public inbox for gcc-patches@gcc.gnu.org
 help / color / mirror / Atom feed
* [PATCH] c++: Tweak for -Wpessimizing-move in templates [PR89780]
@ 2022-08-04 18:46 Marek Polacek
  2022-08-06 23:02 ` Jason Merrill
  0 siblings, 1 reply; 4+ messages in thread
From: Marek Polacek @ 2022-08-04 18:46 UTC (permalink / raw)
  To: GCC Patches, Jason Merrill

In my previous patches I've been extending our std::move warnings,
but this tweak actually dials it down a little bit.  As reported in
bug 89780, it's questionable to warn about expressions in templates
that were type-dependent, but aren't anymore because we're instantiating
the template.  As in,

  template <typename T>
  Dest withMove() {
    T x;
    return std::move(x);
  }

  template Dest withMove<Dest>(); // #1
  template Dest withMove<Source>(); // #2

Saying that the std::move is pessimizing for #1 is not incorrect, but
it's not useful, because removing the std::move would then pessimize #2.
So the user can't really win.  At the same time, disabling the warning
just because we're in a template would be going too far, I still want to
warn for

  template <typename>
  Dest withMove() {
    Dest x;
    return std::move(x);
  }

because the std::move therein will be pessimizing for any instantiation.

So I'm using the suppress_warning machinery to that effect.
Problem: I had to add a new group to nowarn_spec_t, otherwise
suppressing the -Wpessimizing-move warning would disable a whole bunch
of other warnings, which we really don't want.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

	PR c++/89780

gcc/cp/ChangeLog:

	* pt.cc (tsubst_copy_and_build) <case CALL_EXPR>: Maybe suppress
	-Wpessimizing-move.
	* typeck.cc (maybe_warn_pessimizing_move): Don't issue warnings
	if they are suppressed.
	(check_return_expr): Disable -Wpessimizing-move when returning
	a dependent expression.

gcc/ChangeLog:

	* diagnostic-spec.cc (nowarn_spec_t::nowarn_spec_t): Handle
	OPT_Wpessimizing_move and OPT_Wredundant_move.
	* diagnostic-spec.h (nowarn_spec_t): Add NW_REDUNDANT enumerator.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/Wpessimizing-move3.C: Remove dg-warning.
	* g++.dg/cpp0x/Wpessimizing-move7.C: Likewise.
	* g++.dg/cpp0x/Wredundant-move2.C: Likewise.
	* g++.dg/cpp0x/Wpessimizing-move9.C: New test.
---
 gcc/cp/pt.cc                                  |  3 +
 gcc/cp/typeck.cc                              | 20 +++--
 gcc/diagnostic-spec.cc                        |  7 +-
 gcc/diagnostic-spec.h                         |  4 +-
 .../g++.dg/cpp0x/Wpessimizing-move3.C         |  2 +-
 .../g++.dg/cpp0x/Wpessimizing-move7.C         |  2 +-
 .../g++.dg/cpp0x/Wpessimizing-move9.C         | 89 +++++++++++++++++++
 gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C |  4 +-
 8 files changed, 119 insertions(+), 12 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 6c581fe0fb7..fe7e809fc2d 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -21215,6 +21215,9 @@ tsubst_copy_and_build (tree t,
 		  CALL_EXPR_ORDERED_ARGS (call) = ord;
 		  CALL_EXPR_REVERSE_ARGS (call) = rev;
 		}
+	    if (warning_suppressed_p (t, OPT_Wpessimizing_move))
+	      /* This also suppresses -Wredundant-move.  */
+	      suppress_warning (ret, OPT_Wpessimizing_move);
 	  }
 
 	RETURN (ret);
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 2650beb780e..70a5efc45de 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -10430,9 +10430,10 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
 	  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"))
+	      if (!warning_suppressed_p (expr, OPT_Wpessimizing_move)
+		  && warning_at (loc, OPT_Wpessimizing_move,
+				 "moving a temporary object prevents copy "
+				 "elision"))
 		inform (loc, "remove %<std::move%> call");
 	    }
 	  /* The rest of the warnings is only relevant for when we are
@@ -10443,14 +10444,16 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
 	  else if (can_do_nrvo_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"))
+	      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 %<std::move%> 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
@@ -10695,6 +10698,11 @@ check_return_expr (tree retval, bool *no_warning)
       /* We don't know if this is an lvalue or rvalue use, but
 	 either way we can mark it as read.  */
       mark_exp_read (retval);
+      /* Disable our std::move warnings when we're returning
+	 a dependent expression (c++/89780).  */
+      if (retval && TREE_CODE (retval) == CALL_EXPR)
+	/* This also suppresses -Wredundant-move.  */
+	suppress_warning (retval, OPT_Wpessimizing_move);
       return retval;
     }
 
diff --git a/gcc/diagnostic-spec.cc b/gcc/diagnostic-spec.cc
index 4341ccfaae9..aece89619e7 100644
--- a/gcc/diagnostic-spec.cc
+++ b/gcc/diagnostic-spec.cc
@@ -96,7 +96,7 @@ nowarn_spec_t::nowarn_spec_t (opt_code opt)
     case OPT_Winit_self:
     case OPT_Wuninitialized:
     case OPT_Wmaybe_uninitialized:
-	m_bits = NW_UNINIT;
+      m_bits = NW_UNINIT;
       break;
 
     case OPT_Wdangling_pointer_:
@@ -105,6 +105,11 @@ nowarn_spec_t::nowarn_spec_t (opt_code opt)
       m_bits = NW_DANGLING;
       break;
 
+    case OPT_Wpessimizing_move:
+    case OPT_Wredundant_move:
+      m_bits = NW_REDUNDANT;
+      break;
+
     default:
       /* A catchall group for everything else.  */
       m_bits = NW_OTHER;
diff --git a/gcc/diagnostic-spec.h b/gcc/diagnostic-spec.h
index 28e5e5ccc75..e5f1c127d4f 100644
--- a/gcc/diagnostic-spec.h
+++ b/gcc/diagnostic-spec.h
@@ -45,9 +45,11 @@ public:
      NW_DANGLING = 1 << 5,
      /* All other unclassified warnings.  */
      NW_OTHER = 1 << 6,
+     /* Warnings about redundant calls.  */
+     NW_REDUNDANT = 1 << 7,
      /* All groups of warnings.  */
      NW_ALL = (NW_ACCESS | NW_LEXICAL | NW_NONNULL
-	       | NW_UNINIT | NW_VFLOW | NW_DANGLING | NW_OTHER)
+	       | NW_UNINIT | NW_VFLOW | NW_DANGLING | NW_REDUNDANT | NW_OTHER)
    };
 
   nowarn_spec_t (): m_bits () { }
diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C
index a1af1230b68..c81f29a4ba6 100644
--- a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C
+++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C
@@ -39,7 +39,7 @@ Tp1
 fn2 ()
 {
   Tp2 t;
-  return std::move (t); // { dg-warning "moving a local object in a return statement prevents copy elision" }
+  return std::move (t);
 }
 
 template<typename Tp1, typename Tp2>
diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
index a17c7a87b7d..ebbcb2122a6 100644
--- a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
+++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
@@ -46,7 +46,7 @@ template <typename T1, typename T2>
 T1
 fn3 ()
 {
-  return std::move (T2{}); // { dg-warning "moving a temporary object prevents copy elision" }
+  return std::move (T2{});
 }
 
 void
diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C
new file mode 100644
index 00000000000..9519695eda7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C
@@ -0,0 +1,89 @@
+// PR c++/89780
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wpessimizing-move -Wredundant-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 Dest {
+    Dest() = default;
+    Dest(Dest&&);
+    Dest(const Dest&);
+};
+struct Source : Dest {};
+
+template <typename T>
+Dest withMove() {
+  T x;
+  return std::move(x);
+}
+
+template Dest withMove<Dest>();
+template Dest withMove<Source>();
+
+template<typename T>
+Dest bar () {
+  return std::move(T());
+}
+
+template Dest bar<Dest>();
+template Dest bar<Source>();
+
+template<typename T>
+Dest baz (T x) {
+  return std::move(x);
+}
+
+void
+call_baz ()
+{
+  Dest d;
+  Source s;
+  baz (d);
+  baz (s);
+}
+
+template<typename>
+Dest foo ()
+{
+  Dest d;
+  return std::move(d); // { dg-warning "moving a local object in a return statement prevents copy elision" }
+}
+
+template Dest foo<int>();
+
+template<typename>
+Dest qux () {
+  return std::move(Dest()); // { dg-warning "moving a temporary object prevents copy elision" }
+}
+
+template Dest qux<int>();
+
+template<typename>
+Dest qui (Dest x) {
+  return std::move(x); // { dg-warning "redundant move in return statement" }
+}
+
+void
+call_qui ()
+{
+  Dest d;
+  qui<int> (d);
+}
diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C
index f181afeeb84..6e0aa4b0277 100644
--- a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C
+++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C
@@ -37,14 +37,14 @@ template<typename Tp1, typename Tp2>
 Tp1
 fn2 (Tp2 t)
 {
-  return std::move (t); // { dg-warning "redundant move in return statement" }
+  return std::move (t);
 }
 
 template<typename Tp1, typename Tp2>
 Tp1
 fn3 (Tp2 t)
 {
-  return std::move (t); // { dg-warning "redundant move in return statement" }
+  return std::move (t);
 }
 
 int

base-commit: be58bf98e98bb431ed26ca8be84586075fe8be82
prerequisite-patch-id: f4862bc588ce5fed1d21016fecc4b61feb71eba5
prerequisite-patch-id: ce490c3c0b991fa7f27315387949c145c66223a4
-- 
2.37.1


^ permalink raw reply	[flat|nested] 4+ messages in thread
end of thread, other threads:[~2022-08-11 22:25 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-08-04 18:46 [PATCH] c++: Tweak for -Wpessimizing-move in templates [PR89780] Marek Polacek
2022-08-06 23:02 ` Jason Merrill
2022-08-08 19:51   ` [PATCH v2] " Marek Polacek
2022-08-11 22:24     ` Jason Merrill

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).