RFR: JDK-8187601: Unrolling more when SLP auto-vectorization failed
Zhongwei Yao
zhongwei.yao at linaro.org
Fri Sep 29 08:25:39 UTC 2017
Hi, Vladimir,
Sorry for my late response!
And yes, it solves my case.
But I found specjvm2008 doesn't have a stable result, especially for
benchmark case like startup.xxx, scimark.xxx.large etc. And I have
found obvious performance regress in the rest of benchmark cases. What
do you think?
On 21 September 2017 at 00:18, Vladimir Kozlov
<vladimir.kozlov at oracle.com> wrote:
> Nice.
>
> Did you verified that it fixed your case?
>
> Would be nice to run specjvm2008 to make sure performance did not regress.
>
> Thanks,
> Vladimir
>
>
> On 9/20/17 4:07 AM, Zhongwei Yao wrote:
>>
>> Thanks for your suggestions!
>>
>> I've updated the patch that uses pass_slp and do_unroll_only flags
>> without adding a new flag. Please take a look:
>>
>> http://cr.openjdk.java.net/~zyao/8187601/webrev.01/
>>
>>
>>
>> On 20 September 2017 at 01:54, Vladimir Kozlov
>> <vladimir.kozlov at oracle.com> wrote:
>>>
>>>
>>>
>>> On 9/18/17 10:59 PM, Zhongwei Yao wrote:
>>>>
>>>>
>>>> Hi, Vladimir,
>>>>
>>>> On 19 September 2017 at 00:17, Vladimir Kozlov
>>>> <vladimir.kozlov at oracle.com> wrote:
>>>>>
>>>>>
>>>>> Why not use existing set_notpassed_slp() instead of
>>>>> mark_slp_vec_failed()?
>>>>
>>>>
>>>>
>>>> Due to 2 reasons, I have not chosen existing passed_slp flag:
>>>
>>>
>>>
>>> My point is that if we don't find vectors in a loop (as in your case) we
>>> should ignore whole SLP analysis.
>>>
>>> In best case scenario SuperWord::unrolling_analysis() should determine if
>>> there are vectors candidates. For example, check if array's index is
>>> depend
>>> on loop's index variable.
>>>
>>> An other way is to call SuperWord::unrolling_analysis() only after we did
>>> vector analysis.
>>>
>>> It is more complicated changes and out of scope of this. There is also
>>> side
>>> effect I missed before which may prevent using set_notpassed_slp():
>>> LoopMaxUnroll is changed based on SLP analysis before has_passed_slp()
>>> check.
>>>
>>> Note, set_notpassed_slp() is also used to additional unroll already
>>> vectorized loops:
>>>
>>>
>>> http://hg.openjdk.java.net/jdk10/hs/hotspot/file/5ab7a67bc155/src/share/vm/opto/superword.cpp#l2421
>>>
>>> May be you should also call mark_do_unroll_only() when you set
>>> set_major_progress() for _packset.length() == 0 to avoid loop_opts_cnt
>>> problem you pointed. Can you look on this?
>>>
>>> I am not against adding new is_slp_vec_failed() but I want first to
>>> investigate if we can re-use existing functions.
>>>
>>> Thanks,
>>> Vladimir
>>>
>>>
>>>> 1. If we set_notpassed_slp() when _packset.length() == 0 in
>>>> SuperWord::output(), then in the IdealLoopTree::policy_unroll()
>>>> checking:
>>>>
>>>> if (cl->has_passed_slp()) {
>>>> if (slp_max_unroll_factor >= future_unroll_ct) return true;
>>>> // Normal case: loop too big
>>>> return false;
>>>> }
>>>>
>>>> we will ignore the case: "cl->has_passed_slp() &&
>>>> slp_max_unroll_factor < future_unroll_ct && !cl->is_slp_vec_failed()"
>>>> as alos exposed in my patch:
>>>>
>>>> if (cl->has_passed_slp()) {
>>>> if (slp_max_unroll_factor >= future_unroll_ct) return true;
>>>> - // Normal case: loop too big
>>>> - return false;
>>>> + // When SLP vectorization failed, we could do more unrolling
>>>> + // optimizations if body size is less than limit size. Otherwise,
>>>> + // return false due to loop is too big.
>>>> + if (!cl->is_slp_vec_failed()) return false;
>>>> }
>>>>
>>>> However, I have not found a case to support this condition yet.
>>>>
>>>> 2. As replied below, in:
>>>>>
>>>>>
>>>>> - } else if (cl->is_main_loop()) {
>>>>> + } else if (cl->is_main_loop() && !cl->is_slp_vec_failed()) {
>>>>> sw.transform_loop(lpt, true);
>>>>
>>>>
>>>> I need to check whether cl->is_slp_vec_failed() is true.Such
>>>> checking becomes explicit when using SLPAutoVecFailed flag.
>>>>
>>>>>
>>>>> Why you need next additional check?:
>>>>>
>>>>> - } else if (cl->is_main_loop()) {
>>>>> + } else if (cl->is_main_loop() && !cl->is_slp_vec_failed()) {
>>>>> sw.transform_loop(lpt, true);
>>>>>
>>>>
>>>> The additional check prevents the case that when
>>>> cl->is_slp_vec_failed() is true, then SuperWord::output() will
>>>> set_major_progress() at the beginning (because _packset.length() == 0
>>>> is true when cl->is_slp_vec_failed() is true). Then the "phase ideal
>>>> loop iteration" will not stop untill loop_opts_cnt reachs 0, which is
>>>> not we want.
>>>
>>>
>>>
>>>
>>>>
>>>>>
>>>>> Thanks,
>>>>> Vladimir
>>>>>
>>>>>
>>>>> On 9/18/17 2:58 AM, Zhongwei Yao wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> [Forward from aarch64-port-dev to hotspot-compiler-dev]
>>>>>>
>>>>>> Hi, all,
>>>>>>
>>>>>> Bug:
>>>>>> https://bugs.openjdk.java.net/browse/JDK-8187601
>>>>>>
>>>>>> Webrev:
>>>>>> http://cr.openjdk.java.net/~zyao/8187601/webrev.00
>>>>>>
>>>>>> In the current implementation, the loop unrolling times are determined
>>>>>> by vector size and element size when SuperWordLoopUnrollAnalysis is
>>>>>> true (both X86 and aarch64 are true for now).
>>>>>>
>>>>>> This unrolling policy generates less optimized code when SLP
>>>>>> auto-vectorization fails (as following example shows).
>>>>>>
>>>>>> In this patch, I modify the current unrolling policy to do more
>>>>>> unrolling when SLP auto-vectorization fails. So the loop will be
>>>>>> unrolled until reaching the unroll times limitation.
>>>>>>
>>>>>> Here is one example:
>>>>>> public static void accessArrayConstants(int[] array) {
>>>>>> for (int j = 0; j < 1024; j++) {
>>>>>> array[0]++;
>>>>>> array[1]++;
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> Before this patch, the loop will be unrolled by 4 times. 4 is
>>>>>> determined by: AArch64's vector size 128 bits / array element size 32
>>>>>> bits = 4. On X86, vector size is 256 bits. So the unroll times are 8.
>>>>>>
>>>>>> Below is the generated code by C2 on AArch64:
>>>>>>
>>>>>> ==== generated code start ====
>>>>>> 0x0000ffff6caf3180: ldr w10, [x1,#16] ;
>>>>>> 0x0000ffff6caf3184: add w13, w10, #0x1
>>>>>> 0x0000ffff6caf3188: str w13, [x1,#16] ;
>>>>>> 0x0000ffff6caf318c: ldr w12, [x1,#20] ;
>>>>>> 0x0000ffff6caf3190: add w13, w10, #0x4
>>>>>> 0x0000ffff6caf3194: add w10, w12, #0x4
>>>>>> 0x0000ffff6caf3198: str w13, [x1,#16] ;
>>>>>> 0x0000ffff6caf319c: add w11, w11, #0x4 ;
>>>>>> 0x0000ffff6caf31a0: str w10, [x1,#20] ;
>>>>>> 0x0000ffff6caf31a4: cmp w11, #0x3fd
>>>>>> 0x0000ffff6caf31a8: b.lt 0x0000ffff6caf3180 ;
>>>>>> ==== generated code end ====
>>>>>>
>>>>>> After applied this patch, it is unrolled 16 times:
>>>>>>
>>>>>> ==== generated code start ====
>>>>>> 0x0000ffffb0aa6100: ldr w10, [x1,#16] ;
>>>>>> 0x0000ffffb0aa6104: add w13, w10, #0x1
>>>>>> 0x0000ffffb0aa6108: str w13, [x1,#16] ;
>>>>>> 0x0000ffffb0aa610c: ldr w12, [x1,#20] ;
>>>>>> 0x0000ffffb0aa6110: add w13, w10, #0x10
>>>>>> 0x0000ffffb0aa6114: add w10, w12, #0x10
>>>>>> 0x0000ffffb0aa6118: str w13, [x1,#16] ;
>>>>>> 0x0000ffffb0aa611c: add w11, w11, #0x10 ;
>>>>>> 0x0000ffffb0aa6120: str w10, [x1,#20] ;
>>>>>> 0x0000ffffb0aa6124: cmp w11, #0x3f1
>>>>>> 0x0000ffffb0aa6128: b.lt 0x0000ffffb0aa6100 ;
>>>>>> ==== generated code end ====
>>>>>>
>>>>>> This patch passes jtreg tests both on AArch64 and X86.
>>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>
>>
>>
>>
>
--
Best regards,
Zhongwei
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