loop-invariant: Fix -fcompare-debug failure [PR103837]

In the following testcase we have a -fcompare-debug failure, because
can_move_invariant_reg doesn't ignore DEBUG_INSNs in its decisions.
In the testcase we have due to uninitialized variable:
  loop_header
    debug_insn using pseudo84
    pseudo84 = invariant
    insn using pseudo84
  end loop
and with -g decide not to move the pseudo84 = invariant before the
loop header; in this case not resetting the debug insns might be fine.
But, we could have also:
  pseudo84 = whatever
  loop_header
    debug_insn using pseudo84
    pseudo84 = invariant
    insn using pseudo84
  end loop
and in that case not resetting the debug insns would result in wrong-debug.
And, we don't really have generally a good substitution on what pseudo84
contains, it could inherit various values from different paths.
So, the following patch ignores DEBUG_INSNs in the decisions, and if there
are any that previously prevented the optimization, resets them before
return true.

2021-12-28  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/103837
	* loop-invariant.c (can_move_invariant_reg): Ignore DEBUG_INSNs in
	the decisions whether to return false or continue and right before
	returning true reset those debug insns that previously caused
	returning false.

	* gcc.dg/pr103837.c: New test.

2 files changed, 34 insertions, 4 deletions

diff --git a/gcc/loop-invariant.c b/gcc/loop-invariant.c
index 5eee2e5c9f8..be951554337 100644
--- a/gcc/loop-invariant.c
+++ b/gcc/loop-invariant.c
@@ -1691,6 +1691,7 @@ can_move_invariant_reg (class loop *loop, struct invariant *inv, rtx reg)
   unsigned int dest_regno, defs_in_loop_count = 0;
   rtx_insn *insn = inv->insn;
   basic_block bb = BLOCK_FOR_INSN (inv->insn);
+  auto_vec <rtx_insn *, 16> debug_insns_to_reset;
 
   /* We ignore hard register and memory access for cost and complexity reasons.
      Hard register are few at this stage and expensive to consider as they
@@ -1725,10 +1726,13 @@ can_move_invariant_reg (class loop *loop, struct invariant *inv, rtx reg)
 	continue;
 
       /* Don't move if a use is not dominated by def in insn.  */
-      if (use_bb == bb && DF_INSN_LUID (insn) >= DF_INSN_LUID (use_insn))
-	return false;
-      if (!dominated_by_p (CDI_DOMINATORS, use_bb, bb))
-	return false;
+      if ((use_bb == bb && DF_INSN_LUID (insn) >= DF_INSN_LUID (use_insn))
+	  || !dominated_by_p (CDI_DOMINATORS, use_bb, bb))
+	{
+	  if (!DEBUG_INSN_P (use_insn))
+	    return false;
+	  debug_insns_to_reset.safe_push (use_insn);
+	}
     }
 
   /* Check for other defs.  Any other def in the loop might reach a use
@@ -1751,6 +1755,13 @@ can_move_invariant_reg (class loop *loop, struct invariant *inv, rtx reg)
 	return false;
     }
 
+  /* Reset debug uses if a use is not dominated by def in insn.  */
+  for (auto use_insn : debug_insns_to_reset)
+    {
+      INSN_VAR_LOCATION_LOC (use_insn) = gen_rtx_UNKNOWN_VAR_LOC ();
+      df_insn_rescan (use_insn);
+    }
+
   return true;
 }
 
diff --git a/gcc/testsuite/gcc.dg/pr103837.c b/gcc/testsuite/gcc.dg/pr103837.c
new file mode 100644
index 00000000000..43593d52b6c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr103837.c
@@ -0,0 +1,19 @@
+/* PR rtl-optimization/103837 */
+/* { dg-do compile } */
+/* { dg-options "-Og -fcompare-debug -fmove-loop-invariants -fnon-call-exceptions -fexceptions -fdelete-dead-exceptions -fno-tree-dce -w" } */
+
+unsigned long int
+foo (int x)
+{
+  double a;
+  int b;
+  unsigned long int ret = a;
+
+  for (;;)
+    {
+      b = !!((int) a);
+      a = x;
+    }
+
+  return ret;
+}