[PATCH] libstdc++: Optimize std::basic_string_view::starts_with

[Jonathan Wakely]

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.
---
 libstdc++-v3/include/std/string_view | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/string_view 
b/libstdc++-v3/include/std/string_view
index a7c5a126461..740aa9344f0 100644
--- 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
-- 
2.45.1