FULL PRODUCT VERSION :
C:\Users\ajeetkumarv\Desktop\Nashorn Bug>java -version
java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 10.0.14393]

A DESCRIPTION OF THE PROBLEM :
I've commented the mismatches between outputs of versions in below code.

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
import javax.script.Invocable;

public class NashornTest {

public static void main(String[] args) throws ScriptException, NoSuchMethodException{

final String CODE =
" function testFunction(num1, num2) {"
// this prints true for java version 1.8.0_66 but false in 1.8.0_121
// we tried converting them to primitves also using Long.longValue() but results false in 121
+" print(num1 == num2);" // returns false in version 121 but true in version 66
+" print(num1.longValue() == num2.longValue());" // returns false in version 121 but true in version 66
+" }";

final ScriptEngine ENGINE = new ScriptEngineManager().getEngineByName("nashorn");
ENGINE.eval(CODE);
Invocable invocable = (Invocable) ENGINE;

invocable.invokeFunction("testFunction", 37170L, 37170L);

}

}

REGRESSION. Last worked in version 8u102

ADDITIONAL REGRESSION INFORMATION:
C:\Program Files\Java\jdk1.8.0_66\bin>java -version
java version "1.8.0_66"
Java(TM) SE Runtime Environment (build 1.8.0_66-b18)
Java HotSpot(TM) 64-Bit Server VM (build 25.66-b18, mixed mode)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Just compile and run below code for java version 1.8_0_66 and for version 1.8_0_121

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
import javax.script.Invocable;

public class NashornTest {

public static void main(String[] args) throws ScriptException, NoSuchMethodException{

final String CODE =
" function testFunction(num1, num2) {"
// this prints true for java version 1.8.0_66 but false in 1.8.0_121
// we tried converting them to primitves also using Long.longValue() but results false in 121
+" print(num1 == num2);" // returns false in version 121 but true in version 66
+" print(num1.longValue() == num2.longValue());" // returns false in version 121 but true in version 66
+" }";

final ScriptEngine ENGINE = new ScriptEngineManager().getEngineByName("nashorn");
ENGINE.eval(CODE);
Invocable invocable = (Invocable) ENGINE;

invocable.invokeFunction("testFunction", 37170L, 37170L);

}

}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
From within Nashorn engine:
I'm expecting when I compare long values num1 == num2 it should be true as in version 1.8_0_66.

I tested them with converting them to primitive using num1.longValue() == num2.longValue() but in 1.8_0_121 gives false
ACTUAL -
I see two long values when compare within Nashorn gave me false in version 121 inspite of having same value. this gave me true in version 66

ERROR MESSAGES/STACK TRACES THAT OCCUR :
No crashes

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
final String CODE =
" function testFunction(num1, num2) {"
// this prints true for java version 1.8.0_66 but false in 1.8.0_121
// we tried converting them to primitves also using Long.longValue() but results false in 121
+" print(num1 == num2);" // returns false in version 121 but true in version 66
+" print(num1.longValue() == num2.longValue());" // returns false in version 121 but true in version 66
+" }";

final ScriptEngine ENGINE = new ScriptEngineManager().getEngineByName("nashorn");
ENGINE.eval(CODE);
Invocable invocable = (Invocable) ENGINE;

invocable.invokeFunction("testFunction", 37170L, 37170L);
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
As per my observation equals method work.
But why does num1*1 == num2*1 works, I did this just to see if this converts them to primitives and it works. but num1.longValue() doesn't