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, 13 insertions, 1 deletions

diff --git a/gcc/gimple.h b/gcc/gimple.h
index 0420d6d..305d98f 100644
--- a/gcc/gimple.h
+++ b/gcc/gimple.h
@@ -4728,6 +4728,18 @@ is_gimple_debug (const gimple *gs)
 }
 
 
+/* Return the first nondebug statement in GIMPLE sequence S.  */
+
+static inline gimple *
+gimple_seq_first_nondebug_stmt (gimple_seq s)
+{
+  gimple_seq_node n = gimple_seq_first (s);
+  while (n && is_gimple_debug (n))
+    n = n->next;
+  return n;
+}
+
+
 /* Return the last nondebug statement in GIMPLE sequence S.  */
 
 static inline gimple *
@@ -4737,7 +4749,7 @@ gimple_seq_last_nondebug_stmt (gimple_seq s)
   for (n = gimple_seq_last (s);
        n && is_gimple_debug (n);
        n = n->prev)
-    if (n->prev == s)
+    if (n == s)
       return NULL;
   return n;
 }