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[130.44.146.16]) by smtp.gmail.com with ESMTPSA id d19-20020a0cf0d3000000b0063623c266easm4402794qvl.3.2023.07.25.08.15.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 25 Jul 2023 08:15:09 -0700 (PDT) Message-ID: <006e3a0f-07a3-c99c-647b-11271ab2adec@redhat.com> Date: Tue, 25 Jul 2023 11:15:07 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH v2] c++: fix ICE with constexpr ARRAY_REF [PR110382] To: Marek Polacek Cc: GCC Patches , Patrick Palka References: <20230721223835.630543-1-polacek@redhat.com> <77708a9e-b677-e655-a3cc-a9af6d0b1321@redhat.com> From: Jason Merrill In-Reply-To: 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_H4,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_NONE,TXREP,T_SCC_BODY_TEXT_LINE 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 7/24/23 18:37, Marek Polacek wrote: > On Sat, Jul 22, 2023 at 12:28:59AM -0400, Jason Merrill wrote: >> On 7/21/23 18:38, Marek Polacek wrote: >>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/13? >>> >>> -- >8 -- >>> >>> This code in cxx_eval_array_reference has been hard to get right. >>> In r12-2304 I added some code; in r13-5693 I removed some of it. >>> >>> Here the problematic line is "S s = arr[0];" which causes a crash >>> on the assert in verify_ctor_sanity: >>> >>> gcc_assert (!ctx->object || !DECL_P (ctx->object) >>> || ctx->global->get_value (ctx->object) == ctx->ctor); >>> >>> ctx->object is the VAR_DECL 's', which is correct here. The second >>> line points to the problem: we replaced ctx->ctor in >>> cxx_eval_array_reference: >>> >>> new_ctx.ctor = build_constructor (elem_type, NULL); // #1 >> >> ...and this code doesn't also clear(/set) new_ctx.object like everywhere >> else in constexpr.cc that sets new_ctx.ctor. Fixing that should make the >> testcase work. > > Right, but then we'd be back pre-r12-2304 or r13-5693... > > ...except it should work to always clear the object, like below. > >>> which I think we shouldn't have; the CONSTRUCTOR we created in >>> cxx_eval_constant_expression/DECL_EXPR >>> >>> new_ctx.ctor = build_constructor (TREE_TYPE (r), NULL); >>> >>> had the right type. >> >> Indeed, and using it rather than building a new one seems like a valid >> optimization for trunk. > > Agreed, I kept it. > >> I also notice that the DECL_EXPR code calls unshare_constructor, which >> should be unnecessary if init == ctx->ctor? > > It looks like init == ctx->ctor only happens only with this new testcase. > I'm not sure it's worth it adding code for such a rare case? > >>> We still need #1 though. E.g., in constexpr-96241.C, we never >>> set ctx.ctor/object before calling cxx_eval_array_reference, so >>> we have to build a CONSTRUCTOR there. And in constexpr-101371-2.C >>> we have a ctx.ctor, but it has the wrong type, so we need a new one. >>> >>> PR c++/110382 >>> >>> gcc/cp/ChangeLog: >>> >>> * constexpr.cc (cxx_eval_array_reference): Create a new constructor >>> only when we don't already have a matching one. >>> >>> gcc/testsuite/ChangeLog: >>> >>> * g++.dg/cpp1y/constexpr-110382.C: New test. >>> --- >>> gcc/cp/constexpr.cc | 5 ++++- >>> gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C | 17 +++++++++++++++++ >>> 2 files changed, 21 insertions(+), 1 deletion(-) >>> create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C >>> >>> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc >>> index fb94f3cefcb..518b7c7a2d5 100644 >>> --- a/gcc/cp/constexpr.cc >>> +++ b/gcc/cp/constexpr.cc >>> @@ -4291,7 +4291,10 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t, >>> else >>> val = build_value_init (elem_type, tf_warning_or_error); >>> - if (!SCALAR_TYPE_P (elem_type)) >>> + if (!SCALAR_TYPE_P (elem_type) >>> + /* Create a new constructor only if we don't already have one that >>> + is suitable. */ >>> + && !(ctx->ctor && same_type_p (elem_type, TREE_TYPE (ctx->ctor)))) >> >> We generally use same_type_ignoring_top_level_qualifiers_p in the constexpr >> code. > > True, changed. > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? > > For 13, I guess I should only clear the object and leave out the > same_type_ bit. > > -- >8 -- > This code in cxx_eval_array_reference has been hard to get right. > In r12-2304 I added some code; in r13-5693 I removed some of it. > > Here the problematic line is "S s = arr[0];" which causes a crash > on the assert in verify_ctor_sanity: > > gcc_assert (!ctx->object || !DECL_P (ctx->object) > || ctx->global->get_value (ctx->object) == ctx->ctor); > > ctx->object is the VAR_DECL 's', which is correct here. The second > line points to the problem: we replaced ctx->ctor in > cxx_eval_array_reference: > > new_ctx.ctor = build_constructor (elem_type, NULL); // #1 > > which I think we shouldn't have; the CONSTRUCTOR we created in > cxx_eval_constant_expression/DECL_EXPR > > new_ctx.ctor = build_constructor (TREE_TYPE (r), NULL); > > had the right type. > > We still need #1 though. E.g., in constexpr-96241.C, we never > set ctx.ctor/object before calling cxx_eval_array_reference, so > we have to build a CONSTRUCTOR there. And in constexpr-101371-2.C > we have a ctx.ctor, but it has the wrong type, so we need a new one. > > We can fix the problem by always clearing the object, and, as an > optimization, only create/free a new ctor when actually needed. > > PR c++/110382 > > gcc/cp/ChangeLog: > > * constexpr.cc (cxx_eval_array_reference): Create a new constructor > only when we don't already have a matching one. Clear the object > when the type is non-scalar. > > gcc/testsuite/ChangeLog: > > * g++.dg/cpp1y/constexpr-110382.C: New test. > --- > gcc/cp/constexpr.cc | 17 +++++++++++++++-- > gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C | 17 +++++++++++++++++ > 2 files changed, 32 insertions(+), 2 deletions(-) > create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C > > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc > index fb94f3cefcb..054812bcb72 100644 > --- a/gcc/cp/constexpr.cc > +++ b/gcc/cp/constexpr.cc > @@ -4291,15 +4291,28 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree t, > else > val = build_value_init (elem_type, tf_warning_or_error); > > + bool new_ctor; > if (!SCALAR_TYPE_P (elem_type)) > { > new_ctx = *ctx; > - new_ctx.ctor = build_constructor (elem_type, NULL); > + /* We clear the object here. We used to replace it with T, but that > + caused problems (101371, 108158); and anyway, T is the initializer, > + not the target object. */ > + new_ctx.object = NULL_TREE; > + /* Create a new constructor only if we don't already have a suitable > + one. */ > + new_ctor = (!ctx->ctor > + || !same_type_ignoring_top_level_qualifiers_p > + (elem_type, TREE_TYPE (ctx->ctor))); > + if (new_ctor) > + new_ctx.ctor = build_constructor (elem_type, NULL); I think clearing .object should (only) happen along with changing .ctor; if we leave .ctor alone, we should also leave the matching .object alone. > ctx = &new_ctx; > } > + else > + new_ctor = false; > t = cxx_eval_constant_expression (ctx, val, lval, non_constant_p, > overflow_p); > - if (!SCALAR_TYPE_P (elem_type) && t != ctx->ctor) > + if (new_ctor && t != ctx->ctor) > free_constructor (ctx->ctor); > return t; > } > diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C > new file mode 100644 > index 00000000000..317c5ecfcd5 > --- /dev/null > +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C > @@ -0,0 +1,17 @@ > +// PR c++/110382 > +// { dg-do compile { target c++14 } } > + > +struct S { > + double a = 0; > +}; > + > +constexpr double > +g () > +{ > + S arr[1]; > + S s = arr[0]; > + (void) arr[0]; > + return s.a; > +} > + > +int main() { return g (); } > > base-commit: 96482ffe60d9bdec802fcad705c69641b2a3e040