Ajit Agarwal <aagar...@linux.ibm.com> writes:
> Hello Richard:
>
> On 11/06/24 5:15 pm, Richard Sandiford wrote:
>> Ajit Agarwal <aagar...@linux.ibm.com> writes:
>>> Hello Richard:
>>> On 11/06/24 4:56 pm, Ajit Agarwal wrote:
>>>> Hello Richard:
>>>>
>>>> On 11/06/24 4:36 pm, Richard Sandiford wrote:
>>>>> Ajit Agarwal <aagar...@linux.ibm.com> writes:
>>>>>>>>>> After LRA reload:
>>>>>>>>>>
>>>>>>>>>> (insn 9299 2472 2412 187 (set (reg:V2DF 51 19 [orig:240
>>>>>>>>>> vect__302.545 ] [240])
>>>>>>>>>> (mem:V2DF (plus:DI (reg:DI 8 8 [orig:1285 ivtmp.886 ] [1285])
>>>>>>>>>> (const_int 16 [0x10])) [1 MEM <vector(2)
>>>>>>>>>> real(kind=8)> [(real(kind=8) *)_4188]+16 S16 A64]))
>>>>>>>>>> "shell_lam.fppized.f":238:72 1190 {vsx_movv2df_64bit}
>>>>>>>>>> (nil))
>>>>>>>>>> (insn 2412 9299 2477 187 (set (reg:V2DF 51 19 [orig:240
>>>>>>>>>> vect__302.545 ] [240])
>>>>>>>>>> (neg:V2DF (fma:V2DF (reg:V2DF 39 7 [ MEM <vector(2)
>>>>>>>>>> real(kind=8)> [(real(kind=8) *)_4050]+16 ])
>>>>>>>>>> (reg:V2DF 44 12 [3119])
>>>>>>>>>> (neg:V2DF (reg:V2DF 51 19 [orig:240 vect__302.545 ]
>>>>>>>>>> [240]))))) {*vsx_nfmsv2df4}
>>>>>>>>>> (nil))
>>>>>>>>>>
>>>>>>>>>> (insn 2473 9311 9312 187 (set (reg:V2DF 38 6 [orig:905 vect__302.545
>>>>>>>>>> ] [905])
>>>>>>>>>> (neg:V2DF (fma:V2DF (reg:V2DF 44 12 [3119])
>>>>>>>>>> (reg:V2DF 38 6 [orig:2561 MEM <vector(2)
>>>>>>>>>> real(kind=8)> [(real(kind=8) *)_4050] ] [2561])
>>>>>>>>>> (neg:V2DF (reg:V2DF 47 15 [5266])))))
>>>>>>>>>> {*vsx_nfmsv2df4}
>>>>>>>>>> (nil))
>>>>>>>>>>
>>>>>>>>>> In the above allocated code it assign registers 51 and 47 and they
>>>>>>>>>> are not sequential.
>>>>>>>>>
>>>>>>>>> The reload for 2412 looks valid. What was the original pre-reload
>>>>>>>>> version of insn 2473? Also, what happened to insn 2472? Was it
>>>>>>>>> deleted?
>>>>>>>>>
>>>>>>>>
>>>>>>>> This is preload version of 2473:
>>>>>>>>
>>>>>>>> (insn 2473 2396 2478 161 (set (reg:V2DF 905 [ vect__302.545 ])
>>>>>>>> (neg:V2DF (fma:V2DF (reg:V2DF 4283 [3119])
>>>>>>>> (subreg:V2DF (reg:OO 2561 [ MEM <vector(2)
>>>>>>>> real(kind=8)> [(real(kind=8) *)_4050] ]) 0)
>>>>>>>> (neg:V2DF (subreg:V2DF (reg:OO 2572 [
>>>>>>>> vect__300.543_236 ]) 0))))) {*vsx_nfmsv2df4}
>>>>>>>> (expr_list:REG_DEAD (reg:OO 2572 [ vect__300.543_236 ])
>>>>>>>> (expr_list:REG_DEAD (reg:OO 2561 [ MEM <vector(2)
>>>>>>>> real(kind=8)> [(real(kind=8) *)_4050] ])
>>>>>>>> (nil))))
>>>>>>>>
>>>>>>>> insn 2472 is replaced with 9299 after reload.
>>>>>>>
>>>>>>> You'd have to check the dumps to be sure, but I think 9299 is instead
>>>>>>> generated as an input reload of 2412, rather than being a replacement
>>>>>>> of insn 2472. T
>>>>>>
>>>>>> Yes it is generated for 2412. The predecessor of 2412 is load from
>>>>>> plus offset as in 2412 we have subreg:V2DF (reg OO 2572) 16).
>>>>>>
>>>>>> This is not correct as we are not generating lxvp and it is
>>>>>> normal load lxv.
>>>>>> As normal load is generated in predecessor insn of 2412 with
>>>>>> plus constant offset it breaks the correctness.
>>>>>
>>>>> Not using lxvp is a deliberate choice though.
>>>>>
>>>>> If a (reg:OO R) has been spilled, there's no requirement for LRA
>>>>> to load both halves of R when only one half is needed. LRA just
>>>>> loads what it needs into whichever registers happen to be free.
>>>>>
>>>>> If the reload of R instead used lxvp, LRA would be forced to free
>>>>> up another register for the other half of R, even though that value
>>>>> would never be used.
>>>>>
>>>>
>>>> If a (reg:OO R ) 16 is loaded when it is spilled then loaded value
>>>> will be from plus offset 16 instead it should be loaded value
>>>> from zero offset. As in load fusion pass we are replacing
>>>> (reg:V2DI R) with subreg (reg:OO R) 16 and hence loaded value
>>>> is from plus 16 offsets and thats why its breaking the correctness.
>>>>
>>>> Similarly we are replacing (reg:V2DI R) 16 with subreg (reg:OO R) 0
>>>> and loaded value is from 16 offset instead its loading from zero
>>>> offset and thats why we are breaking the correctness.
>>>>
>>>
>>> If a (reg:OO R ) 16 is loaded when it is spilled then loaded value
>>> will be from plus offset 16 instead it should be loaded value
>>> from zero offset. As in load fusion pass we are replacing
>>> (reg:V2DI R) with subreg (reg:OO R) 16 and hence loaded value
>>> is from plus 16 offsets instead it should load from zero offset.
>>> Thats why its breaking the correctness.
>>>
>>> Similarly we are replacing (reg:V2DI R) 16 with subreg (reg:OO R) 0
>>> and loaded value is from 16 offset instead its loading from zero
>>> offset and thats why we are breaking the correctness.
>>
>> I don't understand, sorry. (subreg:V2DI (reg:OO R) 0) is always
>>
>> (a) the first hard register in (reg:OO R), when the whole of R
>> is stored in hard registers
>> (b) at address offset 0 from the start of (reg:OO R), when R is
>> spilled to memory
>>
>> Similarly, (subreg:V2DI (reg:OO R) 16) is always
>>
>> (c) the second hard register in (reg:OO R), when the whole of R
>> is stored in hard registers
>> (d) at address offset 16 from the start of (reg:OO R), when R is
>> spilled to memory
>>
>
> Yes but we are replacing use of loaded value from plus 16 offset
> with subreg (reg OO ) 0 and similarly we are replacing use of loaded
> value from 0 offset with subreg (reg OO ) 16 as we are swapping
> the use operand.
>
> When it is spilled its vice versa subreg (reg OO ) 16 should be
> loaded from 0 offset and subreg (reg OO) 0 should be loaded
> from 16 offset as we are swapping the use operand.
>
> This is the semantics of lxvp.
Hmm, OK. Does that mean that:
lxvp A,B
loads A+1 from B and A from B+16? (I couldn't find an online
description of the instruction btw -- is there one?)
If lxvp does behave like that, it shouldn't be described as a normal move:
[(set (match_operand:OO 0 "nonimmediate_operand" "=wa,ZwO,wa")
(match_operand:OO 1 "input_operand" "ZwO,wa,wa"))]
since the rules I listed above are target-independent.
Alternatively, if lxvp is described as a normal move,
with the 128-bit pieces swapped compared to GCC's normal order,
TARGET_CAN_CHANGE_MODE_CLASS must prevent subregs that select
one half of the register (or less). But that would defeat exactly
the optimisation that you're trying to do.
Thanks,
Richard
