c-family: Fix PR94272 -fcompare-debug issue even for C [PR99230]

The following testcase results in -fcompare-debug failure.
The problem is the similar like in PR94272
https://gcc.gnu.org/pipermail/gcc-patches/2020-March/542562.html
When genericizing, with -g0 we have just a TREE_SIDE_EFFECTS DO_STMT
in a branch of if, while with -g we have that wrapped into
TREE_SIDE_EFFECTS STATEMENT_LIST containing DEBUG_BEGIN_STMT and that
DO_STMT.
The do loop is empty with 0 condition, so c_genericize_control_stmt
turns it into an empty statement (without TREE_SIDE_EFFECTS).
For -g0 that means that suddenly the if branch doesn't have side effects
and is expanded differently.  But with -g we still have TREE_SIDE_EFFECTS
STATEMENT_LIST containing DEBUG_BEGIN_STMT and non-TREE_SIDE_EFFECTS stmt.
The following patch fixes that by detecting this case and removing
TREE_SIDE_EFFECTS.

And, so that we don't duplicate the same code, changes the C++ FE to
just call the c_genericize_control_stmt function that can now handle it.

2021-03-20  Jakub Jelinek  <jakub@redhat.com>

	PR debug/99230
	* c-gimplify.c (c_genericize_control_stmt): Handle STATEMENT_LIST.

	* cp-gimplify.c (cp_genericize_r) <case STATEMENT_LIST>: Remove
	special code, instead call c_genericize_control_stmt.

	* gcc.dg/pr99230.c: New test.

1 files changed, 29 insertions, 0 deletions

diff --git a/gcc/c-family/c-gimplify.c b/gcc/c-family/c-gimplify.c
index e1dfca2..39c969d 100644
--- a/gcc/c-family/c-gimplify.c
+++ b/gcc/c-family/c-gimplify.c
@@ -497,6 +497,35 @@ c_genericize_control_stmt (tree *stmt_p, int *walk_subtrees, void *data,
       genericize_omp_for_stmt (stmt_p, walk_subtrees, data, func, lh);
       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++;
+	      walk_tree_1 (tsi_stmt_ptr (i), func, data, NULL, lh);
+	      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:
       break;
     }