On Tue, Aug 16, 2022 at 03:23:18PM -0400, Jason Merrill wrote: > 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?
IIRC it just felt appropriate since we only want to invoke maybe_warn_ on the full expression, not any subexpressions -- we're looking to see if the outermost expr is a std::move. Also, we want to warn for all types, not just classes. But I can move the call into some place in check_initializer if you prefer. Marek