[PATCH v6 3/8] C++: Support clang compatible [[musttail]] (PR83324)

[Andi Kleen <ak@linux.intel.com>]

This patch implements a clang compatible [[musttail]] attribute for
returns.

musttail is useful as an alternative to computed goto for interpreters.
With computed goto the interpreter function usually ends up very big
which causes problems with register allocation and other per function
optimizations not scaling. With musttail the interpreter can be instead
written as a sequence of smaller functions that call each other. To
avoid unbounded stack growth this requires forcing a sibling call, which
this attribute does. It guarantees an error if the call cannot be tail
called which allows the programmer to fix it instead of risking a stack
overflow. Unlike computed goto it is also type-safe.

It turns out that David Malcolm had already implemented middle/backend
support for a musttail attribute back in 2016, but it wasn't exposed
to any frontend other than a special plugin.

This patch adds a [[gnu::musttail]] attribute for C++ that can be added
to return statements. The return statement must be a direct call
(it does not follow dependencies), which is similar to what clang
implements. It then uses the existing must tail infrastructure.

For compatibility it also detects clang::musttail

One problem is that tree-tailcall usually fails when optimization
is disabled, which implies the attribute only really works with
optimization on. But that seems to be a reasonable limitation.

Passes bootstrap and full test

	PR83324

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_statement): Handle musttail.
	(cp_parser_jump_statement): Dito.
	(cp_parser_std_attribute): Dito.
---
 gcc/cp/parser.cc | 42 +++++++++++++++++++++++++++++++++++++-----
 1 file changed, 37 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 66ce161252c7..d25c1a1ac6a5 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -2467,7 +2467,7 @@ static tree cp_parser_perform_range_for_lookup
 static tree cp_parser_range_for_member_function
   (tree, tree);
 static tree cp_parser_jump_statement
-  (cp_parser *);
+  (cp_parser *, tree &);
 static void cp_parser_declaration_statement
   (cp_parser *);
 
@@ -12734,13 +12734,17 @@ cp_parser_statement (cp_parser* parser, tree in_statement_expr,
 						     NULL_TREE, false);
 	  break;
 
+	case RID_RETURN:
+	  std_attrs = process_stmt_hotness_attribute (std_attrs, attrs_loc);
+	  statement = cp_parser_jump_statement (parser, std_attrs);
+	  break;
+
 	case RID_BREAK:
 	case RID_CONTINUE:
-	case RID_RETURN:
 	case RID_CO_RETURN:
 	case RID_GOTO:
 	  std_attrs = process_stmt_hotness_attribute (std_attrs, attrs_loc);
-	  statement = cp_parser_jump_statement (parser);
+	  statement = cp_parser_jump_statement (parser, std_attrs);
 	  break;
 
 	  /* Objective-C++ exception-handling constructs.  */
@@ -14797,10 +14801,11 @@ cp_parser_init_statement (cp_parser *parser, tree *decl)
    jump-statement:
      goto * expression ;
 
+   STD_ATTRS are the statement attributes. They can be modified.
    Returns the new BREAK_STMT, CONTINUE_STMT, RETURN_EXPR, or GOTO_EXPR.  */
 
 static tree
-cp_parser_jump_statement (cp_parser* parser)
+cp_parser_jump_statement (cp_parser* parser, tree &std_attrs)
 {
   tree statement = error_mark_node;
   cp_token *token;
@@ -14877,6 +14882,33 @@ cp_parser_jump_statement (cp_parser* parser)
 	  /* If the next token is a `;', then there is no
 	     expression.  */
 	  expr = NULL_TREE;
+
+	if (keyword == RID_RETURN && expr)
+	  {
+	    bool musttail_p = false;
+	    if (lookup_attribute ("gnu", "musttail", std_attrs))
+	      {
+		musttail_p = true;
+		std_attrs = remove_attribute ("gnu", "musttail", std_attrs);
+	      }
+	    // support this for compatibility
+	    if (lookup_attribute ("clang", "musttail", std_attrs))
+	      {
+		musttail_p = true;
+		std_attrs = remove_attribute ("clang", "musttail", std_attrs);
+	      }
+	    if (musttail_p)
+	      {
+		tree t = expr;
+		if (t && TREE_CODE (t) == TARGET_EXPR)
+		  t = TARGET_EXPR_INITIAL (t);
+		if (t && TREE_CODE (t) != CALL_EXPR)
+		  error_at (token->location, "cannot tail-call: return value must be a call");
+		else
+		  CALL_EXPR_MUST_TAIL_CALL (t) = 1;
+	      }
+	  }
+
 	/* Build the return-statement, check co-return first, since type
 	   deduction is not valid there.  */
 	if (keyword == RID_CO_RETURN)
@@ -30189,7 +30221,7 @@ cp_parser_std_attribute (cp_parser *parser, tree attr_ns)
     /* Maybe we don't expect to see any arguments for this attribute.  */
     const attribute_spec *as
       = lookup_attribute_spec (TREE_PURPOSE (attribute));
-    if (as && as->max_length == 0)
+    if ((as && as->max_length == 0) || is_attribute_p ("musttail", attr_id))
       {
 	error_at (token->location, "%qE attribute does not take any arguments",
 		  attr_id);
-- 
2.44.0