Consider the following code. The address of each method is entered into a
vtable that's indexed by opcode. A method can be found by a single pointer
calculation vtable[ opcode ]. It interprets an opcode without the runtime
overhead of a switch statement.
// opcodemethod.cc
#include <stdio.h>
typedef bool (*TMethod)(void);
static const int MAX_OPCODE = 3;
static TMethod vtable[ MAX_OPCODE ];
static bool method1() {
printf( "Method 1\n" );
return true;
}
static bool method2() {
printf( "Method 2\n" );
return true;
}
static bool method3() {
printf( "Method 3\n" );
return true;
}
static bool invokeOpcode( int index ) {
TMethod item = vtable[ index ];
return item();
}
void example() {
vtable[ 0 ] = method1;
vtable[ 1 ] = method2;
vtable[ 2 ] = method3;
if ( invokeOpcode( 0 ) ) {
if ( invokeOpcode( 1 ) ) {;
invokeOpcode( 2 );
}
}
}
Since all of these methods must have a similar signature, building a formal
type definition lets the compiler do additional type checking. Inside
interp.cc, frame::runOpcode() ends up short and sweet, something like this:
typedef bool (frame::*TOpcodeMethod)(exception_to_throw &e);
static TOpcodeMethod vtable[ bc::MAX_OPCODE ];
bool frame::runOpcode(exception_to_throw &e)
TOpcodeMethod item = vtable[ cur_code->code[ myPC ] ];
#ifdef PARANOID
if ( item == NULL ) {
kprintf( "frame.runOpcode(): VM corrupted, exiting.\n" );
abort();
}
#endif
myPC++;
return item( e );
}
void frame::loadVTable() {
vtable[ bc::aaload ] = aaload;
vtable[ bc::aastore ] = aastore;
:
vtable[ bc::aload ] = aload;
vtable[ bc::aload_0 ] = aload_0;
vtable[ bc::aload_1 ] = aload_1;
vtable[ bc::aload_2 ] = aload_2;
vtable[ bc::aload_3 ] = aload_3;
vtable[ bc::anewarray ] = anewarray;
vtable[ bc::areturn ] = areturn;
vtable[ bc::arraylength ] = arraylength;
:
vtable[ bc::dup ] = dup;
vtable[ bc::dup_xl ] = dup_x1;
vtable[ bc::dup_x2 ] = dup_x2;
vtable[ bc::dup2 ] = dup2;
vtable[ bc::dup2_x1 ] = dup2_x1;
vtable[ bc::dup2_x2 ] = dup2_x2;
vtable[ bc::swap ] = swap;
}
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