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C++: Support clang compatible [[musttail]] (PR83324)

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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
	* cp-tree.h (AGGR_INIT_EXPR_MUST_TAIL): Add.
	* parser.cc (cp_parser_statement): Handle musttail.
	(cp_parser_jump_statement): Dito.
	* pt.cc (tsubst_expr): Copy CALL_EXPR_MUST_TAIL_CALL.
	* semantics.cc (simplify_aggr_init_expr): Handle musttail.
This commit is contained in:
Andi Kleen 2024-01-23 23:44:48 -08:00 • committed by Andi Kleen
parent 2861eb34e3
commit 2bd8177256
6 changed files with 63 additions and 4 deletions
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20
gcc/c-family/c-attribs.cc
View file

@ -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

1
gcc/c-family/c-common.h
View file

@ -1648,6 +1648,7 @@ 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);

4
gcc/cp/cp-tree.h
View file

@ -4228,6 +4228,10 @@ templated_operator_saved_lookups (tree t)
#define AGGR_INIT_FROM_THUNK_P(NODE) \
(AGGR_INIT_EXPR_CHECK (NODE)->base.protected_flag)
/* Nonzero means that the call was marked musttail. */
#define AGGR_INIT_EXPR_MUST_TAIL(NODE) \
(AGGR_INIT_EXPR_CHECK (NODE)->base.static_flag)
/* AGGR_INIT_EXPR accessors. These are equivalent to the CALL_EXPR
accessors, except for AGGR_INIT_EXPR_SLOT (which takes the place of
CALL_EXPR_STATIC_CHAIN). */

32
gcc/cp/parser.cc
View file

@ -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 *);
@ -12757,7 +12757,7 @@ 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. */
@ -14845,10 +14845,11 @@ 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;
@ -14925,6 +14926,31 @@ 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)
{
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);
}
tree ret_expr = expr;
if (ret_expr && TREE_CODE (ret_expr) == TARGET_EXPR)
ret_expr = TARGET_EXPR_INITIAL (ret_expr);
if (ret_expr && TREE_CODE (ret_expr) == AGGR_INIT_EXPR)
AGGR_INIT_EXPR_MUST_TAIL (ret_expr) = musttail_p;
else
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)

9
gcc/cp/pt.cc
View file

@ -21094,12 +21094,19 @@ 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;
else if (TREE_CODE (call) == AGGR_INIT_EXPR)
AGGR_INIT_EXPR_MUST_TAIL (call) = mtc;
}
if (warning_suppressed_p (t, OPT_Wpessimizing_move))
/* This also suppresses -Wredundant-move. */

1
gcc/cp/semantics.cc
View file

@ -4979,6 +4979,7 @@ simplify_aggr_init_expr (tree *tp)
= CALL_EXPR_OPERATOR_SYNTAX (aggr_init_expr);
CALL_EXPR_ORDERED_ARGS (call_expr) = CALL_EXPR_ORDERED_ARGS (aggr_init_expr);
CALL_EXPR_REVERSE_ARGS (call_expr) = CALL_EXPR_REVERSE_ARGS (aggr_init_expr);
CALL_EXPR_MUST_TAIL_CALL (call_expr) = AGGR_INIT_EXPR_MUST_TAIL (aggr_init_expr);
if (style == ctor)
{