In CompSci 100, you'll learn that there are three components to a loop: an initial condition, a body of work, and a test against the work. In CompEng 100, you'll learn that there is really only one construct at the lowest level: if a simple condition is met, jump elsewhere. All the fussy academics of loop structure are just abstractions to which you can adhere for safety.
Almost every hardware instruction processor suffers a performance hit when they're told in one instruction that they won't in fact be running the NEXT instruction next. Today's processors do a lot of thinking ahead, and predicting what registers or memory locations they'll have to start fetching, even before they've reached the instruction. All that thinking ahead is moot if the condition requires a jump.
Duff's Device is just another way of writing a loop, using the barest features of the processor: jump to new locations, but minimize how often that happens.
It's meaningless in Perl, except as an anecdote, because the virtual machine has so many branches at the machine code level no matter what code you write, and no additional expensive penalty for jumping around in Perl code.
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[ e d @ h a l l e y . c c ]
In reply to Re: Duff's Device
by halley
in thread Duff's Device
by davido
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