On 20/07/18 12:44 +0300, niXman wrote:
Hi,
Patch in attachments.
Thanks. How did you verify it's an optimization?
The to_string functions always pass at least __n=16 to __to_xstring,
which is larger than the SSO buffer in std::__cxx11::string, and so
forces a memory allocation.
The current code uses alloca and if the result fits in the SSO buffer
there is no allocation. e.g. to_string(999999) doesn't allocate from
the heap at all for the new std::__cxx11::string.
For the old COW string this doesn't make much difference, because we
always allocate. Your patch will mean we return a string with excess
capacity, rather than one of exactly capacity()==length(). That isn't
a problem.
Tested on x86_64-linux-gnu.
--
Regards, niXman
___________________________________________________
C++ for Bitcoins: github.com/niXman
Index: libstdc++-v3/include/ext/string_conversions.h
===================================================================
--- libstdc++-v3/include/ext/string_conversions.h (revision 262879)
+++ libstdc++-v3/include/ext/string_conversions.h (working copy)
@@ -100,19 +100,18 @@
__builtin_va_list), std::size_t __n,
const _CharT* __fmt, ...)
{
- // XXX Eventually the result should be constructed in-place in
- // the __cxx11 string, likely with the help of internal hooks.
- _CharT* __s = static_cast<_CharT*>(__builtin_alloca(sizeof(_CharT)
- * __n));
+ _String __str(__n, 0);
__builtin_va_list __args;
__builtin_va_start(__args, __fmt);
- const int __len = __convf(__s, __n, __fmt, __args);
+ const std::size_t __len = __convf(&__str[0], __n, __fmt, __args);
__builtin_va_end(__args);
- return _String(__s, __s + __len);
+ __str.resize(__len);
+
+ return __str;
}
_GLIBCXX_END_NAMESPACE_VERSION
