Sunday, July 19, 2015

Just-In-Time Compiler Optimizations (Know your JVM)

JIT comes in these flavors:
 C1 (Client compiler) -client option
 C2 (Server compiler)-server option
 -XX:+TieredCompilation - Better decision of compilers.
Common Optimizations done by Just-In-Time (JIT) Compiler do:
 1. Eliminate dead codes and Expression optimization.
 int someCalculation(int x1, int x2, int x3) {
         int res1 = x1+x2;
         int res2 = x1-x2;
         int res3 = x1+x3;
         return (res1+res2)/2; 
will be converted to
int someCalculation(int x1, int x2, int x3) {
 return x1; 
 2. Inline Method
- Substitute body of the method (<35 bytes of JVM bytecode) - This provides the best optimization by JIT - A better inline that C++ 
For Example: 
int compute(int var) { int result; if(var > 5) { result = computeFurther(var); } else { result = 100; } return result; } 
If you call myVal = compute(3); it will get converted into myVal = 100;
3. Caching Technique:
Point findMid(Point p1, Point p2) { Point p; p.x = (p1.x + p2.x)/2; p.y = (p1.y + p2.y)/2; return p;
p1.x, p2.x -> It can convert into temp1, temp2 and can be cached.
4. Monomorphic dispatch:
public class Birds { private String color; public String getColor() { return color; } } myColor = birds.getColor(); 
If there is no other override of this method, it will convert into
public class Birds { String color; }
mycolor = birds.color; 
5. Null Checks Removal:
x = point.x; y = point.y; At JVM it is equivalents to: if(point==null) throw new NullPointerException(); else { x = point.x; y = point.y; }  
But if the code will not throw NullPointer for more than threshold reference, it will remove the if check.
6. Threading Optimizations:
- Eliminate locks if monitor is not reachable from other threads - Join adjacent synchronized blocks on the same object
7. Loop Optimizations: 
- Combining loops – Two loops can be combined if taking equivalent time. - Inversion loops – Change while into do-while. (why, just give a javap -c) - Tiling loops – Re-organize loop so that it will fix in cache. 
VM Args:
Xint – Interpreter mode Xcomp – Compiled mode Xmixed – Interpreter + Compiler -server → C2 compiler -client → C1 compiler -XX:+TieredCompilation → C1 + C2 (used by 32/64 bit mode) 
Logging Options:
-XX:+UnlockDiagnosticVMOptions -XX:+LogCompilation -XX:LogFile=<path to file> -XX:MaxInlineSize=<size> -XX:FreqInlineSize=<size> 

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