RFR: JDK-8187601: Unrolling more when SLP auto-vectorization failed
Vladimir Kozlov
vladimir.kozlov at oracle.com
Wed Sep 20 16:18:00 UTC 2017
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.
>>>>>
>>>>
>>>
>>>
>>>
>>
>
>
>
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