On Wed, Feb 15, 2023 at 02:39:16PM -0500, Jason Merrill wrote:
> On 2/9/23 09:39, Marek Polacek wrote:
> > In constexpr-nsdmi3.C, with -fno-elide-constructors, we don't elide
> > the Y::Y(const Y&) call used to initialize o.c. So store_init_value
> > -> cxx_constant_init must constexpr-evaluate the call to Y::Y(const Y&)
> > in cxx_eval_call_expression. It's a trivial function, so we do the
> > "Shortcut trivial constructor/op=" code and rather than evaluating
> > the function, we just create an assignment
> >
> > o.c = *(const struct Y &) (const struct Y *) &(&<PLACEHOLDER_EXPR struct
> > X>)->b
> >
> > which is a MODIFY_EXPR, so the preeval code in cxx_eval_store_expression
> > clears .ctor and .object, therefore we can't replace the PLACEHOLDER_EXPR
> > whereupon we crash at
> >
> > /* A placeholder without a referent. We can get here when
> > checking whether NSDMIs are noexcept, or in massage_init_elt;
> > just say it's non-constant for now. */
> > gcc_assert (ctx->quiet);
> >
> > The PLACEHOLDER_EXPR can also be on the LHS as in constexpr-nsdmi10.C.
> > I don't think we can do much here, but I noticed that the whole
> > trivial_fn_p (fun) block is only entered when -fno-elide-constructors.
> > This is true since GCC 9; it wasn't easy to bisect what changes made it
> > so, but r240845 is probably one of them. -fno-elide-constructors is an
> > option for experiments only so it's not clear to me why we'd still want
> > to shortcut trivial constructor/op=. I propose to remove the code and
> > add a checking assert to make sure we're not getting a trivial_fn_p
> > unless -fno-elide-constructors.
>
> Hmm, trivial op= doesn't ever hit this code?
With -fno-elide-constructors we hit the trivial_fn_p block twice in
constexpr-nsdmi9.C, once for "constexpr Y::Y(const Y&)" and then for
"constexpr Y& Y::operator=(Y&&)". So it does hit the code, but only
with -fno-elide-constructors.
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? I don't
> > think I want to backport this.
> >
> > PR c++/101073
> >
> > gcc/cp/ChangeLog:
> >
> > * constexpr.cc (cxx_eval_call_expression): Replace shortcutting trivial
> > constructor/op= with a checking assert.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp0x/constexpr-nsdmi3.C: New test.
> > * g++.dg/cpp1y/constexpr-nsdmi10.C: New test.
> > ---
> > gcc/cp/constexpr.cc | 25 +++----------------
> > gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi3.C | 17 +++++++++++++
> > .../g++.dg/cpp1y/constexpr-nsdmi10.C | 18 +++++++++++++
> > 3 files changed, 38 insertions(+), 22 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi3.C
> > create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-nsdmi10.C
> >
> > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > index 564766c8a00..1d53dcf0f20 100644
> > --- a/gcc/cp/constexpr.cc
> > +++ b/gcc/cp/constexpr.cc
> > @@ -2865,28 +2865,9 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
> > tree t,
> > ctx = &new_ctx;
> > }
> > - /* Shortcut trivial constructor/op=. */
> > - if (trivial_fn_p (fun))
> > - {
> > - tree init = NULL_TREE;
> > - if (call_expr_nargs (t) == 2)
> > - init = convert_from_reference (get_nth_callarg (t, 1));
> > - else if (TREE_CODE (t) == AGGR_INIT_EXPR
> > - && AGGR_INIT_ZERO_FIRST (t))
> > - init = build_zero_init (DECL_CONTEXT (fun), NULL_TREE, false);
> > - if (init)
> > - {
> > - tree op = get_nth_callarg (t, 0);
> > - if (is_dummy_object (op))
> > - op = ctx->object;
> > - else
> > - op = build1 (INDIRECT_REF, TREE_TYPE (TREE_TYPE (op)), op);
> > - tree set = build2 (MODIFY_EXPR, TREE_TYPE (op), op, init);
>
> I think the problem is using MODIFY_EXPR instead of INIT_EXPR to represent a
> constructor; that's why cxx_eval_store_expression thinks it's OK to
> preevaluate. This should properly use those two tree codes for op= and
> ctor, respectively.
Maybe it was so that the RHS in SET could refer to the op in the LHS?
> > - new_ctx.call = &new_call;
> > - return cxx_eval_constant_expression (&new_ctx, set, lval,
> > - non_constant_p, overflow_p);
> > - }
> > - }
> > + /* We used to shortcut trivial constructor/op= here, but nowadays
> > + we can only get a trivial function here with -fno-elide-constructors.
> > */
> > + gcc_checking_assert (!trivial_fn_p (fun) || !flag_elide_constructors);
>
> ...but if this optimization is so rarely triggered, this simplification is
> OK too.
I'd say that's better so that we don't have to update the code (like
r234345 did).
m~
