procyberian/gcc
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

c++: Implement C++ DR 2406 - [[fallthrough]] attribute and iteration statements

Browse source 
The following patch implements
CWG 2406 - [[fallthrough]] attribute and iteration statements
The genericization of some loops leaves nothing at all or just a label
after a body of a loop, so if the loop is later followed by
case or default label in a switch, the fallthrough statement isn't
diagnosed.

The following patch implements it by marking the IFN_FALLTHROUGH call
in such a case, such that during gimplification it can be pedantically
diagnosed even if it is followed by case or default label or some normal
labels followed by case/default labels.

While looking into this, I've discovered other problems.
expand_FALLTHROUGH_r is removing the IFN_FALLTHROUGH calls from the IL,
but wasn't telling that to walk_gimple_stmt/walk_gimple_seq_mod, so
the callers would then skip the next statement after it, and it would
return non-NULL if the removed stmt was last in the sequence.  This could
lead to wi->callback_result being set even if it didn't appear at the very
end of switch sequence.
The patch makes use of wi->removed_stmt such that the callers properly
know what happened, and use different way to handle the end of switch
sequence case.

That change discovered a bug in the gimple-walk handling of
wi->removed_stmt.  If that flag is set, the callback is telling the callers
that the current statement has been removed and so the innermost
walk_gimple_seq_mod shouldn't gsi_next.  The problem is that
wi->removed_stmt is only reset at the start of a walk_gimple_stmt, but that
can be too late for some cases.  If we have two nested gimple sequences,
say GIMPLE_BIND as the last stmt of some gimple seq, we remove the last
statement inside of that GIMPLE_BIND, set wi->removed_stmt there, don't
do gsi_next correctly because already gsi_remove moved us to the next stmt,
there is no next stmt, so we return back to the caller, but wi->removed_stmt
is still set and so we don't do gsi_next even in the outer sequence, despite
the GIMPLE_BIND (etc.) not being removed.  That means we walk the
GIMPLE_BIND with its whole sequence again.
The patch fixes that by resetting wi->removed_stmt after we've used that
flag in walk_gimple_seq_mod.  Nothing really uses that flag after the
outermost walk_gimple_seq_mod, it is just a private notification that
the stmt callback has removed a stmt.

2023-11-17  Jakub Jelinek  <jakub@redhat.com>

	PR c++/107571
gcc/
	* gimplify.cc (expand_FALLTHROUGH_r): Use wi->removed_stmt after
	gsi_remove, change the way of passing fallthrough stmt at the end
	of sequence to expand_FALLTHROUGH.  Diagnose IFN_FALLTHROUGH
	with GF_CALL_NOTHROW flag.
	(expand_FALLTHROUGH): Change loc into array of 2 location_t elts,
	don't test wi.callback_result, instead check whether first
	elt is not UNKNOWN_LOCATION and in that case pedwarn with the
	second location.
	* gimple-walk.cc (walk_gimple_seq_mod): Clear wi->removed_stmt
	after the flag has been used.
	* internal-fn.def (FALLTHROUGH): Mention in comment the special
	meaning of the TREE_NOTHROW/GF_CALL_NOTHROW flag on the calls.
gcc/c-family/
	* c-gimplify.cc (genericize_c_loop): For C++ mark IFN_FALLTHROUGH
	call at the end of loop body as TREE_NOTHROW.
gcc/testsuite/
	* g++.dg/DRs/dr2406.C: New test.
This commit is contained in:
Jakub Jelinek 2023-11-17 15:43:31 +01:00
parent 97fc8851f6
commit 52eedfa009
5 changed files with 143 additions and 9 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

21
gcc/c-family/c-gimplify.cc
View file

@ -307,6 +307,27 @@ genericize_c_loop (tree *stmt_p, location_t start_locus, tree cond, tree body,
}
append_to_statement_list (body, &stmt_list);
if (c_dialect_cxx ()
&& stmt_list
&& TREE_CODE (stmt_list) == STATEMENT_LIST)
{
tree_stmt_iterator tsi = tsi_last (stmt_list);
if (!tsi_end_p (tsi))
{
tree t = *tsi;
while (TREE_CODE (t) == CLEANUP_POINT_EXPR
|| TREE_CODE (t) == EXPR_STMT
|| CONVERT_EXPR_CODE_P (TREE_CODE (t)))
t = TREE_OPERAND (t, 0);
/* For C++, if iteration statement body ends with fallthrough
statement, mark it such that we diagnose it even if next
statement would be labeled statement with case/default label. */
if (TREE_CODE (t) == CALL_EXPR
&& !CALL_EXPR_FN (t)
&& CALL_EXPR_IFN (t) == IFN_FALLTHROUGH)
TREE_NOTHROW (t) = 1;
}
}
finish_bc_block (&stmt_list, bc_continue, clab);
if (incr)
{

12
gcc/gimple-walk.cc
View file

@ -56,11 +56,21 @@ walk_gimple_seq_mod (gimple_seq *pseq, walk_stmt_fn callback_stmt,
gcc_assert (wi);
wi->callback_result = ret;
return wi->removed_stmt ? NULL : gsi_stmt (gsi);
gimple *g;
if (!wi->removed_stmt)
g = gsi_stmt (gsi);
else
{
g = NULL;
wi->removed_stmt = false;
}
return g;
}
if (!wi->removed_stmt)
gsi_next (&gsi);
else
wi->removed_stmt = false;
}
if (wi)

33
gcc/gimplify.cc
View file

@ -2791,17 +2791,33 @@ expand_FALLTHROUGH_r (gimple_stmt_iterator *gsi_p, bool *handled_ops_p,
*handled_ops_p = false;
break;
case GIMPLE_CALL:
static_cast<location_t *>(wi->info)[0] = UNKNOWN_LOCATION;
if (gimple_call_internal_p (stmt, IFN_FALLTHROUGH))
{
location_t loc = gimple_location (stmt);
gsi_remove (gsi_p, true);
wi->removed_stmt = true;
/* nothrow flag is added by genericize_c_loop to mark fallthrough
statement at the end of some loop's body. Those should be
always diagnosed, either because they indeed don't precede
a case label or default label, or because the next statement
is not within the same iteration statement. */
if ((stmt->subcode & GF_CALL_NOTHROW) != 0)
{
pedwarn (loc, 0, "attribute %<fallthrough%> not preceding "
"a case label or default label");
break;
}
if (gsi_end_p (*gsi_p))
{
*static_cast<location_t *>(wi->info) = gimple_location (stmt);
return integer_zero_node;
static_cast<location_t *>(wi->info)[0] = BUILTINS_LOCATION;
static_cast<location_t *>(wi->info)[1] = loc;
break;
}
bool found = false;
location_t loc = gimple_location (stmt);
gimple_stmt_iterator gsi2 = *gsi_p;
stmt = gsi_stmt (gsi2);
@ -2851,6 +2867,7 @@ expand_FALLTHROUGH_r (gimple_stmt_iterator *gsi_p, bool *handled_ops_p,
}
break;
default:
static_cast<location_t *>(wi->info)[0] = UNKNOWN_LOCATION;
break;
}
return NULL_TREE;
@ -2862,14 +2879,16 @@ static void
expand_FALLTHROUGH (gimple_seq *seq_p)
{
struct walk_stmt_info wi;
location_t loc;
location_t loc[2];
memset (&wi, 0, sizeof (wi));
wi.info = (void *) &loc;
loc[0] = UNKNOWN_LOCATION;
loc[1] = UNKNOWN_LOCATION;
wi.info = (void *) &loc[0];
walk_gimple_seq_mod (seq_p, expand_FALLTHROUGH_r, NULL, &wi);
if (wi.callback_result == integer_zero_node)
if (loc[0] != UNKNOWN_LOCATION)
/* We've found [[fallthrough]]; at the end of a switch, which the C++
standard says is ill-formed; see [dcl.attr.fallthrough]. */
pedwarn (loc, 0, "attribute %<fallthrough%> not preceding "
pedwarn (loc[1], 0, "attribute %<fallthrough%> not preceding "
"a case label or default label");
}

4
gcc/internal-fn.def
View file

@ -533,7 +533,9 @@ DEF_INTERNAL_FN (ATOMIC_AND_FETCH_CMP_0, ECF_LEAF, NULL)
DEF_INTERNAL_FN (ATOMIC_OR_FETCH_CMP_0, ECF_LEAF, NULL)
DEF_INTERNAL_FN (ATOMIC_XOR_FETCH_CMP_0, ECF_LEAF, NULL)
/* To implement [[fallthrough]]. */
/* To implement [[fallthrough]]. If the TREE_NOTHROW or GF_CALL_NOTHROW flag
is set on the call (normally redundant with ECF_NOTHROW), it marks
[[fallthrough]] at the end of C++ loop body. */
DEF_INTERNAL_FN (FALLTHROUGH, ECF_LEAF | ECF_NOTHROW, NULL)
/* To implement __builtin_launder. */

82
gcc/testsuite/g++.dg/DRs/dr2406.C Normal file
View file

@ -0,0 +1,82 @@
// DR 2406 - [[fallthrough]] attribute and iteration statements
// PR c++/107571
// { dg-do compile { target c++11 } }
// { dg-options "-pedantic-errors -Wimplicit-fallthrough" }
void bar ();
void baz ();
void qux ();
void
foo (int n)
{
switch (n)
{
case 1:
case 2:
bar ();
[[fallthrough]];
case 3:
do
{
[[fallthrough]]; // { dg-error "attribute 'fallthrough' not preceding a case label or default label" }
}
while (false);
case 6:
do
{
[[fallthrough]]; // { dg-error "attribute 'fallthrough' not preceding a case label or default label" }
}
while (n--);
case 7:
while (false)
{
[[fallthrough]]; // { dg-error "attribute 'fallthrough' not preceding a case label or default label" }
}
case 5:
baz (); // { dg-warning "this statement may fall through" }
case 4: // { dg-message "here" }
qux ();
[[fallthrough]]; // { dg-error "attribute 'fallthrough' not preceding a case label or default label" }
}
}
void
corge (int n)
{
switch (n)
{
case 1:
{
int i = 0;
do
{
[[fallthrough]]; // { dg-error "attribute 'fallthrough' not preceding a case label or default label" }
}
while (false);
}
case 2:
bar ();
break;
default:
break;
}
}
void
fred (int n)
{
switch (n)
{
case 1:
{
int i = 0;
[[fallthrough]];
}
case 2:
bar ();
break;
default:
break;
}
}