Re: [PATCH] c++: ICE with -fno-elide-constructors and trivial fn [PR101073]

[Jason Merrill <jason@redhat.com>]

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?

> 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.

> -	  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.

>     bool non_constant_args = false;
>     new_call.bindings
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi3.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi3.C
> new file mode 100644
> index 00000000000..ec1c4e53387
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi3.C
> @@ -0,0 +1,17 @@
> +// PR c++/101073
> +// { dg-do compile { target c++11 } }
> +// { dg-additional-options "-fno-elide-constructors" }
> +
> +struct Y
> +{
> +  int a;
> +};
> +
> +struct X
> +{
> +  Y b = Y{1};
> +  Y c = this->b;
> +};
> +
> +constexpr X o = { };
> +static_assert(o.b.a == 1 && o.c.a == 1, "");
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-nsdmi10.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-nsdmi10.C
> new file mode 100644
> index 00000000000..35cb8acc15b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-nsdmi10.C
> @@ -0,0 +1,18 @@
> +// PR c++/101073
> +// { dg-do compile { target c++14 } }
> +// { dg-additional-options "-fno-elide-constructors" }
> +// A copy of constexpr-nsdmi9.C.
> +
> +struct Y
> +{
> +  int a;
> +};
> +
> +struct X
> +{
> +  Y b = (c={5});
> +  Y c = (b={1});
> +};
> +
> +constexpr X o = { };
> +static_assert(o.b.a == 1 && o.c.a == 1, "");
> 
> base-commit: b24e9c083093a9e1b1007933a184c02f7ff058db