Description Bill Tracey 2007-07-11 13:01:49 EDT

In profiling startup of one of the applications I support I saw the class initializer for org.eclipse.swt.internal.image.JPEGFileFormat accounted for >2% of instructions executed in the scenario.  This seemed excessive -- upon investigation discovered code of the form: 

static int[] someClassTable; 

static { 
    someClassTable = new int[2048]; 
    for ( int i = 0; i < someClassTable.length; i++ ) { 
        someClassTable[i] = some computation; 
    } 
} 

It turns out the access to the class variable from within the class initializer is quite expensive -- changing the code to store thru a local: 

static { 
    int[] local_ClassTable = new int[2048]; 
    // someClassTable = new int[2048]; 
    for ( int i = 0; i < someClassTable.length; i++ ) { 
        local_ClassTable[i] = some computation; 
    } 
    someClassTable = local_ClassTable; 
} 

Applying this change to initializeBitCountTable(), initializeRGBYCbCrTables()
initializeYCbCrRGBTables() results in an instruction count of approx 0.7 million 
for the JPEGFileFormat class initializer.  Without the change the instruction count is in excess of 120 million. 

I can provide the changed code via email.