PR libstdc++/91788 improve codegen for std::variant<T...>::index()

If __index_type is a smaller type than size_t, then the result of
size_t(__index_type(-1)) is not equal to size_t(-1), but to an incorrect
value such as size_t(255) or size_t(65535). The old implementation of
variant<T...>::index() uses (size_t(__index_type(_M_index + 1)) - 1)
which is always correct, but generates suboptimal code for many common
cases.

When the __index_type is size_t or valueless variants are not possible
we can just return the value directly.

When the number of alternatives is sufficiently small the result of
converting the _M_index value to the corresponding signed type will be
either non-negative or -1. In those cases converting to the signed type
and then to size_t will either produce the correct positive value or
will sign extend -1 to (size_t)-1 as desired.

For the remaining case we keep the existing arithmetic operations to
ensure the correct result.

	PR libstdc++/91788 (partial)
	* include/std/variant (variant::index()): Improve codegen for cases
	where conversion to size_t already works correctly.

From-SVN: r276056

2 files changed, 17 insertions, 1 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 545247e..54fe95f 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,9 @@
+2019-09-23  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/91788 (partial)
+	* include/std/variant (variant::index()): Improve codegen for cases
+	where conversion to size_t already works correctly.
+
 2019-09-23  Andreas Schwab  <schwab@suse.de>
 
 	* config/abi/post/riscv64-linux-gnu/baseline_symbols.txt: Update.
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index d93ea86..c004324 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1518,7 +1518,17 @@ namespace __variant
       { return !this->_M_valid(); }
 
       constexpr size_t index() const noexcept
-      { return size_t(typename _Base::__index_type(this->_M_index + 1)) - 1; }
+      {
+	using __index_type = typename _Base::__index_type;
+	if constexpr (is_same_v<__index_type, size_t>)
+	  return this->_M_index;
+	else if constexpr (__detail::__variant::__never_valueless<_Types...>())
+	  return this->_M_index;
+	else if constexpr (sizeof...(_Types) <= __index_type(-1) / 2)
+	  return make_signed_t<__index_type>(this->_M_index);
+	else
+	  return size_t(__index_type(this->_M_index + 1)) - 1;
+      }
 
       void
       swap(variant& __rhs)