df: Don't set bbs dirty because of debug insn moves [PR104459]

As mentioned in the PR, we get -fcompare-debug failure, which is caused by
cfg_layout_merge_blocks successfully merging two bbs where both bbs
contained just CODE_LABEL, NOTE_INSN_BASIC_BLOCK and in the -g case both
some debug insns at the end.  cfg_layout_merge_blocks calls
update_bb_for_insn_chain which for the post-label insns in the second block
(except for BARRIERs) calls df_insn_change_bb.  This function changes
the bb of the insns and for notes just punts, but for other insns calls
df_set_bb_dirty.  Now the problem is that because there were only debug
insns and notes in the second block, df_set_bb_dirty is called on both
only in the -g case and not with -g0.  df_set_bb_dirty these days
sets both the BB_MODIFIED flag and marks the bb as dirty, and the former
is what 6 spots in cfgcleanup.cc use in code-generation decisions,
in this case
          may_thread |= (target->flags & BB_MODIFIED) != 0;
in particular.  So, with -g may_thread is true while with -g0 it is not
and we diverge from that point onwards.
I've thought about introducing df_set_bb_dirty_nondebug that wouldn't
set BB_MODIFIED but would mark the bb dirty, but then I went through
history and found changes like:
https://gcc.gnu.org/legacy-ml/gcc-patches/2010-10/msg00059.html
so I've also tried just not calling df_set_bb_dirty for debug insns
at all and it passed x86_64-linux and i686-linux
--enable-checking=yes,rtl,extra,df bootstraps/regtests, so perhaps
that works too.
Now that I look at it again, if we don't need those from %d to %d messages
for debug insns in the dump files, another way to fix it would be just to
change the very first line in the hunk from
  if (!INSN_P (insn))
to
  if (!DEBUG_INSN_P (insn))
Though, df_set_bb_dirty_nondebug which will do everything but
set bb->flags |= BB_MODIFIED is yet another option I can test.
Perhaps even that PR42889 was solely about those 6 decisions in cfgcleanup
(at that point it used df_get_bb_dirty) and not about actually the
recomputation of some of the problems causing different code generations.

2022-02-11  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/104459
	* df-scan.cc (df_insn_change_bb): Don't call df_set_bb_dirty when
	moving DEBUG_INSNs between bbs.

	* gcc.dg/pr104459.c: New test.

2 files changed, 42 insertions, 2 deletions

diff --git a/gcc/df-scan.cc b/gcc/df-scan.cc
index 5df70e95751..9b2375d561b 100644
--- a/gcc/df-scan.cc
+++ b/gcc/df-scan.cc
@@ -1769,13 +1769,15 @@ df_insn_change_bb (rtx_insn *insn, basic_block new_bb)
   if (!INSN_P (insn))
     return;
 
-  df_set_bb_dirty (new_bb);
+  if (!DEBUG_INSN_P (insn))
+    df_set_bb_dirty (new_bb);
   if (old_bb)
     {
       if (dump_file)
 	fprintf (dump_file, "  from %d to %d\n",
 		 old_bb->index, new_bb->index);
-      df_set_bb_dirty (old_bb);
+      if (!DEBUG_INSN_P (insn))
+	df_set_bb_dirty (old_bb);
     }
   else
     if (dump_file)
diff --git a/gcc/testsuite/gcc.dg/pr104459.c b/gcc/testsuite/gcc.dg/pr104459.c
new file mode 100644
index 00000000000..de8a643d9e2
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr104459.c
@@ -0,0 +1,38 @@
+/* PR rtl-optimization/104459 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -funswitch-loops -fno-tree-dce -fcompare-debug -w" } */
+
+void
+foo (int x, int y)
+{
+  unsigned int a;
+
+  for (;;)
+    {
+      short int *p = (short int *) &x;
+      unsigned int q = 0;
+
+      a /= 2;
+      if (a)
+	{
+	  q -= y;
+	  while (q)
+	    ;
+	}
+
+      if (x)
+	{
+	  for (q = 0; q != 1; q += 2)
+	    {
+	      unsigned int n;
+
+	      n = *p ? 0 : q;
+	      y += n < 1;
+
+	      n = a || *p;
+	      if (n % x == 0)
+		y /= x;
+	    }
+	}
+    }
+}