[jit] Fix state issue in gcse.c

[Steven Bosscher]

On Sat, Apr 19, 2014 at 3:24 PM, Steven Bosscher wrote:
> On Tue, Mar 11, 2014 at 8:00 PM, David Malcolm wrote:
>> Investigation revealed the issue to be a CFG from the previous compile
>> being kept alive by this GC root in gcse.c:
>>           static GTY(()) rtx test_insn;
>>
>> This wouldn't it itself be an issue, but one (or more) of the edges had:
>
> But this is an issue! This is not supposed to be possible.
>
> test_insn is not in the insns chain and also not in a basic block, so
> it should never keep a CFG alive.
>
> Your patch papers over a bigger issue: How does test_insn end up
> preventing the CFG from being garbage-collected.


The only way this could happen, that I can think of, is a reference
from SET_SRC into the insn stream (like a label_ref).

Can you please try and see if the patch below fixes your problem?

Ciao!
Steven



        * gcse.c (can_assign_to_reg_without_clobbers_p): Do not let pointers
        from test_insn into GGC space escape via SET_SRC.

Index: gcse.c
===================================================================
--- gcse.c      (revision 209530)
+++ gcse.c      (working copy)
@@ -849,6 +849,7 @@ can_assign_to_reg_without_clobbers_p (rtx x)
 {
   int num_clobbers = 0;
   int icode;
+  bool can_assign = false;

   /* If this is a valid operand, we are OK.  If it's VOIDmode, we aren't.  */
   if (general_operand (x, GET_MODE (x)))
@@ -866,6 +867,7 @@ can_assign_to_reg_without_clobbers_p (rtx x)
                                                   FIRST_PSEUDO_REGISTER * 2),
                                      const0_rtx));
       NEXT_INSN (test_insn) = PREV_INSN (test_insn) = 0;
+      INSN_LOCATION (test_insn) = UNKNOWN_LOCATION;
     }

   /* Now make an insn like the one we would make when GCSE'ing and see if
@@ -874,16 +876,19 @@ can_assign_to_reg_without_clobbers_p (rtx x)
   SET_SRC (PATTERN (test_insn)) = x;

   icode = recog (PATTERN (test_insn), test_insn, &num_clobbers);
-  if (icode < 0)
-    return false;
-
-  if (num_clobbers > 0 && added_clobbers_hard_reg_p (icode))
-    return false;
-
-  if (targetm.cannot_copy_insn_p && targetm.cannot_copy_insn_p (test_insn))
-    return false;
-
-  return true;
+
+  /* If the test insn is valid and doesn't need clobbers, and the target also
+     has no objections, we're good.  */
+  if (icode != 0
+      && (num_clobbers == 0 || !added_clobbers_hard_reg_p (icode))
+      && ! (targetm.cannot_copy_insn_p
+           && targetm.cannot_copy_insn_p (test_insn)))
+    can_assign = true;
+
+  /* Make sure test_insn doesn't have any pointers into GC space.  */
+  SET_SRC (PATTERN (test_insn)) = NULL_RTX;
+
+  return can_assign;
 }

 /* Return nonzero if the operands of expression X are unchanged from the