This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: [PATCH/AARCH64] Improve/correct ThunderX 1 cost model for Arith_shift


On Mon, Jun 19, 2017 at 2:00 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> On Wed, Jun 7, 2017 at 10:16 AM, James Greenhalgh
> <james.greenhalgh@arm.com> wrote:
>> On Fri, Dec 30, 2016 at 10:05:26PM -0800, Andrew Pinski wrote:
>>> Hi,
>>>   Currently for the following function:
>>> int f(int a, int b)
>>> {
>>>   return a + (b <<7);
>>> }
>>>
>>> GCC produces:
>>> add     w0, w0, w1, lsl 7
>>> But for ThunderX 1, it is better if the instruction was split allowing
>>> better scheduling to happen in most cases, the latency is the same.  I
>>> get a small improvement in coremarks, ~1%.
>>>
>>> Currently the code does not take into account Arith_shift even though
>>> the comment:
>>>   /* Strip any extend, leave shifts behind as we will
>>>     cost them through mult_cost.  */
>>> Say it does not strip out the shift, aarch64_strip_extend does and has
>>> always has since the back-end was added to GCC.
>>>
>>> Once I fixed the code around aarch64_strip_extend, I got a regression
>>> for ThunderX 1 as some shifts/extends (left shifts <=4 and/or zero
>>> extends) are considered free so I needed to add a new tuning flag.
>>>
>>> Note I will get an even more improvement for ThunderX 2 CN99XX, but I
>>> have not measured it yet as I have not made the change to
>>> aarch64-cost-tables.h yet as I am waiting for approval of the renaming
>>> patch first before submitting any of the cost table changes.  Also I
>>> noticed this problem with this tuning first and then looked back at
>>> what I needed to do for ThunderX 1.
>>>
>>> OK?  Bootstrapped and tested on aarch64-linux-gnu without any
>>> regressions (both with and without --with-cpu=thunderx).
>>
>> This is mostly OK, but I don't like the name "easy"_shift_extend. Cheap
>> or free seems better. I have some other minor points below.
>
>
> Ok, that seems like a good idea.  I used easy since that was the
> wording our hardware folks had came up with.  I am changing the
> comments to make clearer when this flag should be used.
> I should a new patch out by the end of today.

Due to the LSE ICE which I reported in the other thread, it took me
longer to send out a new patch.
Anyways here is the updated patch with the changes requested.


OK? Bootstrapped and tested on aarch64-linux-gnu with no regressions.


Thanks,
Andrew Pinski


* config/aarch64/aarch64-cost-tables.h (thunderx_extra_costs):
Increment Arith_shift and Arith_shift_reg by 1.
* config/aarch64/aarch64-tuning-flags.def (cheap_shift_extend): New tuning flag.
* config/aarch64/aarch64.c (thunderx_tunings): Enable
AARCH64_EXTRA_TUNE_CHEAP_SHIFT_EXTEND.
(aarch64_strip_extend): Add new argument and test for it.
(aarch64_cheap_mult_shift_p): New function.
(aarch64_rtx_mult_cost): Call aarch64_cheap_mult_shift_p and don't add
a cost if it is true.
Update calls to aarch64_strip_extend.
(aarch64_rtx_costs): Update calls to aarch64_strip_extend.



>
> Thanks,
> Andrew
>
>
>>
>>> Index: config/aarch64/aarch64-tuning-flags.def
>>> ===================================================================
>>> --- config/aarch64/aarch64-tuning-flags.def   (revision 243974)
>>> +++ config/aarch64/aarch64-tuning-flags.def   (working copy)
>>> @@ -35,4 +35,8 @@ two load/stores are not at least 8 byte
>>>  pairs.   */
>>>  AARCH64_EXTRA_TUNING_OPTION ("slow_unaligned_ldpw", SLOW_UNALIGNED_LDPW)
>>>
>>> +/* Logical shift left <=4 with/without zero extend are considered easy
>>> +   extended, also zero extends without the shift. */
>>
>>
>> I'm struggling to parse this comment. "also zero extends without the shift"
>> is what is getting me. I'm also not certain I follow when I should set this
>> flag. If all shifts are cheap/free on my platform, should I set this flag?
>>
>>> +AARCH64_EXTRA_TUNING_OPTION ("easy_shift_extend", EASY_SHIFT_EXTEND)
>>> +
>>>  #undef AARCH64_EXTRA_TUNING_OPTION
>>
>>
>>> +
>>> +/* Return true iff X is an easy shift without a sign extend. */
>>> +
>>
>> Again I don't like calling <= 4 "easy", it feels imprecise.
>>
>> Thanks,
>> James
>>

Attachment: improvecost.diff.txt
Description: Text document


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]