libstdc++: Optimize std::basic_string_view::starts_with

We get smaller code at all optimization levels by not creating a
temporary object, just comparing lengths first and then using
traits_type::compare. This does less work than calling substr then
operator==.

libstdc++-v3/ChangeLog:

	* include/std/string_view (starts_with(basic_string_view)):
	Compare lengths first and then call traits_type::compare
	directly.

diff --git a/libstdc++-v3/include/std/string_view b/libstdc++-v3/include/std/string_view
index a7c5a1264613ad8d4c805023046f3c801e4b9c0b..740aa9344f0147594559c74d90879a689e45c854 100644 (file)
--- a/libstdc++-v3/include/std/string_view
+++ b/libstdc++-v3/include/std/string_view
@@ -385,7 +385,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       [[nodiscard]]
       constexpr bool
       starts_with(basic_string_view __x) const noexcept
-      { return this->substr(0, __x.size()) == __x; }
+      {
+       return _M_len >= __x._M_len
+         && traits_type::compare(_M_str, __x._M_str, __x._M_len) == 0;
+      }
 
       [[nodiscard]]
       constexpr bool