From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [63.128.21.124]) by sourceware.org (Postfix) with ESMTP id 64845386EC54 for ; Mon, 16 Nov 2020 23:52:26 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 64845386EC54 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-292-QAvcISLwOTyXWjD--9ZeOg-1; Mon, 16 Nov 2020 18:52:24 -0500 X-MC-Unique: QAvcISLwOTyXWjD--9ZeOg-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 89E98100746C for ; Mon, 16 Nov 2020 23:52:23 +0000 (UTC) Received: from redhat.com (ovpn-113-80.rdu2.redhat.com [10.10.113.80]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 11BC65D9CC; Mon, 16 Nov 2020 23:52:22 +0000 (UTC) Date: Mon, 16 Nov 2020 18:52:21 -0500 From: Marek Polacek To: Jason Merrill Cc: GCC Patches Subject: Re: [PATCH v2] c++: Extend -Wrange-loop-construct for binding-to-temp [PR94695] Message-ID: <20201116235221.GD3982@redhat.com> References: <20201116033450.1317878-1-polacek@redhat.com> <0b1dccc5-78b6-e2ca-e09b-120dfd5ce3d8@redhat.com> MIME-Version: 1.0 In-Reply-To: <0b1dccc5-78b6-e2ca-e09b-120dfd5ce3d8@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Spam-Status: No, score=-14.4 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H5, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) 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: Mon, 16 Nov 2020 23:52:28 -0000 On Mon, Nov 16, 2020 at 05:02:14PM -0500, Jason Merrill via Gcc-patches wrote: > On 11/15/20 10:34 PM, Marek Polacek wrote: > > [ This year's end-of-stage1 I'm working virtually from American Samoa. ] > > > > This patch finishes the second half of -Wrange-loop-construct I promised > > to implement: it warns when a loop variable in a range-based for-loop is > > initialized with a value of a different type resulting in a copy. For > > instance: > > > > int arr[10]; > > for (const double &x : arr) { ... } > > > > where in every iteration we have to create and destroy a temporary value > > of type double, to which we bind the reference. This could negatively > > impact performance. > > > > As per Clang, this doesn't warn when the range returns a copy, hence the > > glvalue_p check. > > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? > > > > gcc/ChangeLog: > > > > PR c++/94695 > > * doc/invoke.texi: Update the -Wrange-loop-construct description. > > > > gcc/cp/ChangeLog: > > > > PR c++/94695 > > * parser.c (warn_for_range_copy): Warn when the loop variable is > > initialized with a value of a different type resulting in a copy. > > > > gcc/testsuite/ChangeLog: > > > > PR c++/94695 > > * g++.dg/warn/Wrange-loop-construct2.C: New test. > > --- > > gcc/cp/parser.c | 30 ++- > > gcc/doc/invoke.texi | 18 +- > > .../g++.dg/warn/Wrange-loop-construct2.C | 212 ++++++++++++++++++ > > 3 files changed, 256 insertions(+), 4 deletions(-) > > create mode 100644 gcc/testsuite/g++.dg/warn/Wrange-loop-construct2.C > > > > diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c > > index 42f705266bb..e96d0d94c76 100644 > > --- a/gcc/cp/parser.c > > +++ b/gcc/cp/parser.c > > @@ -12681,8 +12681,15 @@ do_range_for_auto_deduction (tree decl, tree range_expr) > > for (const auto &x : range) > > - if this version doesn't make a copy. DECL is the RANGE_DECL; EXPR is the > > - *__for_begin expression. > > + if this version doesn't make a copy. > > + > > + This function also warns when the loop variable is initialized with > > + a value of a different type resulting in a copy: > > + > > + int arr[10]; > > + for (const double &x : arr) > > + > > + DECL is the RANGE_DECL; EXPR is the *__for_begin expression. > > This function is never called when processing_template_decl is on. */ > > static void > > @@ -12700,7 +12707,24 @@ warn_for_range_copy (tree decl, tree expr) > > if (TYPE_REF_P (type)) > > { > > - /* TODO: Implement reference warnings. */ > > + if (!reference_compatible_p (non_reference (type), TREE_TYPE (expr)) > > + && !reference_related_p (non_reference (type), TREE_TYPE (expr)) > > Is there a reason not to use ref_conv_binds_directly_p for this case as > well? No reason. reference_compatible_p also builds up a conversion, so it's not cheaper than ref_conv_binds_directly_p as I mistakenly assumed. Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? -- >8 -- This patch finishes the second half of -Wrange-loop-construct I promised to implement: it warns when a loop variable in a range-based for-loop is initialized with a value of a different type resulting in a copy. For instance: int arr[10]; for (const double &x : arr) { ... } where in every iteration we have to create and destroy a temporary value of type double, to which we bind the reference. This could negatively impact performance. As per Clang, this doesn't warn when the range returns a copy, hence the glvalue_p check. gcc/ChangeLog: PR c++/94695 * doc/invoke.texi: Update the -Wrange-loop-construct description. gcc/cp/ChangeLog: PR c++/94695 * parser.c (warn_for_range_copy): Warn when the loop variable is initialized with a value of a different type resulting in a copy. gcc/testsuite/ChangeLog: PR c++/94695 * g++.dg/warn/Wrange-loop-construct2.C: New test. --- gcc/cp/parser.c | 28 ++- gcc/doc/invoke.texi | 18 +- .../g++.dg/warn/Wrange-loop-construct2.C | 212 ++++++++++++++++++ 3 files changed, 254 insertions(+), 4 deletions(-) create mode 100644 gcc/testsuite/g++.dg/warn/Wrange-loop-construct2.C diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c index 42f705266bb..74584501d11 100644 --- a/gcc/cp/parser.c +++ b/gcc/cp/parser.c @@ -12681,8 +12681,15 @@ do_range_for_auto_deduction (tree decl, tree range_expr) for (const auto &x : range) - if this version doesn't make a copy. DECL is the RANGE_DECL; EXPR is the - *__for_begin expression. + if this version doesn't make a copy. + + This function also warns when the loop variable is initialized with + a value of a different type resulting in a copy: + + int arr[10]; + for (const double &x : arr) + + DECL is the RANGE_DECL; EXPR is the *__for_begin expression. This function is never called when processing_template_decl is on. */ static void @@ -12700,7 +12707,22 @@ warn_for_range_copy (tree decl, tree expr) if (TYPE_REF_P (type)) { - /* TODO: Implement reference warnings. */ + if (!ref_conv_binds_directly_p (type, expr) && glvalue_p (expr)) + { + auto_diagnostic_group d; + if (warning_at (loc, OPT_Wrange_loop_construct, + "loop variable %qD of type %qT binds to a temporary " + "constructed from type %qT", decl, type, + TREE_TYPE (expr))) + { + tree ref = cp_build_qualified_type (TREE_TYPE (expr), + TYPE_QUAL_CONST); + ref = cp_build_reference_type (ref, /*rval*/false); + inform (loc, "use non-reference type %qT to make the copy " + "explicit or %qT to prevent copying", + non_reference (type), ref); + } + } return; } else if (!CP_TYPE_CONST_P (type)) diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index e84a3cf422e..a8d5aa50d82 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -3681,7 +3681,23 @@ void fn () @{ @end smallexample It does not warn when the type being copied is a trivially-copyable type whose -size is less than 64 bytes. This warning is enabled by @option{-Wall}. +size is less than 64 bytes. + +This warning also warns when a loop variable in a range-based for-loop is +initialized with a value of a different type resulting in a copy. For example: + +@smallexample +void fn() @{ + int arr[10]; + for (const double &x : arr) @{ @dots{} @} +@} +@end smallexample + +In the example above, in every iteration of the loop a temporary value of +type @code{double} is created and destroyed, to which the reference +@code{const double &} is bound. + +This warning is enabled by @option{-Wall}. @item -Wredundant-tags @r{(C++ and Objective-C++ only)} @opindex Wredundant-tags diff --git a/gcc/testsuite/g++.dg/warn/Wrange-loop-construct2.C b/gcc/testsuite/g++.dg/warn/Wrange-loop-construct2.C new file mode 100644 index 00000000000..5bb08cc5a1d --- /dev/null +++ b/gcc/testsuite/g++.dg/warn/Wrange-loop-construct2.C @@ -0,0 +1,212 @@ +// PR c++/94695 +// { dg-do compile { target c++11 } } +// { dg-options "-Wrange-loop-construct" } + +struct Foo { }; +struct Bar { + char arr[100]; + Bar(Foo); + Bar(int); + operator int(); +}; + +template +struct It { + T operator*(); + It operator++(); + bool operator!=(const It); +}; + +template +struct Cont { + using I = It; + I begin(); + I end(); +}; + +void +fn1 () +{ + int arr[10]; + Cont cont_int; + + for (const double &x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const double x : arr) { (void) x; } + for (const int &x : arr) { (void) x; } + for (double &&x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (double x : arr) { (void) x; } + + for (const int &&x : cont_int) { (void) x; } + for (const int &x : cont_int) { (void) x; } + for (const int x : cont_int) { (void) x; } + for (int&& x : cont_int) { (void) x; } + for (int x : cont_int) { (void) x; } + + for (const double &&x : cont_int) { (void) x; } + for (const double &x : cont_int) { (void) x; } + for (const double x : cont_int) { (void) x; } + + for (double &&x : cont_int) { (void) x; } + for (double x : cont_int) { (void) x; } + + for (const Bar &&x : cont_int) { (void) x; } + for (const Bar x : cont_int) { (void) x; } +} + +void +fn2 () +{ + Cont cont_int_ref; + + for (const int &x : cont_int_ref) { (void) x; } + for (const int x : cont_int_ref) { (void) x; } + for (int &x : cont_int_ref) { (void) x; } + for (int x : cont_int_ref) { (void) x; } + + for (const double &&x : cont_int_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const double &x : cont_int_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const double x : cont_int_ref) { (void) x; } + for (double &&x : cont_int_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (double x : cont_int_ref) { (void) x; } + + for (const Bar &&x : cont_int_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const Bar &x : cont_int_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const Bar x : cont_int_ref) { (void) x; } + for (Bar &&x : cont_int_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (Bar x : cont_int_ref) { (void) x; } +} + +void +fn3 () +{ + Cont cont_bar; + + for (const Bar &&x : cont_bar) { (void) x; } + for (const Bar &x : cont_bar) { (void) x; } + for (const Bar x : cont_bar) { (void) x; } + for (Bar &&x : cont_bar) { (void) x; } + for (Bar x : cont_bar) { (void) x; } + + for (const int &&x : cont_bar) { (void) x; } + for (const int &x : cont_bar) { (void) x; } + for (const int x : cont_bar) { (void) x; } + for (int &&x : cont_bar) { (void) x; } + for (int x : cont_bar) { (void) x; } +} + +void +fn4 () +{ + Cont cont_bar_ref; + + for (const Bar &x : cont_bar_ref) { (void) x; } + for (Bar &x : cont_bar_ref) { (void) x; } + for (Bar x : cont_bar_ref) { (void) x; } + + for (const int &&x : cont_bar_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const int &x : cont_bar_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const int x : cont_bar_ref) { (void) x; } + for (int &&x : cont_bar_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (int x : cont_bar_ref) { (void) x; } +} + +void +fn5 () +{ + Cont cont_foo; + + for (const Bar &&x : cont_foo) { (void) x; } + for (const Bar &x : cont_foo) { (void) x; } + for (const Bar x : cont_foo) { (void) x; } + for (Bar &&x : cont_foo) { (void) x; } + for (Bar x : cont_foo) { (void) x; } +} + +void +fn6 () +{ + Cont cont_foo_ref; + + for (const Bar &&x : cont_foo_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const Bar &x : cont_foo_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const Bar x : cont_foo_ref) { (void) x; } + for (Bar &&x : cont_foo_ref) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (Bar x : cont_foo_ref) { (void) x; } +} + +void +fn7 () +{ + double arr[2]; + + for (const double &x : arr) { (void) x; } + for (const double x : arr) { (void) x; } + for (double &x : arr) { (void) x; } + for (double x : arr) { (void) x; } + + for (const int &&x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const int &x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const int x : arr) { (void) x; } + for (int &&x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (int x : arr) { (void) x; } + + for (const Bar &&x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const Bar &x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const Bar x : arr) { (void) x; } + for (Bar &&x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (Bar x : arr) { (void) x; } +} + +void +fn8 () +{ + Foo arr[2]; + + for (const Foo &x : arr) { (void) x; } + for (const Foo x : arr) { (void) x; } + for (Foo &x : arr) { (void) x; } + for (Foo x : arr) { (void) x; } + + for (const Bar &&x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const Bar &x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const Bar x : arr) { (void) x; } + for (Bar &&x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (Bar x : arr) { (void) x; } +} + +void +fn9 () +{ + Bar arr[2] = { 1, 2 }; + + for (const Bar &x : arr) { (void) x; } + for (Bar &x : arr) { (void) x; } + for (Bar x : arr) { (void) x; } + + for (const int &&x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const int &x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (const int x : arr) { (void) x; } + for (int &&x : arr) { (void) x; } // { dg-warning "binds to a temporary constructed from type" } + for (int x : arr) { (void) x; } +} + +template +void +fn10 () +{ + Cont cont_bar; + + for (const Bar &x : cont_bar) { (void) x; } + + Cont cont_dep; + for (const T &x : cont_dep) { (void) x; } +} +template void fn10(); + +struct S { + void fn() + { + Cont cont_bar; + for (const Bar &x : cont_bar) { (void) x; } + } +}; base-commit: 814e016318646d06b1662219cc716d502b76d8ce -- 2.28.0