On 2021/6/9 04:11, Segher Boessenkool wrote:
> Hi!
>
> On Fri, Jun 04, 2021 at 09:40:58AM +0800, Xionghu Luo wrote:
>>>> Combine still fail to merge the two instructions:
>>>>
>>>> Trying 6 -> 7:
>>>> 6: r120:KF#0=r125:KF#0<-<0x40
>>>> REG_DEAD r125:KF
>>>> 7: [sfp:DI+r123:DI]=r120:KF#0<-<0x40
>>>> REG_DEAD r120:KF
>>>> Successfully matched this instruction:
>>>> (set (mem/c:V1TI (plus:DI (reg/f:DI 110 sfp)
>>>> (reg:DI 123)) [1 S16 A128])
>>>> (subreg:V1TI (reg:KF 125) 0))
>>>> rejecting combination of insns 6 and 7
>>>> original costs 4 + 4 = 8
>>>> replacement cost 12
>>>
>>> So what instructions were these? Why did the store cost 4 but the new
>>> one costs 12?
>
> The *vsx_le_perm_store_<mode> instruction has the *preferred*
> alternative with cost 12, while the other alternative has cost 8. Why
> is that? That looks like a bug.
> (set_attr "length" "12,8")
12 was introduced by Mike's commit c477a6674364(r6-2577), and all the 5
vsx_le_perm_store_<mode> are set to 12 for modes VSX_D/VSX_W/V8HI/V16QI
/VSX_LE_128, I guess it is split to two rs6000_emit_le_vsx_permute before
reload, but 3 rs6000_emit_le_vsx_permute after reload, so the length is
12, then it seems also not reasonable to change it from 12 to 8? And I am
not sure when the alternative 1 will be chosen?
vsx.md:
;; The post-reload split requires that we re-permute the source
;; register in case it is still live.
(define_split
[(set (match_operand:VSX_LE_128 0 "memory_operand")
(match_operand:VSX_LE_128 1 "vsx_register_operand"))]
"!BYTES_BIG_ENDIAN && TARGET_VSX && reload_completed && !TARGET_P9_VECTOR
&& !altivec_indexed_or_indirect_operand (operands[0], <MODE>mode)"
[(const_int 0)]
{
rs6000_emit_le_vsx_permute (operands[1], operands[1], <MODE>mode);
rs6000_emit_le_vsx_permute (operands[0], operands[1], <MODE>mode);
rs6000_emit_le_vsx_permute (operands[1], operands[1], <MODE>mode);
DONE;
})
>
>>>> By hacking the vsx_le_perm_store_v1ti INSN_COST from 12 to 8,
>>>
>>> It should be the same cost as the other store!
>>
>> vsx_le_permute_v1ti's cost is defined to 4 in vsx.md:
>
> Yes. Why is alternative 0 of *vsx_le_perm_store_<mode> said to have a
> length of 3 insns?
>
>
> Segher
>
--
Thanks,
Xionghu
