http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52285
Steven Bosscher <steven at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |ASSIGNED
CC|steven at gcc dot gnu.org |
AssignedTo|unassigned at gcc dot |steven at gcc dot gnu.org
|gnu.org |
--- Comment #10 from Steven Bosscher <steven at gcc dot gnu.org> 2012-11-13
10:30:27 UTC ---
There are several reasons why RTL LIM cannot currently hoist the (frame) rtx.
The first is that in general it stays away from any HARD_REGISTER_P reg with
a 10-foot pole. For most hard registers this is probably a good strategy:
Anything that's in a "real" hard register at this point is there for a reason
(function return, global reg, whatever) and almost certainly not invariant in
a loop.
Second, RTL LIM only hoists expression that can be assigned to the original
SET_DEST of a single set. In the case of this PR, the insn in case is:
;; UD chains for insn luid 2 uid 15
;; reg 20 { }
;; reg 63 { d2(bb 4 insn 12) }
(insn 15 12 16 4 (set (reg:CCZ 17 flags)
(compare:CCZ (reg/v/f:DI 63 [ p ])
(reg/f:DI 20 frame))) PR52285.c:10 8 {*cmpdi_1}
(nil))
This fails in may_assign_reg_p because (reg:CCZ 17) can't be assigned to (it
is a hard register, and I suppose it has class NO_REGS), so the SET_SRC is
not even considered by find_invariant_insn as a potential invariant. I think
this condition can be relaxed with something like,
Index: loop-invariant.c
===================================================================
--- loop-invariant.c (revision 193454)
+++ loop-invariant.c (working copy)
@@ -874,11 +874,11 @@
dest = SET_DEST (set);
if (!REG_P (dest)
- || HARD_REGISTER_P (dest))
+ || HARD_REGISTER_P (dest)
+ || !may_assign_reg_p (dest))
simple = false;
- if (!may_assign_reg_p (SET_DEST (set))
- || !check_maybe_invariant (SET_SRC (set)))
+ if (!check_maybe_invariant (SET_SRC (set)))
return;
/* If the insn can throw exception, ...
Finally, RTL LIM cannot hoist parts of expressions. It only hoists the
SET_SRC as a whole, or nothing at all. I have patches for that, originally
developed to hoist addresses out of MEMs. I'll dust them off and see if
I can make it handle "(reg:frame + CONST_INT)" and other expressions that
involve eliminable regs.
