From: Jonathan Wakely <jwakely@redhat.com>
Date: Thu, 30 May 2024 19:36:42 +0000 (+0100)
Subject: libstdc++: Optimize std::basic_string_view::starts_with
X-Git-Tag: basepoints/gcc-16~8559
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=482f97e79fc29ea2d61f1425b32564a668b51e1c;p=thirdparty%2Fgcc.git

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 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