[Bug target/63679] [5.0 Regression][AArch64] Failure to constant fold.

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63679

--- Comment #3 from Tejas Belagod <belagod at gcc dot gnu.org> ---
When I try 5.0 with -fno-tree-vectorize, I get:

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  # .MEM_4 = VDEF <.MEM_3(D)>
  aD.2496 = *.LC0D.2503;
  # VUSE <.MEM_4>
  _10 = aD.2496[0];
  # VUSE <.MEM_4>
  _22 = aD.2496[1];
  sum_23 = _10 + _22;
  # VUSE <.MEM_4>
  _29 = aD.2496[2];
  sum_30 = sum_23 + _29;
  # VUSE <.MEM_4>
  _36 = aD.2496[3];
  sum_37 = sum_30 + _36;
  # VUSE <.MEM_4>
  _43 = aD.2496[4];
  sum_44 = sum_37 + _43;
  # VUSE <.MEM_4>
  _50 = aD.2496[5];
  sum_51 = sum_44 + _50;
  # VUSE <.MEM_4>
  _57 = aD.2496[6];
  sum_58 = sum_51 + _57;
  # VUSE <.MEM_4>
  _6 = aD.2496[7];
  sum_7 = _6 + sum_58;
  # .MEM_9 = VDEF <.MEM_4>
  aD.2496 ={v} {CLOBBER};
  # VUSE <.MEM_9>
  return sum_7;
;;    succ:       EXIT

This:

  # .MEM_4 = VDEF <.MEM_3(D)>
  aD.2496 = *.LC0D.2503;

is what's mainly different from 4.9. 5.0 seems to use a TImode load to
initialize the stack with the const array.

(insn 10 9 11 (set (mem/c:TI (reg:DI 91) [1 aD.2496+0 S16 A128])
        (reg:TI 93)) foo.c:4 -1
     (nil))

(insn 11 10 12 (set (reg:TI 94)
        (mem/u/c:TI (plus:DI (reg:DI 92)
                (const_int 16 [0x10])) [0  S16 A32])) foo.c:4 -1
     (nil))

(insn 12 11 0 (set (mem/c:TI (plus:DI (reg:DI 91)
                (const_int 16 [0x10])) [1 aD.2496+16 S16 A128])
        (reg:TI 94)) foo.c:4 -1
     (nil))

;; sum_23 = _10 + _22;

(insn 13 12 14 (set (reg:SI 95)
        (mem/c:SI (plus:DI (reg/f:DI 68 virtual-stack-vars)
                (const_int -32 [0xffffffffffffffe0])) [1 aD.2496+0 S4 A128]))
foo.c:9 -1
     (nil))


When DSE wants to optimize it away, it fails to extract SI values from the
TImode stores:

**scanning insn=14
cselib lookup (reg/f:DI 64 sfp) => 3:3
cselib value 6:4299 0x2f6de50 (plus:DI (reg/f:DI 64 sfp)
    (const_int -28 [0xffffffffffffffe4]))

cselib lookup (plus:DI (reg/f:DI 64 sfp)
        (const_int -28 [0xffffffffffffffe4])) => 6:4299
  mem: (plus:DI (reg/f:DI 64 sfp)
    (const_int -28 [0xffffffffffffffe4]))

   after canon_rtx address: (plus:DI (reg/f:DI 64 sfp)
    (const_int -28 [0xffffffffffffffe4]))
  gid=0 offset=-28 
 processing const load gid=0[-28..-24)
trying to replace SImode load in insn 14 from TImode store in insn 10
(lshiftrt:DI (reg:DI 105)
    (const_int 32 [0x20]))

Hot cost: 8 (final)
 -- could not extract bits of stored value
removing from active insn=10 has store
mems_found = 0, cannot_delete = true
cselib lookup (mem/c:SI (plus:DI (reg/f:DI 64 sfp)
            (const_int -28 [0xffffffffffffffe4])) [1 aD.2496+4 S4 A32]) => 0:0

**scanning insn=15
....