------- Comment #14 from steven at gcc dot gnu dot org 2010-02-04 15:19 -------
Part of the problem comes from the way IVOPTS optimizes the memory access:
;; Generating RTL for gimple basic block 3
;; D.1814_10 = MEM[base: D.1846_29];
(insn 52 51 0 tst.c:6 (set (reg:SI 172 [ D.1814 ])
(mem:SI (reg:SI 179 [ ivtmp.20 ]) [2 *D.1846_29 S4 A32])) -1 (nil))
;; ivtmp.20_24 = ivtmp.20_25 + 4;
(insn 53 52 0 tst.c:6 (set (reg:SI 179 [ ivtmp.20 ])
(plus:SI (reg:SI 179 [ ivtmp.20 ])
(const_int 4 [0x4]))) -1 (nil))
;; MEM[base: D.1847_30, offset: 4294967292] = D.1816_13;
(insn 54 53 55 tst.c:6 (set (reg:SI 189)
(mult:SI (reg:SI 172 [ D.1814 ])
(reg:SI 180 [ pretmp.11 ]))) -1 (nil))
(insn 55 54 0 tst.c:6 (set (mem:SI (plus:SI (reg:SI 179 [ ivtmp.20 ])
(const_int -4 [0xfffffffffffffffc])) [2 *D.1847_30 S4 A32])
(reg:SI 189)) -1 (nil))
;; if (ivtmp.20_24 != D.1849_32)
(insn 57 55 58 tst.c:4 (set (reg:CC 24 cc)
(compare:CC (reg:SI 179 [ ivtmp.20 ])
(reg:SI 183 [ D.1849 ]))) -1 (nil))
(jump_insn 58 57 0 tst.c:4 (set (pc)
(if_then_else (ne (reg:CC 24 cc)
(const_int 0 [0x0]))
(label_ref 56)
(pc))) -1 (expr_list:REG_BR_PROB (const_int 9844 [0x2674])
(nil)))
This yields the sequence:
52 r172:SI=[r179:SI]
53 r179:SI=r179:SI+0x4
54 r189:SI=r172:SI*r180:SI
55 [r179:SI-0x4]=r189:SI
and we never get rid of this again. There is another bug related to this one:
bug 31849. The patch there probably needs dusting off, and then I'll see if it
improves things for this bug as well.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36712
