RFR: JDK-8187601: Unrolling more when SLP auto-vectorization failed

[Zhongwei Yao]

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