From: Marek Polacek Date: Wed, 3 Aug 2022 23:47:30 +0000 (-0400) Subject: c++: Tweak for -Wpessimizing-move in templates [PR89780] X-Git-Tag: basepoints/gcc-14~5111 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=6602a2b2dee16af6e2d451c704789356042b5881;p=thirdparty%2Fgcc.git c++: Tweak for -Wpessimizing-move in templates [PR89780] In my previous patches I've been extending our std::move warnings, but this tweak actually dials it down a little bit. As reported in bug 89780, it's questionable to warn about expressions in templates that were type-dependent, but aren't anymore because we're instantiating the template. As in, template Dest withMove() { T x; return std::move(x); } template Dest withMove(); // #1 template Dest withMove(); // #2 Saying that the std::move is pessimizing for #1 is not incorrect, but it's not useful, because removing the std::move would then pessimize #2. So the user can't really win. At the same time, disabling the warning just because we're in a template would be going too far, I still want to warn for template Dest withMove() { Dest x; return std::move(x); } because the std::move therein will be pessimizing for any instantiation. So I'm using the suppress_warning machinery to that effect. Problem: I had to add a new group to nowarn_spec_t, otherwise suppressing the -Wpessimizing-move warning would disable a whole bunch of other warnings, which we really don't want. PR c++/89780 gcc/cp/ChangeLog: * pt.cc (tsubst_copy_and_build) : Maybe suppress -Wpessimizing-move. * typeck.cc (maybe_warn_pessimizing_move): Don't issue warnings if they are suppressed. (check_return_expr): Disable -Wpessimizing-move when returning a dependent expression. gcc/ChangeLog: * diagnostic-spec.cc (nowarn_spec_t::nowarn_spec_t): Handle OPT_Wpessimizing_move and OPT_Wredundant_move. * diagnostic-spec.h (nowarn_spec_t): Add NW_REDUNDANT enumerator. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/Wpessimizing-move3.C: Remove dg-warning. * g++.dg/cpp0x/Wredundant-move2.C: Likewise. --- diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc index 6c581fe0fb7..fe7e809fc2d 100644 --- a/gcc/cp/pt.cc +++ b/gcc/cp/pt.cc @@ -21215,6 +21215,9 @@ tsubst_copy_and_build (tree t, CALL_EXPR_ORDERED_ARGS (call) = ord; CALL_EXPR_REVERSE_ARGS (call) = rev; } + if (warning_suppressed_p (t, OPT_Wpessimizing_move)) + /* This also suppresses -Wredundant-move. */ + suppress_warning (ret, OPT_Wpessimizing_move); } RETURN (ret); diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc index 19bb7f74321..e95ce8fad14 100644 --- a/gcc/cp/typeck.cc +++ b/gcc/cp/typeck.cc @@ -10447,14 +10447,16 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p) else if (can_do_nrvo_p (arg, type)) { auto_diagnostic_group d; - if (warning_at (loc, OPT_Wpessimizing_move, - "moving a local object in a return statement " - "prevents copy elision")) + if (!warning_suppressed_p (expr, OPT_Wpessimizing_move) + && warning_at (loc, OPT_Wpessimizing_move, + "moving a local object in a return statement " + "prevents copy elision")) inform (loc, "remove % call"); } /* Warn if the move is redundant. It is redundant when we would do maybe-rvalue overload resolution even without std::move. */ else if (warn_redundant_move + && !warning_suppressed_p (expr, OPT_Wredundant_move) && (moved = treat_lvalue_as_rvalue_p (arg, /*return*/true))) { /* Make sure that overload resolution would actually succeed @@ -10699,6 +10701,11 @@ check_return_expr (tree retval, bool *no_warning) /* We don't know if this is an lvalue or rvalue use, but either way we can mark it as read. */ mark_exp_read (retval); + /* Disable our std::move warnings when we're returning + a dependent expression (c++/89780). */ + if (retval && TREE_CODE (retval) == CALL_EXPR) + /* This also suppresses -Wredundant-move. */ + suppress_warning (retval, OPT_Wpessimizing_move); return retval; } diff --git a/gcc/diagnostic-spec.cc b/gcc/diagnostic-spec.cc index 4341ccfaae9..aece89619e7 100644 --- a/gcc/diagnostic-spec.cc +++ b/gcc/diagnostic-spec.cc @@ -96,7 +96,7 @@ nowarn_spec_t::nowarn_spec_t (opt_code opt) case OPT_Winit_self: case OPT_Wuninitialized: case OPT_Wmaybe_uninitialized: - m_bits = NW_UNINIT; + m_bits = NW_UNINIT; break; case OPT_Wdangling_pointer_: @@ -105,6 +105,11 @@ nowarn_spec_t::nowarn_spec_t (opt_code opt) m_bits = NW_DANGLING; break; + case OPT_Wpessimizing_move: + case OPT_Wredundant_move: + m_bits = NW_REDUNDANT; + break; + default: /* A catchall group for everything else. */ m_bits = NW_OTHER; diff --git a/gcc/diagnostic-spec.h b/gcc/diagnostic-spec.h index 28e5e5ccc75..e5f1c127d4f 100644 --- a/gcc/diagnostic-spec.h +++ b/gcc/diagnostic-spec.h @@ -45,9 +45,11 @@ public: NW_DANGLING = 1 << 5, /* All other unclassified warnings. */ NW_OTHER = 1 << 6, + /* Warnings about redundant calls. */ + NW_REDUNDANT = 1 << 7, /* All groups of warnings. */ NW_ALL = (NW_ACCESS | NW_LEXICAL | NW_NONNULL - | NW_UNINIT | NW_VFLOW | NW_DANGLING | NW_OTHER) + | NW_UNINIT | NW_VFLOW | NW_DANGLING | NW_REDUNDANT | NW_OTHER) }; nowarn_spec_t (): m_bits () { } diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C index a1af1230b68..c81f29a4ba6 100644 --- a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C @@ -39,7 +39,7 @@ Tp1 fn2 () { Tp2 t; - return std::move (t); // { dg-warning "moving a local object in a return statement prevents copy elision" } + return std::move (t); } template diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C new file mode 100644 index 00000000000..898040e6bfc --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C @@ -0,0 +1,89 @@ +// PR c++/89780 +// { dg-do compile { target c++11 } } +// { dg-options "-Wpessimizing-move -Wredundant-move" } + +// Define std::move. +namespace std { + template + struct remove_reference + { typedef _Tp type; }; + + template + struct remove_reference<_Tp&> + { typedef _Tp type; }; + + template + struct remove_reference<_Tp&&> + { typedef _Tp type; }; + + template + constexpr typename std::remove_reference<_Tp>::type&& + move(_Tp&& __t) noexcept + { return static_cast::type&&>(__t); } +} + +struct Dest { + Dest() = default; + Dest(Dest&&); + Dest(const Dest&); +}; +struct Source : Dest {}; + +template +Dest withMove() { + T x; + return std::move(x); +} + +template Dest withMove(); +template Dest withMove(); + +template +Dest bar () { + return std::move(T()); // { dg-warning "moving a temporary object prevents copy elision" } +} + +template Dest bar(); +template Dest bar(); + +template +Dest baz (T x) { + return std::move(x); +} + +void +call_baz () +{ + Dest d; + Source s; + baz (d); + baz (s); +} + +template +Dest foo () +{ + Dest d; + return std::move(d); // { dg-warning "moving a local object in a return statement prevents copy elision" } +} + +template Dest foo(); + +template +Dest qux () { + return std::move(Dest()); // { dg-warning "moving a temporary object prevents copy elision" } +} + +template Dest qux(); + +template +Dest qui (Dest x) { + return std::move(x); // { dg-warning "redundant move in return statement" } +} + +void +call_qui () +{ + Dest d; + qui (d); +} diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C index f181afeeb84..6e0aa4b0277 100644 --- a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C @@ -37,14 +37,14 @@ template Tp1 fn2 (Tp2 t) { - return std::move (t); // { dg-warning "redundant move in return statement" } + return std::move (t); } template Tp1 fn3 (Tp2 t) { - return std::move (t); // { dg-warning "redundant move in return statement" } + return std::move (t); } int