From 6c136d53e89e32fe9d22e41e5df89c08d3470049 Mon Sep 17 00:00:00 2001 From: Marek Polacek Date: Wed, 17 Aug 2022 13:36:52 -0400 Subject: [PATCH] c++: Extend -Wredundant-move for const-qual objects [PR90428] In this PR, Jon suggested extending the -Wredundant-move warning to warn when the user is moving a const object as in: struct T { }; T f(const T& t) { return std::move(t); } where the std::move is redundant, because T does not have a T(const T&&) constructor (which is very unlikely). Even with the std::move, T(T&&) would not be used because it would mean losing the const. Instead, T(const T&) will be called. I had to restructure the function a bit, but it's better now. This patch depends on my other recent patches to maybe_warn_pessimizing_move. PR c++/90428 gcc/cp/ChangeLog: * typeck.cc (can_do_rvo_p): Rename to ... (can_elide_copy_prvalue_p): ... this. (maybe_warn_pessimizing_move): Extend the -Wredundant-move warning to warn about std::move on a const-qualified object. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/Wredundant-move1.C: Adjust dg-warning. * g++.dg/cpp0x/Wredundant-move9.C: Likewise. * g++.dg/cpp0x/Wredundant-move10.C: New test. --- gcc/cp/typeck.cc | 155 +++++++++++------- gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C | 3 +- .../g++.dg/cpp0x/Wredundant-move10.C | 61 +++++++ gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C | 3 +- 4 files changed, 162 insertions(+), 60 deletions(-) create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc index e95ce8fad14..992ebfd99fb 100644 --- a/gcc/cp/typeck.cc +++ b/gcc/cp/typeck.cc @@ -10301,7 +10301,7 @@ can_do_nrvo_p (tree retval, tree functype) prvalue. */ static bool -can_do_rvo_p (tree retval, tree functype) +can_elide_copy_prvalue_p (tree retval, tree functype) { if (functype == error_mark_node) return false; @@ -10415,70 +10415,109 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p) return; } - /* We're looking for *std::move ((T &) &arg). */ - if (REFERENCE_REF_P (expr) - && TREE_CODE (TREE_OPERAND (expr, 0)) == CALL_EXPR) + /* First, check if this is a call to std::move. */ + if (!REFERENCE_REF_P (expr) + || TREE_CODE (TREE_OPERAND (expr, 0)) != CALL_EXPR) + return; + tree fn = TREE_OPERAND (expr, 0); + if (!is_std_move_p (fn)) + return; + tree arg = CALL_EXPR_ARG (fn, 0); + if (TREE_CODE (arg) != NOP_EXPR) + return; + /* If we're looking at *std::move ((T &) &arg), do the pessimizing N/RVO + and implicitly-movable warnings. */ + if (TREE_CODE (TREE_OPERAND (arg, 0)) == ADDR_EXPR) { - tree fn = TREE_OPERAND (expr, 0); - if (is_std_move_p (fn)) + arg = TREE_OPERAND (arg, 0); + arg = TREE_OPERAND (arg, 0); + arg = convert_from_reference (arg); + if (can_elide_copy_prvalue_p (arg, type)) { - tree arg = CALL_EXPR_ARG (fn, 0); - tree moved; - if (TREE_CODE (arg) != NOP_EXPR) - return; - arg = TREE_OPERAND (arg, 0); - if (TREE_CODE (arg) != ADDR_EXPR) - return; - arg = TREE_OPERAND (arg, 0); - arg = convert_from_reference (arg); - if (can_do_rvo_p (arg, type)) + auto_diagnostic_group d; + if (warning_at (loc, OPT_Wpessimizing_move, + "moving a temporary object prevents copy elision")) + inform (loc, "remove % call"); + } + /* The rest of the warnings is only relevant for when we are returning + from a function. */ + if (!return_p) + return; + + tree moved; + /* Warn if we could do copy elision were it not for the move. */ + if (can_do_nrvo_p (arg, type)) + { + auto_diagnostic_group d; + 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 + if we removed the std::move call. */ + tree t = convert_for_initialization (NULL_TREE, type, + moved, + (LOOKUP_NORMAL + | LOOKUP_ONLYCONVERTING + | LOOKUP_PREFER_RVALUE), + ICR_RETURN, NULL_TREE, 0, + tf_none); + /* If this worked, implicit rvalue would work, so the call to + std::move is redundant. */ + if (t != error_mark_node + /* Trying to move something const will never succeed unless + there's T(const T&&), which it almost never is, and if + so, T wouldn't be error_mark_node now: the above convert_ + call with LOOKUP_PREFER_RVALUE returns an error if a const T& + overload is selected. */ + || (CP_TYPE_CONST_P (TREE_TYPE (arg)) + && same_type_ignoring_top_level_qualifiers_p + (TREE_TYPE (arg), type))) { auto_diagnostic_group d; - if (warning_at (loc, OPT_Wpessimizing_move, - "moving a temporary object prevents copy " - "elision")) + if (warning_at (loc, OPT_Wredundant_move, + "redundant move in return statement")) inform (loc, "remove % call"); } - /* The rest of the warnings is only relevant for when we are - returning from a function. */ - else if (!return_p) - return; - /* Warn if we could do copy elision were it not for the move. */ - else if (can_do_nrvo_p (arg, type)) - { - auto_diagnostic_group d; - 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 - if we removed the std::move call. */ - tree t = convert_for_initialization (NULL_TREE, type, - moved, - (LOOKUP_NORMAL - | LOOKUP_ONLYCONVERTING - | LOOKUP_PREFER_RVALUE), - ICR_RETURN, NULL_TREE, 0, - tf_none); - /* If this worked, implicit rvalue would work, so the call to - std::move is redundant. */ - if (t != error_mark_node) - { - auto_diagnostic_group d; - if (warning_at (loc, OPT_Wredundant_move, - "redundant move in return statement")) - inform (loc, "remove % call"); - } - } } + } + /* Also try to warn about redundant std::move in code such as + T f (const T& t) + { + return std::move(t); + } + for which EXPR will be something like + *std::move ((const struct T &) (const struct T *) t) + and where the std::move does nothing if T does not have a T(const T&&) + constructor, because the argument is const. It will not use T(T&&) + because that would mean losing the const. */ + else if (TYPE_REF_P (TREE_TYPE (arg)) + && CP_TYPE_CONST_P (TREE_TYPE (TREE_TYPE (arg)))) + { + tree rtype = TREE_TYPE (TREE_TYPE (arg)); + if (!same_type_ignoring_top_level_qualifiers_p (rtype, type)) + return; + /* Check for the unlikely case there's T(const T&&) (we don't care if + it's deleted). */ + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (rtype))) + if (move_fn_p (fn)) + { + tree t = TREE_VALUE (FUNCTION_FIRST_USER_PARMTYPE (fn)); + if (UNLIKELY (CP_TYPE_CONST_P (TREE_TYPE (t)))) + return; + } + auto_diagnostic_group d; + if (warning_at (loc, OPT_Wredundant_move, + "redundant move in return statement")) + inform (loc, "remove % call"); } } diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C index ce4087b476f..c227019cce1 100644 --- a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C @@ -60,7 +60,8 @@ fn4 (const T t) { // t is const: will decay into copy despite std::move, so it's redundant. // We used to warn about this, but no longer since c++/87378. - return std::move (t); // { dg-warning "redundant move" "" { target c++20 } } + // Now we warn again since c++/90428. + return std::move (t); // { dg-warning "redundant move" } } int diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C new file mode 100644 index 00000000000..a215a4774d6 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C @@ -0,0 +1,61 @@ +// PR c++/90428 +// { dg-do compile { target c++11 } } +// { dg-options "-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 T { T(); T(const T&); T(T&&) = delete; }; +struct S : T { }; +struct W { W(const W&); W(W&&) = delete; W(const W&&); }; + +T f1(T t) +{ + const T& rt = t; + return std::move(rt); // { dg-warning "redundant move" } +} + +T f2(const T& t) +{ + return std::move(t); // { dg-warning "redundant move" } +} + +W f3(const W& w) +{ + return std::move(w); +} + +T f4(const S& s) +{ + return std::move(s); +} + +T f5(const T t) +{ + return std::move(t); // { dg-warning "redundant move" } +} + +struct S1 { S1(S1 &&) = delete; S1(const S1&); }; +struct S2: S1 {}; + +S1 f3(const S2 s) +{ + return std::move(s); // { dg-warning "redundant move" "" { target c++20 } } +} diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C index ca1e23b7a4b..489ecd2b9c9 100644 --- a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C @@ -61,7 +61,8 @@ fn4 (const T t) { // t is const: will decay into copy despite std::move, so it's redundant. // We used to warn about this, but no longer since c++/87378. - return std::move (t); // { dg-warning "redundant move" "" { target c++20 } } + // Now we warn again since c++/90428. + return std::move (t); // { dg-warning "redundant move" } } int