c++: Implement -Wrange-loop-construct [PR94695]

This new warning can be used to prevent expensive copies inside range-based
for-loops, for instance:

  struct S { char arr[128]; };
  void fn () {
    S arr[5];
    for (const auto x : arr) {  }
  }

where auto deduces to S and then we copy the big S in every iteration.
Using "const auto &x" would not incur such a copy.  With this patch the
compiler will warn:

q.C:4:19: warning: loop variable 'x' creates a copy from type 'const S' [-Wrange-loop-construct]
    4 |   for (const auto x : arr) {  }
      |                   ^
q.C:4:19: note: use reference type 'const S&' to prevent copying
    4 |   for (const auto x : arr) {  }
      |                   ^
      |                   &

As per Clang, this warning is suppressed for trivially copyable types
whose size does not exceed 64B.  The tricky part of the patch was how
to figure out if using a reference would have prevented a copy.  To
that point, I'm using the new function called ref_conv_binds_directly_p.

This warning is enabled by -Wall.  Further warnings of similar nature
should follow soon.

gcc/c-family/ChangeLog:

	PR c++/94695
	* c.opt (Wrange-loop-construct): New option.

gcc/cp/ChangeLog:

	PR c++/94695
	* call.c (ref_conv_binds_directly_p): New function.
	* cp-tree.h (ref_conv_binds_directly_p): Declare.
	* parser.c (warn_for_range_copy): New function.
	(cp_convert_range_for): Call it.

gcc/ChangeLog:

	PR c++/94695
	* doc/invoke.texi: Document -Wrange-loop-construct.

gcc/testsuite/ChangeLog:

	PR c++/94695
	* g++.dg/warn/Wrange-loop-construct.C: New test.

1 files changed, 64 insertions, 4 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 8905833..cb44227 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -12646,6 +12646,64 @@ do_range_for_auto_deduction (tree decl, tree range_expr)
     }
 }
 
+/* Warns when the loop variable should be changed to a reference type to
+   avoid unnecessary copying.  I.e., from
+
+     for (const auto x : range)
+
+   where range returns a reference, to
+
+     for (const auto &x : range)
+
+   if this version doesn't make a copy.  DECL is the RANGE_DECL; EXPR is the
+   *__for_begin expression.
+   This function is never called when processing_template_decl is on.  */
+
+static void
+warn_for_range_copy (tree decl, tree expr)
+{
+  if (!warn_range_loop_construct
+      || decl == error_mark_node)
+    return;
+
+  location_t loc = DECL_SOURCE_LOCATION (decl);
+  tree type = TREE_TYPE (decl);
+
+  if (from_macro_expansion_at (loc))
+    return;
+
+  if (TYPE_REF_P (type))
+    {
+      /* TODO: Implement reference warnings.  */
+      return;
+    }
+  else if (!CP_TYPE_CONST_P (type))
+    return;
+
+  /* Since small trivially copyable types are cheap to copy, we suppress the
+     warning for them.  64B is a common size of a cache line.  */
+  if (TREE_CODE (TYPE_SIZE_UNIT (type)) != INTEGER_CST
+      || (tree_to_uhwi (TYPE_SIZE_UNIT (type)) <= 64
+	  && trivially_copyable_p (type)))
+    return;
+
+  tree rtype = cp_build_reference_type (type, /*rval*/false);
+  /* If we could initialize the reference directly, it wouldn't involve any
+     copies.  */
+  if (!ref_conv_binds_directly_p (rtype, expr))
+    return;
+
+  auto_diagnostic_group d;
+  if (warning_at (loc, OPT_Wrange_loop_construct,
+		  "loop variable %qD creates a copy from type %qT",
+		  decl, type))
+    {
+      gcc_rich_location richloc (loc);
+      richloc.add_fixit_insert_before ("&");
+      inform (&richloc, "use reference type to prevent copying");
+    }
+}
+
 /* Converts a range-based for-statement into a normal
    for-statement, as per the definition.
 
@@ -12656,7 +12714,7 @@ do_range_for_auto_deduction (tree decl, tree range_expr)
 
       {
 	auto &&__range = RANGE_EXPR;
-	for (auto __begin = BEGIN_EXPR, end = END_EXPR;
+	for (auto __begin = BEGIN_EXPR, __end = END_EXPR;
 	      __begin != __end;
 	      ++__begin)
 	  {
@@ -12756,14 +12814,16 @@ cp_convert_range_for (tree statement, tree range_decl, tree range_expr,
     cp_maybe_mangle_decomp (range_decl, decomp_first_name, decomp_cnt);
 
   /* The declaration is initialized with *__begin inside the loop body.  */
-  cp_finish_decl (range_decl,
-		  build_x_indirect_ref (input_location, begin, RO_UNARY_STAR,
-					tf_warning_or_error),
+  tree deref_begin = build_x_indirect_ref (input_location, begin, RO_UNARY_STAR,
+					   tf_warning_or_error);
+  cp_finish_decl (range_decl, deref_begin,
 		  /*is_constant_init*/false, NULL_TREE,
 		  LOOKUP_ONLYCONVERTING);
   if (VAR_P (range_decl) && DECL_DECOMPOSITION_P (range_decl))
     cp_finish_decomp (range_decl, decomp_first_name, decomp_cnt);
 
+  warn_for_range_copy (range_decl, deref_begin);
+
   return statement;
 }