Optimization for std::to_string()

[Jonathan Wakely]

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