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[130.44.159.43]) by smtp.gmail.com with ESMTPSA id bq20sm1920889qkb.64.2022.01.20.12.23.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Jan 2022 12:23:25 -0800 (PST) Message-ID: <1ace548d-8408-ecc1-b3ea-f70eeca6adf3@redhat.com> Date: Thu, 20 Jan 2022 15:23:24 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [PATCH] c++: ICE with noexcept and canonical types [PR101715] To: Marek Polacek Cc: Nathan Sidwell , GCC Patches References: <20220115002249.366484-1-polacek@redhat.com> <1ef32dc7-8b16-9fa4-7c7c-649632be6768@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=-13.2 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_LOW, RCVD_IN_MSPIKE_H3, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_NONE, TXREP autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 20 Jan 2022 20:23:31 -0000 On 1/18/22 11:05, Marek Polacek wrote: > On Mon, Jan 17, 2022 at 01:48:48PM -0500, Jason Merrill wrote: >> On 1/14/22 19:22, Marek Polacek wrote: >>> This is a "canonical types differ for identical types" ICE, which started >>> with r11-4682. It's a bit tricky to explain. Consider: >>> >>> template struct S { >>> S bar() noexcept(T::value); // #1 >>> S foo() noexcept(T::value); // #2 >>> }; >>> >>> template S S::foo() noexcept(T::value) {} // #3 >>> >>> We ICE because #3 and #2 have the same type, but their canonical types >>> differ: TYPE_CANONICAL (#3) == #2 but TYPE_CANONICAL (#2) == #1. >>> >>> The member functions #1 and #2 have the same type. However, since their >>> noexcept-specifier is deferred, when parsing them, we create a variant for >>> both of them, because DEFERRED_PARSE cannot be compared. In other words, >>> build_cp_fntype_variant's >>> >>> tree v = TYPE_MAIN_VARIANT (type); >>> for (; v; v = TYPE_NEXT_VARIANT (v)) >>> if (cp_check_qualified_type (v, type, type_quals, rqual, raises, late)) >>> return v; >>> >>> will *not* find an existing variant when creating a method_type for #2, so we >>> have to create a new one. >>> >>> But then we perform delayed parsing and call fixup_deferred_exception_variants >>> for #1 and #2. f_d_e_v will replace TYPE_RAISES_EXCEPTIONS with the newly >>> parsed noexcept-specifier. It also sets TYPE_CANONICAL (#2) to #1. Both >>> noexcepts turned out to be the same, so now we have two equivalent variants in >>> the list! I.e., >>> >>> +-----------------+ +-----------------+ +-----------------+ >>> | main | | #2 | | #1 | >>> | S S::(S*) |----->| S S::(S*) |----->| S S::(S*) |----->NULL >>> | - | | noex(T::value) | | noex(T::value) | >>> +-----------------+ +-----------------+ +-----------------+ >>> >>> Then we get to #3. As for #1 and #2, grokdeclarator calls build_memfn_type, >>> which ends up calling build_cp_fntype_variant, which will use the loop >>> above to look for an existing variant. The first one that matches >>> cp_check_qualified_type will be used, so we use #2 rather than #1, and the >>> TYPE_CANONICAL mismatch follows. Hopefully that makes sense. >> >> Why doesn't the TYPE_CANONICAL (v) == v check prevent this? > > In other words, I think you're asking: why did fixup_deferred_exception_variants > set TYPE_CANONICAL (#2) to #1 (which then differs from TYPE_CANONICAL (#3), > which is #2)? I meant to ask why TYPE_CANONICAL (#3) got set to #2 instead of #1? And to answer my own question, it's because the check I mention is in fixup_deferred_exception_variants, and #3 doesn't go through there at all; the loop in build_cp_fntype_variant assumes no duplicate variants, which your patch fixes. > The method_type for #1 (I'll mark is as #1 here) is built with it being its own > canonical type. > > The first call to fixup_deferred_exception_variants does not change it: in > there, VARIANT is #1, the loop with 'TYPE_CANONICAL (v) == v' cannot find > an existing variant that would match, so when we do > > v = build_cp_fntype_variant (TYPE_CANONICAL (variant), > rqual, cr, false); > we get #1 so > TYPE_CANONICAL (variant) = v; > is just > TYPE_CANONICAL (#1) = #1; > so no change. > > The second call to fixup_deferred_exception_variants: here we're working with > VARIANT #2. Now we again scan the list of variants {main, #2, #1} where we > find a match for #2: #1. #1's TYPE_CANONICAL is #1 as per above, so we set > TYPE_CANONICAL (#2) = #1; > which I think is correct. > > > I think TYPE_CANONICAL (#3) should also be #1, not #2, which my patch attempts > to do. > > > Hope this explanation makes some sense, please ask away if it doesn't! > >>> As for the fix, I didn't think I could rewrite the method_type #2 with #1 >>> because the type may have escaped via decltype. So my approach is to >>> elide #2 from the list, so when looking for a matching variant, we always >>> find #1 (#2 remains live though, which admittedly sounds sort of dodgy). >>> >>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11? >>> >>> PR c++/101715 >>> >>> gcc/cp/ChangeLog: >>> >>> * tree.c (fixup_deferred_exception_variants): Remove duplicate >>> variants after parsing the exception specifications. >>> >>> gcc/testsuite/ChangeLog: >>> >>> * g++.dg/cpp0x/noexcept72.C: New test. >>> * g++.dg/cpp0x/noexcept73.C: New test. >>> --- >>> gcc/cp/tree.c | 16 +++++++++++++++- >>> gcc/testsuite/g++.dg/cpp0x/noexcept72.C | 21 +++++++++++++++++++++ >>> gcc/testsuite/g++.dg/cpp0x/noexcept73.C | 13 +++++++++++++ >>> 3 files changed, 49 insertions(+), 1 deletion(-) >>> create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept72.C >>> create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept73.C >>> >>> diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c >>> index 7f7de86b4e8..2efad49e7c1 100644 >>> --- a/gcc/cp/tree.c >>> +++ b/gcc/cp/tree.c >>> @@ -2804,8 +2804,9 @@ fixup_deferred_exception_variants (tree type, tree raises) >>> /* Though sucky, this walk will process the canonical variants >>> first. */ >>> + tree prev = NULL_TREE; >>> for (tree variant = TYPE_MAIN_VARIANT (type); >>> - variant; variant = TYPE_NEXT_VARIANT (variant)) >>> + variant; prev = variant, variant = TYPE_NEXT_VARIANT (variant)) >>> if (TYPE_RAISES_EXCEPTIONS (variant) == original) >>> { >>> gcc_checking_assert (variant != TYPE_MAIN_VARIANT (type)); >>> @@ -2827,6 +2828,19 @@ fixup_deferred_exception_variants (tree type, tree raises) >>> v = build_cp_fntype_variant (TYPE_CANONICAL (variant), >>> rqual, cr, false); >>> TYPE_CANONICAL (variant) = v; >>> + >>> + /* If VARIANT became a duplicate (cp_check_qualified_type-wise) >>> + of an existing variant in the variant list of TYPE after we >>> + have parsed its exception specification, elide it. Otherwise, >>> + build_cp_fntype_variant would use it, leading to "canonical >>> + types differ for identical types." */ >>> + for (v = TYPE_MAIN_VARIANT (type); v; v = TYPE_NEXT_VARIANT (v)) >>> + if (v != variant >>> + /* The main variant will not have TYPE_RAISES_EXCEPTIONS >>> + so PREV should never be null. */ >>> + && cp_check_qualified_type (v, variant, var_quals, >>> + rqual, cr, false)) >>> + TYPE_NEXT_VARIANT (prev) = TYPE_NEXT_VARIANT (variant); I think we don't two loops through the variants. It ought to work to replace the existing loop with yours; if we find v, we prune and use its TYPE_CANONICAL. Jason