diff options
author | 2023-09-11 11:20:53 -0700 | |
---|---|---|
committer | 2023-09-11 11:22:15 -0700 | |
commit | f3eae68bd1d17df708f2b59a43ad7e837a616e6a (patch) | |
tree | d292bc842785f0d734d10cbc0e35431547924543 /absl/strings | |
parent | 317085adc4e4e474eb4546fa9d37f74c61d95f7f (diff) |
Fixes StrCat() performance regression when not using libc++
65d7b6d changed StrCat() to not use an intermediate buffer when the
result fits in the SSO buffer, but only libc++ has an SSO buffer large
enough for this optimization to work.
PiperOrigin-RevId: 564447163
Change-Id: I0c7fa4afed3369b36e13e7d1691eb7f933ea0091
Diffstat (limited to 'absl/strings')
-rw-r--r-- | absl/strings/str_cat.h | 23 |
1 files changed, 20 insertions, 3 deletions
diff --git a/absl/strings/str_cat.h b/absl/strings/str_cat.h index 57a5823f..554c89de 100644 --- a/absl/strings/str_cat.h +++ b/absl/strings/str_cat.h @@ -498,11 +498,28 @@ inline std::string SingleArgStrCat(unsigned long long x) { inline std::string SingleArgStrCat(float x) { return FloatToString(x); } inline std::string SingleArgStrCat(double x) { return FloatToString(x); } - -template <typename T, typename = std::enable_if_t<std::is_arithmetic<T>{} && - !std::is_same<T, char>{}>> +// As of September 2023, the SingleArgStrCat() optimization is only enabled for +// libc++. The reasons for this are: +// 1) The SSO size for libc++ is 23, while libstdc++ and MSSTL have an SSO size +// of 15. Since IntegerToString unconditionally resizes the string to 22 bytes, +// this causes both libstdc++ and MSSTL to allocate. +// 2) strings_internal::STLStringResizeUninitialized() only has an +// implementation that avoids initialization when using libc++. This isn't as +// relevant as (1), and the cost should be benchmarked if (1) ever changes on +// libstc++ or MSSTL. +#ifdef _LIBCPP_VERSION +#define ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE true +#else +#define ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE false +#endif + +template <typename T, typename = std::enable_if_t< + ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE && + std::is_arithmetic<T>{} && !std::is_same<T, char>{}>> using EnableIfFastCase = T; +#undef ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE + } // namespace strings_internal ABSL_MUST_USE_RESULT inline std::string StrCat() { return std::string(); } |