gimplify: Fix -fcompare-debug differences caused by gimplify_body [PR94281]

The following testcase FAILs, because gimplify_body adds a GIMPLE_NOP only
when there are no statements in the function and with -g there is a
DEBUG_BEGIN_STMT, so it doesn't add it and due to -fno-tree-dce that never
gets removed afterwards.  Similarly, if the body seq after gimplification
contains some DEBUG_BEGIN_STMTs plus a single gbind, then we could behave
differently between -g0 and -g, by using that gbind as the body in the -g0
case and not in the -g case.
This patch fixes that by ignoring DEBUG_BEGIN_STMTs (other debug stmts can't
appear at this point yet thankfully) during decisions and if we pick the
single gbind and there are DEBUG_BEGIN_STMTs next to it, it moves them into
the gbind.
While debugging this, I found also a bug in the gimple_seq_last_nondebug_stmt
function, for a seq that has a single non-DEBUG_BEGIN_STMT statement
followed by one or more DEBUG_BEGIN_STMTs it would return NULL rather than
the first statement.

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

	PR debug/94281
	* gimple.h (gimple_seq_first_nondebug_stmt): New function.
	(gimple_seq_last_nondebug_stmt): Don't return NULL if seq contains
	a single non-debug stmt followed by one or more debug stmts.
	* gimplify.c (gimplify_body): Use gimple_seq_first_nondebug_stmt
	instead of gimple_seq_first_stmt, use gimple_seq_first_nondebug_stmt
	and gimple_seq_last_nondebug_stmt instead of gimple_seq_first and
	gimple_seq_last to check if outer_stmt gbind could be reused and
	if yes and it is surrounded by any debug stmts, move them into the
	gbind body.

	* g++.dg/debug/pr94281.C: New test.

1 files changed, 32 insertions, 3 deletions

diff --git a/gcc/gimplify.c b/gcc/gimplify.c
index ef531f3..787435c 100644
--- a/gcc/gimplify.c
+++ b/gcc/gimplify.c
@@ -14849,7 +14849,7 @@ gimplify_body (tree fndecl, bool do_parms)
   /* Gimplify the function's body.  */
   seq = NULL;
   gimplify_stmt (&DECL_SAVED_TREE (fndecl), &seq);
-  outer_stmt = gimple_seq_first_stmt (seq);
+  outer_stmt = gimple_seq_first_nondebug_stmt (seq);
   if (!outer_stmt)
     {
       outer_stmt = gimple_build_nop ();
@@ -14859,8 +14859,37 @@ gimplify_body (tree fndecl, bool do_parms)
   /* The body must contain exactly one statement, a GIMPLE_BIND.  If this is
      not the case, wrap everything in a GIMPLE_BIND to make it so.  */
   if (gimple_code (outer_stmt) == GIMPLE_BIND
-      && gimple_seq_first (seq) == gimple_seq_last (seq))
-    outer_bind = as_a <gbind *> (outer_stmt);
+      && (gimple_seq_first_nondebug_stmt (seq)
+	  == gimple_seq_last_nondebug_stmt (seq)))
+    {
+      outer_bind = as_a <gbind *> (outer_stmt);
+      if (gimple_seq_first_stmt (seq) != outer_stmt
+	  || gimple_seq_last_stmt (seq) != outer_stmt)
+	{
+	  /* If there are debug stmts before or after outer_stmt, move them
+	     inside of outer_bind body.  */
+	  gimple_stmt_iterator gsi = gsi_for_stmt (outer_stmt, &seq);
+	  gimple_seq second_seq = NULL;
+	  if (gimple_seq_first_stmt (seq) != outer_stmt
+	      && gimple_seq_last_stmt (seq) != outer_stmt)
+	    {
+	      second_seq = gsi_split_seq_after (gsi);
+	      gsi_remove (&gsi, false);
+	    }
+	  else if (gimple_seq_first_stmt (seq) != outer_stmt)
+	    gsi_remove (&gsi, false);
+	  else
+	    {
+	      gsi_remove (&gsi, false);
+	      second_seq = seq;
+	      seq = NULL;
+	    }
+	  gimple_seq_add_seq_without_update (&seq,
+					     gimple_bind_body (outer_bind));
+	  gimple_seq_add_seq_without_update (&seq, second_seq);
+	  gimple_bind_set_body (outer_bind, seq);
+	}
+    }
   else
     outer_bind = gimple_build_bind (NULL_TREE, seq, NULL);