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[v9,04/10] C++: Support clang compatible [[musttail]] (PR83324)

Message ID 20240708170031.1621184-5-ak@linux.intel.com
State New
Headers show
Series [v9,01/10] Improve must tail in RTL backend | expand

Commit Message

Andi Kleen July 8, 2024, 4:55 p.m. UTC 
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

Passes bootstrap and full test

gcc/cp/ChangeLog:

	PR c/83324
	* parser.cc (cp_parser_statement): Handle musttail.
	(cp_parser_jump_statement): Dito.
	* pt.cc (tsubst_expr): Copy CALL_EXPR_MUST_TAIL_CALL.
---
 gcc/cp/parser.cc | 34 +++++++++++++++++++++++++++++++---
 gcc/cp/pt.cc     |  7 ++++++-
 2 files changed, 37 insertions(+), 4 deletions(-)

Comments

Andi Kleen July 13, 2024, 5:33 p.m. UTC | #1 
Updated version with common code for C/C++ extracted in c-family.
Other than that no changes.
Is this version ok to commit?

---


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

Passes bootstrap and full test

gcc/c-family/ChangeLog:

	* c-attribs.cc (set_musttail_on_return): New function.
	* c-common.h (set_musttail_on_return): Declare new function.

gcc/cp/ChangeLog:

	PR c/83324
	* parser.cc (cp_parser_statement): Handle musttail.
	(cp_parser_jump_statement): Dito.
	* pt.cc (tsubst_expr): Copy CALL_EXPR_MUST_TAIL_CALL.
---
 gcc/c-family/c-attribs.cc | 20 ++++++++++++++++++++
 gcc/c-family/c-common.h   |  1 +
 gcc/cp/parser.cc          | 26 +++++++++++++++++++++++---
 gcc/cp/pt.cc              |  7 ++++++-
 4 files changed, 50 insertions(+), 4 deletions(-)

diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc
index 5adc7b775eaf..685f212683f4 100644
--- a/gcc/c-family/c-attribs.cc
+++ b/gcc/c-family/c-attribs.cc
@@ -672,6 +672,26 @@ attribute_takes_identifier_p (const_tree attr_id)
     return targetm.attribute_takes_identifier_p (attr_id);
 }
 
+/* Set a musttail attribute MUSTTAIL_P on return expression RETVAL
+   at LOC.  */
+
+void
+set_musttail_on_return (tree retval, location_t loc, bool musttail_p)
+{
+  if (retval && musttail_p)
+    {
+      tree t = retval;
+      if (TREE_CODE (t) == TARGET_EXPR)
+	t = TARGET_EXPR_INITIAL (t);
+      if (TREE_CODE (t) != CALL_EXPR)
+	error_at (loc, "cannot tail-call: return value must be a call");
+      else
+	CALL_EXPR_MUST_TAIL_CALL (t) = 1;
+    }
+  else if (musttail_p && !retval)
+    error_at (loc, "cannot tail-call: return value must be a call");
+}
+
 /* Verify that argument value POS at position ARGNO to attribute NAME
    applied to function FN (which is either a function declaration or function
    type) refers to a function parameter at position POS and the expected type
diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
index e84c9c47513b..079c9dc5f08b 100644
--- a/gcc/c-family/c-common.h
+++ b/gcc/c-family/c-common.h
@@ -1646,6 +1646,7 @@ extern tree handle_noreturn_attribute (tree *, tree, tree, int, bool *);
 extern tree handle_musttail_attribute (tree *, tree, tree, int, bool *);
 extern bool has_attribute (location_t, tree, tree, tree (*)(tree));
 extern tree build_attr_access_from_parms (tree, bool);
+extern void set_musttail_on_return (tree, location_t, bool);
 
 /* In c-format.cc.  */
 extern bool valid_format_string_type_p (tree);
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 31ae9c2fb54d..e2411ee7213c 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 *);
 
@@ -12756,7 +12756,7 @@ cp_parser_statement (cp_parser* parser, tree in_statement_expr,
 	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.  */
@@ -14844,10 +14844,11 @@ 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;
@@ -14924,6 +14925,25 @@ 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);
+	      }
+
+	    set_musttail_on_return (expr, token->location, musttail_p);
+	  }
+
 	/* Build the return-statement, check co-return first, since type
 	   deduction is not valid there.  */
 	if (keyword == RID_CO_RETURN)
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index d1316483e245..3b914089a6e2 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -21113,12 +21113,17 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	    bool op = CALL_EXPR_OPERATOR_SYNTAX (t);
 	    bool ord = CALL_EXPR_ORDERED_ARGS (t);
 	    bool rev = CALL_EXPR_REVERSE_ARGS (t);
-	    if (op || ord || rev)
+	    bool mtc = false;
+	    if (TREE_CODE (t) == CALL_EXPR)
+	      mtc = CALL_EXPR_MUST_TAIL_CALL (t);
+	    if (op || ord || rev || mtc)
 	      if (tree call = extract_call_expr (ret))
 		{
 		  CALL_EXPR_OPERATOR_SYNTAX (call) = op;
 		  CALL_EXPR_ORDERED_ARGS (call) = ord;
 		  CALL_EXPR_REVERSE_ARGS (call) = rev;
+		  if (TREE_CODE (call) == CALL_EXPR)
+		    CALL_EXPR_MUST_TAIL_CALL (call) = mtc;
 		}
 	    if (warning_suppressed_p (t, OPT_Wpessimizing_move))
 	      /* This also suppresses -Wredundant-move.  */
diff mbox series

Patch

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 31ae9c2fb54d..c8ed88f7a91b 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 *);
 
@@ -12756,7 +12756,7 @@  cp_parser_statement (cp_parser* parser, tree in_statement_expr,
 	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.  */
@@ -14844,10 +14844,11 @@  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;
@@ -14924,6 +14925,33 @@  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)
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index d1316483e245..3b914089a6e2 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -21113,12 +21113,17 @@  tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	    bool op = CALL_EXPR_OPERATOR_SYNTAX (t);
 	    bool ord = CALL_EXPR_ORDERED_ARGS (t);
 	    bool rev = CALL_EXPR_REVERSE_ARGS (t);
-	    if (op || ord || rev)
+	    bool mtc = false;
+	    if (TREE_CODE (t) == CALL_EXPR)
+	      mtc = CALL_EXPR_MUST_TAIL_CALL (t);
+	    if (op || ord || rev || mtc)
 	      if (tree call = extract_call_expr (ret))
 		{
 		  CALL_EXPR_OPERATOR_SYNTAX (call) = op;
 		  CALL_EXPR_ORDERED_ARGS (call) = ord;
 		  CALL_EXPR_REVERSE_ARGS (call) = rev;
+		  if (TREE_CODE (call) == CALL_EXPR)
+		    CALL_EXPR_MUST_TAIL_CALL (call) = mtc;
 		}
 	    if (warning_suppressed_p (t, OPT_Wpessimizing_move))
 	      /* This also suppresses -Wredundant-move.  */