Re: [PATCH] c++: Extend -Wpessimizing-move to other contexts

[Marek Polacek <polacek@redhat.com>]

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