RFR(S) 8029302: Performance regression in Math.pow intrinsic

Wed Apr 23 11:06:03 UTC 2014

Hi Vladimir,

 > x**0 = 1
 > x**1 = x
 > x**-1  = 1/x
 > x**0.5 = sqrt(x)
I will file a new bug for these simple cases as well.

 > Also there is check for NaN before all this cases except x**0 = 1:
 >
 > /* +-NaN return x+y */
This case returns NaN if either x (when y != 0) or y is NaN?
If so, yes, we should handle this simple case as well.
(Why do we return x+y? Would not
if (x != x) return x;
if (y != y) return y;
be clearer and faster since we can skip the addition?)

The only case of NaN I will catch when y == 2 is NaN**2. And that should 
result in NaN since we will do x * x (NaN * NaN).

Kind Regards,
Niclas Adlertz

On 04/17/2014 03:45 PM, 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]
>>