(__lhs != nullptr, "operator==(char*, basic_string): received nullptr");
+    size_t __lhs_len = _Traits::length(__lhs);
+    if (__lhs_len != __rhs.size()) return false;
+    return __rhs.compare(0, _String::npos, __lhs, __lhs_len) == 0;
 }
 
 template<class _CharT, class _Traits, class _Allocator>
@@ -3804,7 +3808,11 @@ bool
 operator==(const basic_string<_CharT,_Traits,_Allocator>& __lhs,
            const _CharT* __rhs) _NOEXCEPT
 {
-    return __lhs.compare(__rhs) == 0;
+    typedef basic_string<_CharT, _Traits, _Allocator> _String;
+    _LIBCPP_ASSERT(__rhs != nullptr, "operator==(basic_string, char*): received nullptr");
+    size_t __rhs_len = _Traits::length(__rhs);
+    if (__rhs_len != __lhs.size()) return false;
+    return __lhs.compare(0, _String::npos, __rhs, __rhs_len) == 0;
 }
 
 // operator!=