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Add std::to_underlying to the set of stdlib functions that are always folded

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gcc/cp/ChangeLog:
	* cp-gimplify.cc (cp_fold): Add to_underlying.

gcc/testsuite/ChangeLog:
	* g++.dg/opt/pr96780_cpp23.C: New.

libstdc++-v3/ChangeLog:
	* include/std/utility (to_underlying): Add the __always_inline__ attribute.

Signed-off-by: Ville Voutilainen <ville.voutilainen@gmail.com>
This commit is contained in:
Ville Voutilainen
2025-05-12 23:16:46 +03:00
parent 8519b8ba9d
commit 3ecca8f3ad
3 changed files with 22 additions and 9 deletions
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13
gcc/cp/cp-gimplify.cc
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@@ -3343,19 +3343,16 @@ cp_fold (tree x, fold_flags_t flags)
|| id_equal (DECL_NAME (callee), "addressof")
/* This addressof equivalent is used heavily in libstdc++. */
|| id_equal (DECL_NAME (callee), "__addressof")
|| id_equal (DECL_NAME (callee), "to_underlying")
|| id_equal (DECL_NAME (callee), "as_const")))
{
r = CALL_EXPR_ARG (x, 0);
/* Check that the return and argument types are sane before
folding. */
if (INDIRECT_TYPE_P (TREE_TYPE (x))
&& INDIRECT_TYPE_P (TREE_TYPE (r)))
{
if (!same_type_p (TREE_TYPE (x), TREE_TYPE (r)))
r = build_nop (TREE_TYPE (x), r);
x = cp_fold (r, flags);
break;
}
if (!same_type_p (TREE_TYPE (x), TREE_TYPE (r)))
r = build_nop (TREE_TYPE (x), r);
x = cp_fold (r, flags);
break;
}
int sv = optimize, nw = sv;

16
gcc/testsuite/g++.dg/opt/pr96780_cpp23.C Normal file
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@@ -0,0 +1,16 @@
// PR c++/96780
// Verify calls to std::move/forward are folded away by the frontend.
// { dg-do compile { target c++23 } }
// { dg-additional-options "-ffold-simple-inlines -fdump-tree-gimple" }
#include <utility>
enum class A : char {a};
extern A& x;
void f() {
auto&& x1 = std::to_underlying(x);
}
// { dg-final { scan-tree-dump-not "= std::to_underlying" "gimple" } }

2
libstdc++-v3/include/std/utility
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@@ -201,7 +201,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#ifdef __cpp_lib_to_underlying // C++ >= 23
/// Convert an object of enumeration type to its underlying type.
template<typename _Tp>
[[nodiscard]]
[[nodiscard, __gnu__::__always_inline__]]
constexpr underlying_type_t<_Tp>
to_underlying(_Tp __value) noexcept
{ return static_cast<underlying_type_t<_Tp>>(__value); }