RFR(S) 8029302: Performance regression in Math.pow intrinsic
Vladimir Kozlov
vladimir.kozlov at oracle.com
Thu Apr 17 15:45:12 UTC 2014
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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