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

[Jason Merrill via Gcc-patches]

On 8/16/22 14:09, Marek Polacek wrote:

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.

Makes sense.  The patch is OK.

But I can move the call into some place in check_initializer if you prefer.

Marek