[committed] libstdc++: Micro-optimize construction of named std::locale

[committed] libstdc++: Micro-optimize construction of named std::locale

[Jonathan Wakely]

Tested x86_64-linux. Pushed to trunk.

-- >8 --

This shaves about 100ns off the std::locale constructor for named
locales (which is only about 1% of the total time).

Using !*s instead of !strcmp(s, "") doesn't make any difference as GCC
optimizes that already even at -O1. !strcmp(s, "C") is optimized at -O2
so replacing that with s[0] == 'C' && s[1] == '\0' only matters for the
--enable-libstdcxx-debug builds. But !strcmp(s, "POSIX") always makes a
call to strcmp at any optimization level. We make that strcmp call,
maybe several times, for any locale name except for "C" (which will be
matched before we get to the check for "POSIX").

For most targets, locale names begin with a lowercase letter and the
only one that begins with 'P' is "POSIX". Replacing !strcmp(s, "POSIX")
with s[0] == 'P' && !strcmp(s+1, "OSIX") means that we avoid calling
strcmp unless the string really does match "POSIX".

Maybe more importantly, I find is_C_locale(s) easier to read than
strcmp(s, "C") == 0 || strcmp(s, "POSIX") == 0, and !is_C_locale(s)
easier to read than strcmp(s, "C") != 0 && strcmp(s, "POSIX") != 0.

libstdc++-v3/ChangeLog:

	* src/c++98/localename.cc (is_C_locale): New function.
	(locale::locale(const char*)): Use is_C_locale.
---
 libstdc++-v3/src/c++98/localename.cc | 39 ++++++++++++++++------------
 1 file changed, 23 insertions(+), 16 deletions(-)

diff --git a/libstdc++-v3/src/c++98/localename.cc b/libstdc++-v3/src/c++98/localename.cc
index 25e6d966dca..68cb81d0709 100644
--- a/libstdc++-v3/src/c++98/localename.cc
+++ b/libstdc++-v3/src/c++98/localename.cc
@@ -36,24 +36,37 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   using namespace __gnu_cxx;
 
+  static inline bool
+  is_C_locale(const char* s)
+  {
+    switch (s[0])
+    {
+    case 'C':
+      return s[1] == '\0';
+    case 'P':
+      return !std::strcmp(s+1, "OSIX");
+    default:
+      return false;
+    }
+  }
+
   locale::locale(const char* __s) : _M_impl(0)
   {
     if (__s)
       {
 	_S_initialize();
-	if (std::strcmp(__s, "C") == 0 || std::strcmp(__s, "POSIX") == 0)
+	if (is_C_locale(__s))
 	  (_M_impl = _S_classic)->_M_add_reference();
-	else if (std::strcmp(__s, "") != 0)
+	else if (*__s)
 	  _M_impl = new _Impl(__s, 1);
 	else
 	  {
 	    // Get it from the environment.
 	    char* __env = std::getenv("LC_ALL");
 	    // If LC_ALL is set we are done.
-	    if (__env && std::strcmp(__env, "") != 0)
+	    if (__env && *__env)
 	      {
-		if (std::strcmp(__env, "C") == 0
-		    || std::strcmp(__env, "POSIX") == 0)
+		if (is_C_locale(__env))
 		  (_M_impl = _S_classic)->_M_add_reference();
 		else
 		  _M_impl = new _Impl(__env, 1);
@@ -63,9 +76,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		// LANG may set a default different from "C".
 		string __lang;
 		__env = std::getenv("LANG");
-		if (!__env || std::strcmp(__env, "") == 0
-		    || std::strcmp(__env, "C") == 0
-		    || std::strcmp(__env, "POSIX") == 0)
+		if (!__env || !*__env || is_C_locale(__env))
 		  __lang = "C";
 		else
 		  __lang = __env;
@@ -77,17 +88,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		  for (; __i < _S_categories_size; ++__i)
 		    {
 		      __env = std::getenv(_S_categories[__i]);
-		      if (__env && std::strcmp(__env, "") != 0
-			  && std::strcmp(__env, "C") != 0
-			  && std::strcmp(__env, "POSIX") != 0)
+		      if (__env && *__env && !is_C_locale(__env))
 			break;
 		    }
 		else
 		  for (; __i < _S_categories_size; ++__i)
 		    {
 		      __env = std::getenv(_S_categories[__i]);
-		      if (__env && std::strcmp(__env, "") != 0
-			  && __lang != __env)
+		      if (__env && *__env && __lang != __env)
 			break;
 		    }
 
@@ -113,14 +121,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		      {
 			__env = std::getenv(_S_categories[__i]);
 			__str += _S_categories[__i];
-			if (!__env || std::strcmp(__env, "") == 0)
+			if (!__env || !*__env)
 			  {
 			    __str += '=';
 			    __str += __lang;
 			    __str += ';';
 			  }
-			else if (std::strcmp(__env, "C") == 0
-				 || std::strcmp(__env, "POSIX") == 0)
+			else if (is_C_locale(__env))
 			  __str += "=C;";
 			else
 			  {
-- 
2.41.0