[Bug target/27959] [4.1 regression] s390x miscompilation due to clobbering literal pool base reg

["krebbel at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #3 from krebbel at gcc dot gnu dot org  2006-06-09 11:41 -------
On s390 we use a trick to make the literal pool base register
available "on demand". It is defined as eliminable register
which can be eliminated to itself with offset 0. When a reload round
finds a r13 (literal pool base) it asks the back end whether it can
be eliminated and the back end says no. So reload throws any pseudos
out of r13 and marks it as bad_spill_regs_global hence it won't be
used as a reload register.

In your example there is no literal pool reference when entering reload.
So the initial elimination step marks r13 as eliminable and therefore free
to be used in reloads and it actually is used in a reload. In the second 
round reload forces a constant into the literal pool what creates a 
literal pool reference. When update_eliminables is called r13 is marked 
as cant_eliminate and the respective bit in bad_spill_regs_global is set. 
So far so good.

Unfortunately reload seems to think that registers which were once good to
be used in reloads will be good for future reloads as well - thats my
impression at least. The bitmap used_spill_regs is not reset between the
reload rounds. used_spill_regs is used in finish_spills to recalculate the
spill_regs array which then contains r13 since it was used as reload reg in the
first round.

The following patch deletes all registers which are recognized by
update_eliminables as eliminable_previous but cant_eliminate now from the
used_spill_regs bitmap and fixes your testcase.

Since reload is always a bit dodgy and tends to be unpredictable I would be
interested in Ulrichs opinion about the patch. He knows much better whats
going on in reload. So Ulrich do think this makes sense?


Index: gcc/reload1.c
===================================================================
*** gcc/reload1.c.orig  2006-06-09 13:12:39.000000000 +0200
--- gcc/reload1.c       2006-06-09 13:14:54.000000000 +0200
*************** reload (rtx first, int global)
*** 987,992 ****
--- 987,994 ----
        HARD_REG_SET to_spill;
        CLEAR_HARD_REG_SET (to_spill);
        update_eliminables (&to_spill);
+       AND_COMPL_HARD_REG_SET(used_spill_regs, to_spill);
+ 
        for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
          if (TEST_HARD_REG_BIT (to_spill, i))
            {


-- 

krebbel at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2006-06-09 11:41:57
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27959