On Tue, 13 May 2025, Ville Voutilainen wrote:
> On Tue, 13 May 2025 at 23:34, Jason Merrill <[email protected]> wrote:
> >
> > On 5/13/25 4:29 PM, Ville Voutilainen wrote:
> > > On Tue, 13 May 2025 at 23:23, Jason Merrill <[email protected]> wrote:
> > >>> /* 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 (id_equal (DECL_NAME (callee), "to_underlying")
> > >>> + || (INDIRECT_TYPE_P (TREE_TYPE (x))
> > >>> + && INDIRECT_TYPE_P (TREE_TYPE (r))))
> > >>
> > >> Maybe instead of INDIRECT_TYPE_P, check ! AGGREGATE_TYPE_P ? I'm not
> > >> sure how much sanity checking we need here, maybe we should just trust
> > >> the library.
> > >
> > > I.. ..don't think AGGREGATE_TYPE_P is right?
> >
> > I was suggesting *not* AGGREGATE_TYPE_P, because build_nop doesn't work
> > for such types.
>
> Ah, yes, got it. I did spot the 'not' but was wondering whether that
> covers all the right cases, but for references and enumerators, I
> guess
> it would.
>
> > > It seems to me that this
> > > sanity check is checking that it's a cast between
> > > references for the other ones, and that's just unsuitable for
> > > to_underlying. I would suggest just removing that sanity
> > > check and trusting the library.
> >
> > That's fine too.
>
> I'll test the attached.
> diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
> index d2423fd1848..4e5d383daeb 100644
> --- a/gcc/cp/cp-gimplify.cc
> +++ b/gcc/cp/cp-gimplify.cc
> @@ -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. */
This comment should be removed too, along with the sanity check.
> - 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;
> diff --git a/gcc/testsuite/g++.dg/opt/pr96780_cpp23.C
> b/gcc/testsuite/g++.dg/opt/pr96780_cpp23.C
> new file mode 100644
> index 00000000000..ba4a837dded
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/opt/pr96780_cpp23.C
> @@ -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" } }
> diff --git a/libstdc++-v3/include/std/utility
> b/libstdc++-v3/include/std/utility
> index 1c15c75f92b..8a85ccfd09b 100644
> --- a/libstdc++-v3/include/std/utility
> +++ b/libstdc++-v3/include/std/utility
> @@ -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); }
> >
