On 8/2/22 16:04, Marek Polacek wrote:
In my recent patch which enhanced -Wpessimizing-move so that it warns
about class prvalues too I said that I'd like to extend it so that it
warns in more contexts where a std::move can prevent copy elision, such
as:
T t = std::move(T());
T t(std::move(T()));
T t{std::move(T())};
T t = {std::move(T())};
void foo (T);
foo (std::move(T()));
This patch does that by adding two maybe_warn_pessimizing_move calls.
These must happen before we've converted the initializers otherwise the
std::move will be buried in a TARGET_EXPR.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
PR c++/106276
gcc/cp/ChangeLog:
* call.cc (build_over_call): Call maybe_warn_pessimizing_move.
* cp-tree.h (maybe_warn_pessimizing_move): Declare.
* decl.cc (build_aggr_init_full_exprs): Call
maybe_warn_pessimizing_move.
* typeck.cc (maybe_warn_pessimizing_move): Handle TREE_LIST and
CONSTRUCTOR. Add a bool parameter and use it. Adjust a diagnostic
message.
(check_return_expr): Adjust the call to maybe_warn_pessimizing_move.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/Wpessimizing-move7.C: Add dg-warning.
* g++.dg/cpp0x/Wpessimizing-move8.C: New test.
---
gcc/cp/call.cc | 5 +-
gcc/cp/cp-tree.h | 1 +
gcc/cp/decl.cc | 3 +-
gcc/cp/typeck.cc | 58 ++++++++++++-----
.../g++.dg/cpp0x/Wpessimizing-move7.C | 16 ++---
.../g++.dg/cpp0x/Wpessimizing-move8.C | 65 +++++++++++++++++++
6 files changed, 120 insertions(+), 28 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move8.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 01a7be10077..370137ebd6d 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -9627,10 +9627,13 @@ build_over_call (struct z_candidate *cand, int flags,
tsubst_flags_t complain)
if (!conversion_warning)
arg_complain &= ~tf_warning;
+ if (arg_complain & tf_warning)
+ maybe_warn_pessimizing_move (arg, type, /*return_p*/false);
+
val = convert_like_with_context (conv, arg, fn, i - is_method,
arg_complain);
val = convert_for_arg_passing (type, val, arg_complain);
-
+
if (val == error_mark_node)
return error_mark_node;
else
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 3278b4114bd..5a8af22b509 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8101,6 +8101,7 @@ extern tree finish_right_unary_fold_expr (tree, int);
extern tree finish_binary_fold_expr (tree, tree, int);
extern tree treat_lvalue_as_rvalue_p (tree, bool);
extern bool decl_in_std_namespace_p (tree);
+extern void maybe_warn_pessimizing_move (tree, tree, bool);
/* in typeck2.cc */
extern void require_complete_eh_spec_types (tree, tree);
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 70ad681467e..dc6853a7de1 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -7220,9 +7220,10 @@ check_array_initializer (tree decl, tree type, tree init)
static tree
build_aggr_init_full_exprs (tree decl, tree init, int flags)
-
{
gcc_assert (stmts_are_full_exprs_p ());
+ if (init)
+ maybe_warn_pessimizing_move (init, TREE_TYPE (decl), /*return_p*/false);
This is a surprising place to add this. Why here rather than in
build_aggr_init or check_initializer?
return build_aggr_init (decl, init, flags, tf_warning_or_error);
}
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 9500c4e2fe8..2650beb780e 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -10368,17 +10368,17 @@ treat_lvalue_as_rvalue_p (tree expr, bool return_p)
}
}
-/* Warn about wrong usage of std::move in a return statement. RETVAL
- is the expression we are returning; FUNCTYPE is the type the function
- is declared to return. */
+/* Warn about dubious usage of std::move (in a return statement, if RETURN_P
+ is true). EXPR is the std::move expression; TYPE is the type of the object
+ being initialized. */
-static void
-maybe_warn_pessimizing_move (tree retval, tree functype)
+void
+maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
{
if (!(warn_pessimizing_move || warn_redundant_move))
return;
- location_t loc = cp_expr_loc_or_input_loc (retval);
+ const location_t loc = cp_expr_loc_or_input_loc (expr);
/* C++98 doesn't know move. */
if (cxx_dialect < cxx11)
@@ -10390,14 +10390,32 @@ maybe_warn_pessimizing_move (tree retval, tree
functype)
return;
/* This is only interesting for class types. */
- if (!CLASS_TYPE_P (functype))
+ if (!CLASS_TYPE_P (type))
return;
+ /* A a = std::move (A()); */
+ if (TREE_CODE (expr) == TREE_LIST)
+ {
+ if (list_length (expr) == 1)
+ expr = TREE_VALUE (expr);
+ else
+ return;
+ }
+ /* A a = {std::move (A())};
+ A a{std::move (A())}; */
+ else if (TREE_CODE (expr) == CONSTRUCTOR)
+ {
+ if (CONSTRUCTOR_NELTS (expr) == 1)
+ expr = CONSTRUCTOR_ELT (expr, 0)->value;
+ else
+ return;
+ }
+
/* We're looking for *std::move<T&> ((T &) &arg). */
- if (REFERENCE_REF_P (retval)
- && TREE_CODE (TREE_OPERAND (retval, 0)) == CALL_EXPR)
+ if (REFERENCE_REF_P (expr)
+ && TREE_CODE (TREE_OPERAND (expr, 0)) == CALL_EXPR)
{
- tree fn = TREE_OPERAND (retval, 0);
+ tree fn = TREE_OPERAND (expr, 0);
if (is_std_move_p (fn))
{
tree arg = CALL_EXPR_ARG (fn, 0);
@@ -10409,20 +10427,24 @@ maybe_warn_pessimizing_move (tree retval, tree
functype)
return;
arg = TREE_OPERAND (arg, 0);
arg = convert_from_reference (arg);
- /* Warn if we could do copy elision were it not for the move. */
- if (can_do_nrvo_p (arg, functype))
+ if (can_do_rvo_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"))
+ "moving a temporary object prevents copy "
+ "elision"))
inform (loc, "remove %<std::move%> call");
}
- else if (can_do_rvo_p (arg, functype))
+ /* 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_at (loc, OPT_Wpessimizing_move,
- "moving a temporary object in a return statement "
+ "moving a local object in a return statement "
"prevents copy elision"))
inform (loc, "remove %<std::move%> call");
}
@@ -10433,7 +10455,7 @@ maybe_warn_pessimizing_move (tree retval, tree functype)
{
/* Make sure that overload resolution would actually succeed
if we removed the std::move call. */
- tree t = convert_for_initialization (NULL_TREE, functype,
+ tree t = convert_for_initialization (NULL_TREE, type,
moved,
(LOOKUP_NORMAL
| LOOKUP_ONLYCONVERTING
@@ -10718,7 +10740,7 @@ check_return_expr (tree retval, bool *no_warning)
return NULL_TREE;
if (!named_return_value_okay_p)
- maybe_warn_pessimizing_move (retval, functype);
+ maybe_warn_pessimizing_move (retval, functype, /*return_p*/true);
/* Do any required conversions. */
if (bare_retval == result || DECL_CONSTRUCTOR_P (current_function_decl))
diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
index cd4eaa09aae..a17c7a87b7d 100644
--- a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
+++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
@@ -30,23 +30,23 @@ static A foo ();
A
fn1 ()
{
- return std::move (A{}); // { dg-warning "moving a temporary object in a return
statement prevents copy elision" }
- return std::move (A()); // { dg-warning "moving a temporary object in a return
statement prevents copy elision" }
- return std::move (foo ()); // { dg-warning "moving a temporary object in a return
statement prevents copy elision" }
+ return std::move (A{}); // { dg-warning "moving a temporary object prevents copy
elision" }
+ return std::move (A()); // { dg-warning "moving a temporary object prevents copy
elision" }
+ return std::move (foo ()); // { dg-warning "moving a temporary object prevents
copy elision" }
}
B fn2 ()
{
- return std::move (A());
- return std::move (A{});
- return std::move (foo ());
+ return std::move (A()); // { dg-warning "moving a temporary object prevents copy
elision" }
+ return std::move (A{}); // { dg-warning "moving a temporary object prevents copy
elision" }
+ return std::move (foo ()); // { dg-warning "moving a temporary object prevents
copy elision" }
}
template <typename T1, typename T2>
T1
fn3 ()
{
- return std::move (T2{}); // { dg-warning "moving a temporary object in a return
statement prevents copy elision" }
+ return std::move (T2{}); // { dg-warning "moving a temporary object prevents copy
elision" }
}
void
@@ -58,6 +58,6 @@ do_fn3 ()
char take_buffer;
struct label_text {
- label_text take() { return std::move(label_text(&take_buffer)); } // { dg-warning
"moving a temporary object in a return statement prevents copy elision" }
+ label_text take() { return std::move(label_text(&take_buffer)); } // { dg-warning
"moving a temporary object prevents copy elision" }
label_text(char *);
};
diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move8.C
b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move8.C
new file mode 100644
index 00000000000..51406c8f97f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move8.C
@@ -0,0 +1,65 @@
+// PR c++/106276
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wpessimizing-move" }
+
+// Define std::move.
+namespace std {
+ template<typename _Tp>
+ struct remove_reference
+ { typedef _Tp type; };
+
+ template<typename _Tp>
+ struct remove_reference<_Tp&>
+ { typedef _Tp type; };
+
+ template<typename _Tp>
+ struct remove_reference<_Tp&&>
+ { typedef _Tp type; };
+
+ template<typename _Tp>
+ constexpr typename std::remove_reference<_Tp>::type&&
+ move(_Tp&& __t) noexcept
+ { return static_cast<typename std::remove_reference<_Tp>::type&&>(__t); }
+}
+
+struct A { A(); A(const A&) = delete; A(A&&); };
+struct B { B(A); };
+struct X { };
+
+void foo (A);
+void bar (X);
+
+void
+fn1 ()
+{
+ A a1 = std::move (A()); // { dg-warning "moving a temporary object prevents copy
elision" }
+ A a2 = std::move (A{}); // { dg-warning "moving a temporary object prevents copy
elision" }
+ A a3(std::move (A())); // { dg-warning "moving a temporary object prevents copy
elision" }
+ A a4(std::move (A{})); // { dg-warning "moving a temporary object prevents copy
elision" }
+ A a5{std::move (A())}; // { dg-warning "moving a temporary object prevents copy
elision" }
+ A a6{std::move (A{})}; // { dg-warning "moving a temporary object prevents copy
elision" }
+ A a7 = {std::move (A())}; // { dg-warning "moving a temporary object prevents
copy elision" }
+ A a8 = {std::move (A{})}; // { dg-warning "moving a temporary object prevents
copy elision" }
+
+ B b1 = std::move (A()); // { dg-warning "moving a temporary object prevents copy
elision" }
+ B b2(std::move (A())); // { dg-warning "moving a temporary object prevents copy
elision" }
+ B b3{std::move (A())}; // { dg-warning "moving a temporary object prevents copy
elision" }
+ B b4 = {std::move (A())}; // { dg-warning "moving a temporary object prevents
copy elision" }
+
+ X x1 = std::move (X()); // { dg-warning "moving a temporary object prevents copy
elision" }
+ X x2 = std::move (X{}); // { dg-warning "moving a temporary object prevents copy
elision" }
+ X x3(std::move (X())); // { dg-warning "moving a temporary object prevents copy
elision" }
+ X x4(std::move (X{})); // { dg-warning "moving a temporary object prevents copy
elision" }
+ X x5{std::move (X())}; // { dg-warning "moving a temporary object prevents copy
elision" }
+ X x6{std::move (X{})}; // { dg-warning "moving a temporary object prevents copy
elision" }
+ X x7 = {std::move (X())}; // { dg-warning "moving a temporary object prevents
copy elision" }
+ X x8 = {std::move (X{})}; // { dg-warning "moving a temporary object prevents
copy elision" }
+
+ foo (std::move (A())); // { dg-warning "moving a temporary object prevents copy
elision" }
+ foo (std::move (A{})); // { dg-warning "moving a temporary object prevents copy
elision" }
+ bar (std::move (X())); // { dg-warning "moving a temporary object prevents copy
elision" }
+ bar (std::move (X{})); // { dg-warning "moving a temporary object prevents copy
elision" }
+
+ foo (std::move (a1));
+ bar (std::move (x1));
+}
base-commit: 502605a277d36cee1b0570982a16d97a43eace67
prerequisite-patch-id: f4862bc588ce5fed1d21016fecc4b61feb71eba5
