libstdc++-v3: Optimize 'to_string<int>' with numeric_limits instead of __to_chars_len
Commit Message
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.gehry@bytedance.com>
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h: Use
std::numeric_limits<int>::digits10 instead of __to_chars_len.
Diff:
Comments
Please CC libstdc++ patches to the libstdc++ list, or they won't get
reviewed (because I don't subscribe to gcc-patches).
GCC 5 does implement SSO but it's only used conditionally. Your patch
uses numeric_limits unconditionally, which will result in
over-allocation for COW strings.
There also seems to be a syntax error in the unsigned overload, was
this patch tested?
Minor: the "component" tag in the subject should be just "libstdc++:"
without the "-v3" part.
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;
}