RFR(S) 8029302: Performance regression in Math.pow intrinsic
Vladimir Kozlov
vladimir.kozlov at oracle.com
Thu Apr 24 17:20:00 UTC 2014
Good.
Vladimir
On 4/24/14 2:06 AM, Niclas Adlertz wrote:
> Yes, http://cr.openjdk.java.net/~adlertz/JDK-8029302/webrev01/
>
> I only removed:
> region_node->init_req(2, if_false);
>
> Kind Regards,
> Niclas Adlertz
>
> On 04/23/2014 07:21 PM, Vladimir Kozlov wrote:
>> On 4/23/14 4:07 AM, Niclas Adlertz wrote:
>>> Hi Vladimir,
>>>
>>> > Next line is not needed, this edge will be initialized later:
>>> >
>>> > region_node->init_req(2, if_false);
>>> Thanks.
>>>
>>> > And I am not sure that you should skip result check:
>>> >
>>> > if (result != result)? {
>>> > result = uncommon_trap() or runtime_call();
>>> > }
>>> As I understand it, the reason why we have this check is to see if the
>>> fast_pow() intrinsic computed a NaN result where we expected a non-NaN
>>> result.
>>>
>>> This can happen in two cases as I see it;
>>> 1. When x < 0.0
>>> 2. When x = NaN and y == 0.
>>>
>>> The first case will never happen, since we never call fast_pow with x <
>>> 0.0.
>>> The second case we could do a special case for, as you mentioned in your
>>> previous mail. (x**0 = 1)
>>
>> Based on your explanations current check placement is good. We will not
>> need it for (x**0 = 1) too.
>>
>>>
>>> There might be more cases when fast_pow() can return a NaN result (where
>>> we expect a non-NaN result) which I haven't spotted. If not, we could
>>> add a special case for x**0 and move the check of NaN in the end to
>>> inside the else body inside:
>>> if (x <= 0.0) {
>>> long longy = (long)y;
>>> if ((double)longy == y) { // if y is long
>>> if (y + 1 == y) longy = 0; // huge number: even
>>> result = ((1&longy) == 0)?-DPow(abs(x), y):DPow(abs(x), y);
>>> } else {
>>> // move result != result check here
>>> }
>>> }
>>>
>>
>> Lets consider this when we add additional optimization.
>>
>>> I believe we currently do excessive checking of NaN.
>>> NaN**y where y != 0 should result in NaN, fast_pow() will return NaN
>>> here. Despite this, we will still do the result != result check, it will
>>> be true and we will do a call to the runtime.
>>
>> Since NaN is an edge case it may not be a matter for now. But I agree
>> that we can add a check x == NaN and call runtime immediately before
>> calling fast_pow(). If it does not affect much performance (it is
>> additional branch) we should go for this change.
>>
>>>
>>> In the case of x**2, I don't see how we can create a non expected NaN
>>> result, since the only way we can get a NaN result is NaN**2, which
>>> should result in NaN anyway.
>>
>> Agree.
>>
>> Do you have latest webrev?
>>
>> Vladimir
>>
>>>
>>> Kind Regards,
>>> Niclas Adlertz
>>>
>>> On 04/17/2014 04:14 PM, Vladimir Kozlov wrote:
>>>> About your changes.
>>>>
>>>> Next line is not needed, this edge will be initialized later:
>>>>
>>>> region_node->init_req(2, if_false);
>>>>
>>>> And I am not sure that you should skip result check:
>>>>
>>>> if (result != result)? {
>>>> result = uncommon_trap() or runtime_call();
>>>> }
>>>>
>>>> Thanks,
>>>> Vladimir
>>>>
>>>> On 4/17/14 8:45 AM, Vladimir Kozlov wrote:
>>>>> Niclas,
>>>>>
>>>>> Looking on __ieee754_pow() in sharedRuntimeTrans.cpp and it has other
>>>>> simple cases:
>>>>>
>>>>> x**0 = 1
>>>>> x**1 = x
>>>>> x**-1 = 1/x
>>>>> x**0.5 = sqrt(x)
>>>>>
>>>>> It would be nice to know which are frequently used and implement them
>>>>> too.
>>>>>
>>>>> Also there is check for NaN before all this cases except x**0 = 1:
>>>>>
>>>>> /* +-NaN return x+y */
>>>>>
>>>>> You need to test that new C2 code produces the same results for NaN
>>>>> values.
>>>>>
>>>>> Thanks,
>>>>> Vladimir
>>>>>
>>>>> On 4/17/14 3:10 AM, Niclas Adlertz wrote:
>>>>>> Hi all,
>>>>>>
>>>>>> webrev: http://cr.openjdk.java.net/~adlertz/JDK-8029302/webrev00/
>>>>>> bug: https://bugs.openjdk.java.net/browse/JDK-8029302
>>>>>>
>>>>>> We have a performance regression in Math.pow(x,2) on x64, starting
>>>>>> from 7u40.
>>>>>> In 7u40 we replaced a call to SharedRuntime::dpow with an intrinsic
>>>>>> for Math.pow. This is faster in almost all cases,
>>>>>> except for Math.pow(x,2). (See comments in bug report for more info.)
>>>>>>
>>>>>> I have added a C2 IR check for Math.pow(x,y) when y == 2, and instead
>>>>>> of calling SharedRuntime::dpow when y == 2, I
>>>>>> directly do x * x.
>>>>>>
>>>>>> I've changed the generated C2 IR,
>>>>>>
>>>>>> From (psuedo code):
>>>>>>
>>>>>> if (x <= 0.0) {
>>>>>> long longy = (long)y;
>>>>>> if ((double)longy == y) { // if y is long
>>>>>> if (y + 1 == y) longy = 0; // huge number: even
>>>>>> result = ((1&longy) == 0)?-DPow(abs(x), y):DPow(abs(x), y);
>>>>>> } else {
>>>>>> result = NaN;
>>>>>> }
>>>>>> } else {
>>>>>> result = DPow(x,y);
>>>>>> }
>>>>>> if (result != result)? {
>>>>>> result = uncommon_trap() or runtime_call();
>>>>>> }
>>>>>> return result;
>>>>>>
>>>>>> To (psuedo code):
>>>>>>
>>>>>> if (y == 2) {
>>>>>> return x * x;
>>>>>> } else {
>>>>>> if (x <= 0.0) {
>>>>>> long longy = (long)y;
>>>>>> if ((double)longy == y) { // if y is long
>>>>>> if (y + 1 == y) longy = 0; // huge number: even
>>>>>> result = ((1&longy) == 0)?-DPow(abs(x), y):DPow(abs(x), y);
>>>>>> } else {
>>>>>> result = NaN;
>>>>>> }
>>>>>> } else {
>>>>>> result = DPow(x,y);
>>>>>> }
>>>>>> if (result != result)? {
>>>>>> result = uncommon_trap() or runtime_call();
>>>>>> }
>>>>>> return result;
>>>>>> }
>>>>>>
>>>>>> I have run jtreg tests in jdk/tests/java/lang (with -server, -Xcomp
>>>>>> and -XX:-TieredCompilation) and run JPRT. No
>>>>>> problems encountered.
>>>>>> In particular, java/lang/Math/PowTests passes.
>>>>>>
>>>>>> I re-wrote the performance test included in the bug report
>>>>>> (https://bugs.openjdk.java.net/secure/attachment/17807/Main.java)
>>>>>> to a JMH test;
>>>>>> http://cr.openjdk.java.net/~adlertz/JDK-8029302/webrev00/MyBenchmark.java
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> Below are the performance results. The x^2 case is now much faster
>>>>>> even compared to 7u25. (Since we now skip the call to
>>>>>> SharedRuntime::dpow)
>>>>>>
>>>>>> Numbers from 7u25 b34:
>>>>>> Iteration 1: 46764.923 ops/ms
>>>>>> Iteration 2: 46695.196 ops/ms
>>>>>> Iteration 3: 46647.386 ops/ms
>>>>>> Iteration 4: 46806.854 ops/ms
>>>>>> Iteration 5: 46787.259 ops/ms
>>>>>> Iteration 6: 46788.196 ops/ms
>>>>>> Iteration 7: 46797.500 ops/ms
>>>>>> Iteration 8: 46784.237 ops/ms
>>>>>> Iteration 9: 46782.717 ops/ms
>>>>>> Iteration 10: 46790.678 ops/ms
>>>>>> Iteration 11: 46785.139 ops/ms
>>>>>> Iteration 12: 46798.346 ops/ms
>>>>>> Iteration 13: 46784.595 ops/ms
>>>>>> Iteration 14: 46770.963 ops/ms
>>>>>> Iteration 15: 46789.574 ops/ms
>>>>>> Iteration 16: 46822.452 ops/ms
>>>>>> Iteration 17: 46813.571 ops/ms
>>>>>> Iteration 18: 46747.076 ops/ms
>>>>>> Iteration 19: 46774.254 ops/ms
>>>>>> Iteration 20: 46779.329 ops/ms
>>>>>>
>>>>>> Result : 46775.512 ±(99.9%) 34.788 ops/ms
>>>>>> Statistics: (min, avg, max) = (46647.386, 46775.512, 46822.452),
>>>>>> stdev = 40.061
>>>>>> Confidence interval (99.9%): [46740.725, 46810.300]
>>>>>>
>>>>>>
>>>>>> Numbers from 7u40 b34:
>>>>>> Iteration 1: 9966.052 ops/ms
>>>>>> Iteration 2: 9967.683 ops/ms
>>>>>> Iteration 3: 9967.229 ops/ms
>>>>>> Iteration 4: 9967.266 ops/ms
>>>>>> Iteration 5: 9937.091 ops/ms
>>>>>> Iteration 6: 9966.272 ops/ms
>>>>>> Iteration 7: 9964.679 ops/ms
>>>>>> Iteration 8: 9966.326 ops/ms
>>>>>> Iteration 9: 9964.899 ops/ms
>>>>>> Iteration 10: 9966.920 ops/ms
>>>>>> Iteration 11: 9963.278 ops/ms
>>>>>> Iteration 12: 9967.334 ops/ms
>>>>>> Iteration 13: 9963.351 ops/ms
>>>>>> Iteration 14: 9968.032 ops/ms
>>>>>> Iteration 15: 9964.312 ops/ms
>>>>>> Iteration 16: 9967.080 ops/ms
>>>>>> Iteration 17: 9965.114 ops/ms
>>>>>> Iteration 18: 9966.860 ops/ms
>>>>>> Iteration 19: 9965.375 ops/ms
>>>>>> Iteration 20: 9966.215 ops/ms
>>>>>>
>>>>>> Result : 9964.568 ±(99.9%) 5.743 ops/ms
>>>>>> Statistics: (min, avg, max) = (9937.091, 9964.568, 9968.032),
>>>>>> stdev = 6.613
>>>>>> Confidence interval (99.9%): [9958.826, 9970.311]
>>>>>>
>>>>>>
>>>>>> Numbers from http://hg.openjdk.java.net/jdk9/hs-comp/hotspot without
>>>>>> the y == 2 check:
>>>>>> Iteration 1: 9966.775 ops/ms
>>>>>> Iteration 2: 9964.514 ops/ms
>>>>>> Iteration 3: 9959.708 ops/ms
>>>>>> Iteration 4: 9965.501 ops/ms
>>>>>> Iteration 5: 9958.087 ops/ms
>>>>>> Iteration 6: 9964.471 ops/ms
>>>>>> Iteration 7: 9964.966 ops/ms
>>>>>> Iteration 8: 9965.132 ops/ms
>>>>>> Iteration 9: 9959.055 ops/ms
>>>>>> Iteration 10: 9964.666 ops/ms
>>>>>> Iteration 11: 9965.649 ops/ms
>>>>>> Iteration 12: 9964.309 ops/ms
>>>>>> Iteration 13: 9966.963 ops/ms
>>>>>> Iteration 14: 9956.511 ops/ms
>>>>>> Iteration 15: 9964.881 ops/ms
>>>>>> Iteration 16: 9966.927 ops/ms
>>>>>> Iteration 17: 9951.054 ops/ms
>>>>>> Iteration 18: 9966.512 ops/ms
>>>>>> Iteration 19: 9967.041 ops/ms
>>>>>> Iteration 20: 9967.198 ops/ms
>>>>>>
>>>>>> Result : 9963.496 ±(99.9%) 3.760 ops/ms
>>>>>> Statistics: (min, avg, max) = (9951.054, 9963.496, 9967.198),
>>>>>> stdev = 4.330
>>>>>> Confidence interval (99.9%): [9959.736, 9967.256]
>>>>>>
>>>>>>
>>>>>> Numbers from http://hg.openjdk.java.net/jdk9/hs-comp/hotspot with the
>>>>>> y == 2 check:
>>>>>> Iteration 1: 276969.757 ops/ms
>>>>>> Iteration 2: 276809.529 ops/ms
>>>>>> Iteration 3: 276621.258 ops/ms
>>>>>> Iteration 4: 276352.094 ops/ms
>>>>>> Iteration 5: 276922.865 ops/ms
>>>>>> Iteration 6: 276617.189 ops/ms
>>>>>> Iteration 7: 276941.087 ops/ms
>>>>>> Iteration 8: 276215.547 ops/ms
>>>>>> Iteration 9: 276118.685 ops/ms
>>>>>> Iteration 10: 276550.807 ops/ms
>>>>>> Iteration 11: 276773.424 ops/ms
>>>>>> Iteration 12: 276871.125 ops/ms
>>>>>> Iteration 13: 276059.947 ops/ms
>>>>>> Iteration 14: 277109.329 ops/ms
>>>>>> Iteration 15: 276910.165 ops/ms
>>>>>> Iteration 16: 276138.922 ops/ms
>>>>>> Iteration 17: 276083.749 ops/ms
>>>>>> Iteration 18: 276367.479 ops/ms
>>>>>> Iteration 19: 276563.471 ops/ms
>>>>>> Iteration 20: 276022.425 ops/ms
>>>>>>
>>>>>> Result : 276550.943 ±(99.9%) 309.657 ops/ms
>>>>>> Statistics: (min, avg, max) = (276022.425, 276550.943,
>>>>>> 277109.329), stdev = 356.601
>>>>>> Confidence interval (99.9%): [276241.286, 276860.600]
>>>>>>
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