n ** m is slow
#3818
Comments
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Appears to be because there are so many BigInteger allocations/operations in the Demonstrated by this hack: diff --git a/core/src/main/java/org/jruby/util/Numeric.java b/core/src/main/java/org/jruby/util/Numeric.java
index cab2ebe..087aa32 100644
--- a/core/src/main/java/org/jruby/util/Numeric.java
+++ b/core/src/main/java/org/jruby/util/Numeric.java
@@ -507,6 +507,9 @@ public class Numeric {
y >>= 1;
}
+ if (z * x > z) {
+ z *= x;
+ } else {
BigInteger bigX = BigInteger.valueOf(x);
BigInteger bigXZ = bigX.multiply(bigX);
if (bigXZ.divide(bigX).longValue() != z) {
@@ -515,6 +518,7 @@ public class Numeric {
return v;
}
z = bigXZ.longValue();
+ }
} while(--y != 0);
if (neg) z = -z;
return RubyFixnum.newFixnum(runtime, z);output: |
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Incidentally, I can't provide a proper PR/patch, because I'm totally lost by this logic around the x/y/z values: BigInteger bigX = BigInteger.valueOf(x);
BigInteger bigXZ = bigX.multiply(bigXZ); /* is this right? */
if (bigXZ.divide(bigX).longValue() != z) {
/* .. */
}
z = bigXZ.longValue();In pseudocode this is: xz = x * x #?
if (xz / x != z) # ?? (x*x)/x==x, no? so this is: if (x != z)
# ...
end
z = xz # z = x*x? I expected z = x*zI can't work out what that Is this the real cause of the slowdown? |
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@phluid61 yeah seems like this should be z and not x. Because of that we seem to always going the Bignum algo at that point which will be slow. In MRI, if (MUL_OVERFLOW_FIXNUM_P(x, z)) {
goto bignum;
}So this logic is meant to be a replacement for MRI's overflow check. When I change this to use BigInteger.valueOf(z) we double in speed and avoid going down biginteger path but then we fail one boundary test in ruby/spec. |
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Ok I basically ported MRI's overflow check. Numbers...Before: After: Compared to MRI 2.3: |
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👍 |
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@phluid61 Thanks for bringing this to light. Almost makes me want to generate some microbenches against other numeric operations to see if we have other issues. |
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👍 on more numeric benchmarks!
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Environment
Expected Behavior
**should have comparable benchmark for Integers as*Actual Behavior
Benchmarks show
**is an order of magnitude slower than*for Integers, andInteger ** Integeris slower thanFloat ** Integertest.rb:
MRI:
jruby:
Originally reported here
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