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c++: Fix a -fcompare-debug issue with DEBUG_BEGIN_STMT stmts in STATEMENT_LISTs [PR94272]

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The following testcase FAILs with -fcompare-debug.  The problem is that
the C++ FE initially uses IF_STMTs, tcc_statement which default to
TREE_SIDE_EFFECTS set, but later on is genericized into COND_EXPRs,
tcc_expression which default to TREE_SIDE_EFFECTS ored from all 3 operands.
Furthermore, with -g we emit by default DEBUG_BEGIN_STMTs (TREE_SIDE_EFFECTS
clear) and so end up with a STATEMENT_LIST containing DEBUG_BEGIN_STMT
+ e.g. the IF_STMT, while with -g0 we would end up with just the IF_STMT
alone and in that case there is no STATEMENT_LIST wrapping it.

Now, the STATEMENT_LIST has TREE_SIDE_EFFECTS set to match the IF_STMT,
but if none of the 3 operands (condition and both branches) have
TREE_SIDE_EFFECTS, genericize_if_stmt will replace the IF_STMT with
COND_EXPR without TREE_SIDE_EFFECTS, but with -g only STATEMENT_LIST
wrapping it will keep TREE_SIDE_EFFECTS.  Then during gimplification,
shortcut_cond_expr checks TREE_SIDE_EFFECTS of the operands and as it
is differennt between -g and -g0, will generate different code.

The following patch attempts to fix this by clearing TREE_SIDE_EFFECTS
on STATEMENT_LISTs that initially have it set and contain only
DEBUG_BEGIN_STMT or at most one other statement that lost TREE_SIDE_EFFECTS
during the genericization.

2020-03-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94272
	* cp-gimplify.c (cp_genericize_r): Handle STATEMENT_LIST.

	* g++.dg/debug/pr94272.C: New test.
This commit is contained in:
Jakub Jelinek 2020-03-26 09:18:35 +01:00
parent 9708ca2be4
commit 5a1706f63a
4 changed files with 53 additions and 0 deletions
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5
gcc/cp/ChangeLog
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@ -1,3 +1,8 @@
2020-03-26 Jakub Jelinek <jakub@redhat.com>
PR c++/94272
* cp-gimplify.c (cp_genericize_r): Handle STATEMENT_LIST.
2020-03-25 Patrick Palka <ppalka@redhat.com>
PR c++/94265

29
gcc/cp/cp-gimplify.c
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@ -1754,6 +1754,35 @@ cp_genericize_r (tree *stmt_p, int *walk_subtrees, void *data)
walk_subtrees = 0;
break;
case STATEMENT_LIST:
if (TREE_SIDE_EFFECTS (stmt))
{
tree_stmt_iterator i;
int nondebug_stmts = 0;
bool clear_side_effects = true;
/* Genericization can clear TREE_SIDE_EFFECTS, e.g. when
transforming an IF_STMT into COND_EXPR. If such stmt
appears in a STATEMENT_LIST that contains only that
stmt and some DEBUG_BEGIN_STMTs, without -g where the
STATEMENT_LIST wouldn't be present at all the resulting
expression wouldn't have TREE_SIDE_EFFECTS set, so make sure
to clear it even on the STATEMENT_LIST in such cases. */
for (i = tsi_start (stmt); !tsi_end_p (i); tsi_next (&i))
{
tree t = tsi_stmt (i);
if (TREE_CODE (t) != DEBUG_BEGIN_STMT && nondebug_stmts < 2)
nondebug_stmts++;
cp_walk_tree (tsi_stmt_ptr (i), cp_genericize_r, data, NULL);
if (TREE_CODE (t) != DEBUG_BEGIN_STMT
&& (nondebug_stmts > 1 || TREE_SIDE_EFFECTS (tsi_stmt (i))))
clear_side_effects = false;
}
if (clear_side_effects)
TREE_SIDE_EFFECTS (stmt) = 0;
*walk_subtrees = 0;
}
break;
default:
if (IS_TYPE_OR_DECL_P (stmt))
*walk_subtrees = 0;

5
gcc/testsuite/ChangeLog
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@ -1,3 +1,8 @@
2020-03-26 Jakub Jelinek <jakub@redhat.com>
PR c++/94272
* g++.dg/debug/pr94272.C: New test.
2020-03-26 Felix Yang <felix.yang@huawei.com>
PR tree-optimization/94269

14
gcc/testsuite/g++.dg/debug/pr94272.C Normal file
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@ -0,0 +1,14 @@
// PR c++/94272
// { dg-do compile }
// { dg-options "-O2 -fnon-call-exceptions -fcompare-debug" }
int *c, d, *e;
void
foo ()
{
if (c && d)
;
else if (*e)
;
}