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[130.44.159.43]) by smtp.gmail.com with ESMTPSA id m65-20020a378a44000000b0073b692623c5sm2616150qkd.129.2023.02.15.11.39.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 15 Feb 2023 11:39:18 -0800 (PST) Message-ID: <40b07900-c063-dd12-2840-efe8a886e538@redhat.com> Date: Wed, 15 Feb 2023 14:39:16 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.2 Subject: Re: [PATCH] c++: ICE with -fno-elide-constructors and trivial fn [PR101073] To: Marek Polacek , GCC Patches References: <20230209173922.30789-1-polacek@redhat.com> From: Jason Merrill In-Reply-To: <20230209173922.30789-1-polacek@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-12.6 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,NICE_REPLY_A,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: 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 *) &(&)->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