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libstdc++: Micro-optimize construction of named std::locale

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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.
This commit is contained in:
Jonathan Wakely 2023-07-19 09:56:58 +01:00
parent cc3d7baf27
commit 74c019b50b
1 changed files with 23 additions and 16 deletions
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39
libstdc++-v3/src/c++98/localename.cc
View file

@ -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
{