As the compiler generates machine code it is possible for it to scan the code as it is compiled and to optimise the result. This process operates in a single pass, so the optimisation is limited, but never-the-less useful.
The Forth requires parameters on the stack before executing the function. If these parameters are literals it is often possible to reduce the code e.g..
: TST $03 I/O PORTA C! ;
This would require the following code;
$0208 CALL [ DOLITERAL ]
$020C BRA $0210
$020E $03
$0210 CALL [ DOLITERAL ]
$0214 BRA $0218
$0216 $F80
$0218 RCALL [ C! ]
$021A RETURN
However, the optimizer produces:
$0200 MOVLW $03
$0202 MOVFF WREG , $0F80
$0206 RETURN
This has saved 6 words of storage and several micro seconds of execution time. It would be unusual to make this simple code a definition and so this would normally become in-line code within a larger definition. Also in this example, if the literal 03 was computed at run-time the result is already in TOS so the compiler just MOVFF TOSL , PORTA ,A and then cleans the stack. Still a significant saving.