From: redi <redi@138bc75d-0d04-0410-961f-82ee72b054a4>
Date: Tue, 25 Sep 2018 14:59:16 +0000 (+0000)
Subject: PR libstdc++/87431 optimise valueless_by_exception()
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=bc35ec6d4ca5e90065942577a27af6c6088187fa;p=thirdparty%2Fgcc.git

PR libstdc++/87431 optimise valueless_by_exception()

If a std::variant can never get into valueless state then we don't need
to do a runtime check for a valid alternative.

	PR libstdc++/87431
	* include/std/variant (_Variant_storage<true, _Types...>::_M_valid):
	Avoid runtime test when all alternatives are scalars and so cannot
	throw during initialization.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@264574 138bc75d-0d04-0410-961f-82ee72b054a4
---

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index ff5301774c7f..67b3eb4a94eb 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,10 @@
+2018-09-25  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/87431
+	* include/std/variant (_Variant_storage<true, _Types...>::_M_valid):
+	Avoid runtime test when all alternatives are scalars and so cannot
+	throw during initialization.
+
 2018-09-25  Gerald Pfeifer  <gerald@pfeifer.com>
 
 	* doc/xml/manual/codecvt.xml: Move link to "UTF-8 and Unicode FAQ"
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 5a77e9e2d842..9289eef28cf6 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -439,6 +439,8 @@ namespace __variant
       constexpr bool
       _M_valid() const noexcept
       {
+	if constexpr ((is_scalar_v<_Types> && ...))
+	  return true;
 	return this->_M_index != __index_type(variant_npos);
       }