c++: Fix a -fcompare-debug issue with DEBUG_BEGIN_STMT stmts in STATEMENT_LISTs ...

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.

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 34ccb9fa4c0445106bde23c123cd44b13cc009b5..9ff2525f83dbea385e7d6fc11c89f0de4c3c34d8 100644 (file)
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -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

diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index aa80384e1a4885e204b91a9074c39eca24748f6e..d003f5b7825c5ae1cd99bf9a2c12990363f907ec 100644 (file)
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -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;

diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index f8512d3fb3277623accfdbd059a2a5b2ef102a92..6d5c12f20a3fa3705afeb8656e80351be0e00db6 100644 (file)
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -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

diff --git a/gcc/testsuite/g++.dg/debug/pr94272.C b/gcc/testsuite/g++.dg/debug/pr94272.C
new file mode 100644 (file)
index 0000000..78522e0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/debug/pr94272.C
@@ -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)
+    ;
+}