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

[Niclas Adlertz]

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]