user manual

Generic Loop Hoisting 23
22007E/0—November 1999 AMD Athlon™ Processor x86 Code Optimization
Generalization for Multiple Constant Control Code
To generalize this further for multiple constant control code
some more work may have to be done to create the proper outer
loop. Enumeration of the constant cases will reduce this to a
simple switch statement.
Example 2:
for(i ... ) {
if( CONSTANT0 ) {
DoWork0( i ); //does not affect CONSTANT0
// or CONSTANT1
} else {
DoWork1( i ); //does not affect CONSTANT0
// or CONSTANT1
}
if( CONSTANT1 ) {
DoWork2( i ); //does not affect CONSTANT0
// or CONSTANT1
} else {
DoWork3( i ); //does not affect CONSTANT0
// or CONSTANT1
}
}
The above loop should be transformed into:
#define combine( c1, c2 ) (((c1) << 1) + (c2))
switch( combine( CONSTANT0!=0, CONSTANT1!=0 ) ) {
case combine( 0, 0 ):
for( i ... ) {
DoWork0( i );
DoWork2( i );
}
break;
case combine( 1, 0 ):
for( i ... ) {
DoWork1( i );
DoWork2( i );
}
break;
case combine( 0, 1 ):
for( i ... ) {
DoWork0( i );
DoWork3( i );
}
break;