[PATCH] c++: fold calls to std::move/forward [PR96780]

[Patrick Palka <ppalka@redhat.com>]

A well-formed call to std::move/forward is equivalent to a cast, but the
former being a function call means it comes with bloated debug info, which
persists even after the call has been inlined away, for an operation that
is never interesting to debug.

This patch addresses this problem in a relatively ad-hoc way by folding
calls to std::move/forward into casts as part of the frontend's general
expression folding routine.  After this patch with -O2 and a non-checking
compiler, debug info size for some testcases decreases by about ~10% and
overall compile time and memory usage decreases by ~2%.

Bootstrapped and regtested on x86_64-pc-linux-gnu, is this something we
want to consider for GCC 12?

	PR c++/96780

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fold) <case CALL_EXPR>: When optimizing,
	fold calls to std::move/forward into simple casts.
	* cp-tree.h (is_std_move_p, is_std_forward_p): Declare.
	* typeck.cc (is_std_move_p, is_std_forward_p): Export.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/pr96780.C: New test.
---
 gcc/cp/cp-gimplify.cc              | 18 ++++++++++++++++++
 gcc/cp/cp-tree.h                   |  2 ++
 gcc/cp/typeck.cc                   |  6 ++----
 gcc/testsuite/g++.dg/opt/pr96780.C | 24 ++++++++++++++++++++++++
 4 files changed, 46 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/opt/pr96780.C

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index d7323fb5c09..0b009b631c7 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -2756,6 +2756,24 @@ cp_fold (tree x)
 
     case CALL_EXPR:
       {
+	if (optimize
+	    && (is_std_move_p (x) || is_std_forward_p (x)))
+	  {
+	    /* When optimizing, "inline" calls to std::move/forward by
+	       simply folding them into the corresponding cast.  This is
+	       cheaper than relying on the inliner to do so, and also
+	       means we avoid generating useless debug info for them at all.
+
+	       At this point the argument has already been coerced into a
+	       reference, so it suffices to use a NOP_EXPR to express the
+	       reference-to-reference cast.  */
+	    r = CALL_EXPR_ARG (x, 0);
+	    if (!same_type_p (TREE_TYPE (x), TREE_TYPE (r)))
+	      r = build_nop (TREE_TYPE (x), r);
+	    x = cp_fold (r);
+	    break;
+	  }
+
 	int sv = optimize, nw = sv;
 	tree callee = get_callee_fndecl (x);
 
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 37d462fca6e..ab828730b03 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8089,6 +8089,8 @@ 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 bool is_std_move_p		     (tree);
+extern bool is_std_forward_p		     (tree);
 
 /* in typeck2.cc */
 extern void require_complete_eh_spec_types	(tree, tree);
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index bddc83759ad..a3644f8e7f7 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -62,8 +62,6 @@ static bool maybe_warn_about_returning_address_of_local (tree, location_t = UNKN
 static void error_args_num (location_t, tree, bool);
 static int convert_arguments (tree, vec<tree, va_gc> **, tree, int,
                               tsubst_flags_t);
-static bool is_std_move_p (tree);
-static bool is_std_forward_p (tree);
 
 /* Do `exp = require_complete_type (exp);' to make sure exp
    does not have an incomplete type.  (That includes void types.)
@@ -10207,7 +10205,7 @@ decl_in_std_namespace_p (tree decl)
 
 /* Returns true if FN, a CALL_EXPR, is a call to std::forward.  */
 
-static bool
+bool
 is_std_forward_p (tree fn)
 {
   /* std::forward only takes one argument.  */
@@ -10224,7 +10222,7 @@ is_std_forward_p (tree fn)
 
 /* Returns true if FN, a CALL_EXPR, is a call to std::move.  */
 
-static bool
+bool
 is_std_move_p (tree fn)
 {
   /* std::move only takes one argument.  */
diff --git a/gcc/testsuite/g++.dg/opt/pr96780.C b/gcc/testsuite/g++.dg/opt/pr96780.C
new file mode 100644
index 00000000000..ca24b2802bb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/pr96780.C
@@ -0,0 +1,24 @@
+// PR c++/96780
+// Verify calls to std::move/forward are folded away by the frontend.
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-O -fdump-tree-gimple" }
+
+#include <utility>
+
+struct A;
+
+extern A& a;
+extern const A& ca;
+
+void f() {
+  auto&& x1 = std::move(a);
+  auto&& x2 = std::forward<A>(a);
+  auto&& x3 = std::forward<A&>(a);
+
+  auto&& x4 = std::move(ca);
+  auto&& x5 = std::forward<const A>(ca);
+  auto&& x6 = std::forward<const A&>(ca);
+}
+
+// { dg-final { scan-tree-dump-not "= std::move" "gimple" } }
+// { dg-final { scan-tree-dump-not "= std::forward" "gimple" } }
-- 
2.35.1.354.g715d08a9e5