On Sat, Nov 13, 2010 at 6:56 AM, Paolo Bonzini <bonz...@gnu.org> wrote:
> On 11/13/2010 03:34 PM, H.J. Lu wrote:
>>
>> On Sat, Nov 13, 2010 at 2:27 AM, Paolo Bonzini<bonz...@gnu.org> wrote:
>>>
>>> On 11/12/2010 03:25 PM, H.J. Lu wrote:
>>>>
>>>> IRA may move instructions across an unspec_volatile,
>>>
>>> Do you have a testcase?
>>>
>>
>> x86 has
>>
>> ;; Clear the upper 128bits of AVX registers, equivalent to a NOP
>> ;; if the upper 128bits are unused.
>> (define_insn "avx_vzeroupper"
>> [(unspec_volatile [(match_operand 0 "const_int_operand" "")]
>> UNSPECV_VZEROUPPER)]
>> "TARGET_AVX"
>> "vzeroupper"
>> [(set_attr "type" "sse")
>> (set_attr "modrm" "0")
>> (set_attr "memory" "none")
>> (set_attr "prefix" "vex")
>> (set_attr "mode" "OI")])
>>
>> It is no-nop, but it has to be in the place where it was expanded.
>> Since there is no register operand, IRA moves instructions across
>> it. We have to undo IRA moves in ix86_reorg.
>
> That's because VZEROUPPER (and VZEROALL too, btw) has input and output
Please pay close attention to VZEROALL, which I enclosed below.
> operands that you are not modeling. Undoing these moves in reorg seems very
> wrong to me, even though you need it anyway to delete them.
>
VZEROUPPER is no-nop to executions. But it isn't no-nop for performance.
That is why no instructions should be moved across it. But IRA doesn't support
this.
--
H.J.
--
(define_insn "*avx_vzeroall"
[(match_parallel 0 "vzeroall_operation"
[(unspec_volatile [(const_int 0)] UNSPECV_VZEROALL)])]
"TARGET_AVX"
"vzeroall"
[(set_attr "type" "sse")
(set_attr "modrm" "0")
(set_attr "memory" "none")
(set_attr "prefix" "vex")
(set_attr "mode" "OI")])
