[PATCH] libstdc++-v3: Optimize 'to_string' with numeric_limits instead of __to_chars_len

[刘可 via Gcc-patches]

Hi!
Gcc5 has implemented 'SSO'. The length of small string local buffer is 15,
which
is enough to store an integer. So we can use
'numeric_limits<int>::digits+1' to
get the max length of int instead of  dynamically obtaining the length of
the
integer through __to_chars_len. In this way, I will get a performance
improvement
of about 15%.

Before optimization:
--------------------------------------------------------------------------------
Benchmark                Time                CPU               Iterations
--------------------------------------------------------------------------------
# to_string<int>
Int2String               191785 ns       191780 ns             3645
# to_string<unsigned>
Unsigned2String    159605 ns       159599 ns             4367

After optimization:
--------------------------------------------------------------------------------
Benchmark                Time                CPU               Iterations
--------------------------------------------------------------------------------
# to_string<int>
Int2String               159382 ns       159381 ns             4354
# to_string<unsigned>
Unsigned2String    136744 ns       136742 ns             5144

2020-09-13 Liuke <liuke.ge...@bytedance.com>

libstdc++-v3/ChangeLog:

        * include/bits/basic_string.h: Use
std::numeric_limits<int>::digits10 instead of __to_chars_len.

Diff:
diff --git a/libstdc++-v3/include/bits/basic_string.h
b/libstdc++-v3/include/bits/basic_string.h
index b61fe05efcf..5cbec537b2f 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -39,6 +39,7 @@
 #include <ext/atomicity.h>
 #include <ext/alloc_traits.h>
 #include <debug/debug.h>
+#include <limits>

 #if __cplusplus >= 201103L
 #include <initializer_list>
@@ -3721,7 +3722,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
   {
     const bool __neg = __val < 0;
     const unsigned __uval = __neg ? (unsigned)~__val + 1u : __val;
-    const auto __len = __detail::__to_chars_len(__uval);
+    const auto __len = std::numeric_limits<int>::digits10 + 1;
     string __str(__neg + __len, '-');
     __detail::__to_chars_10_impl(&__str[__neg], __len, __uval);
     return __str;
@@ -3730,7 +3731,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
   inline string
   to_string(unsigned __val)
   {
-    string __str(__detail::__to_chars_len(__val), '\0');
+    string __str(std::numeric_limits<unsigned>::digits10 + 1;, '\0');
     __detail::__to_chars_10_impl(&__str[0], __str.size(), __val);
     return __str;
   }