2014-04-26 10:52:28 -04:00
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Duff's device
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2014-04-26 11:54:15 -04:00
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n. The most dramatic use yet seen of fall through in C, invented by Tom Duff
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when he was at Lucasfilm. Trying to optimize all the instructions he could
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out of an inner loop that copied data serially onto an output port, he
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decided to unroll it. He then realized that the unrolled version could be
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implemented by interlacing the structures of a switch and a loop: register n
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= (count + 7) / 8; /* count 0 assumed */ switch (count % 8) { case 0: do {
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*to = *from++; case 7: *to = *from++; case 6: *to = *from++; case 5: *to =
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*from++; case 4: *to = *from++; case 3: *to = *from++; case 2: *to =
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*from++; case 1: *to = *from++; } while (--n 0); } Shocking though it
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appears to all who encounter it for the first time, the device is actually
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perfectly valid, legal C. C's default fall through in case statements has
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long been its most controversial single feature; Duff observed that This
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code forms some sort of argument in that debate, but I'm not sure whether
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it's for or against. Duff has discussed the device in detail at
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http://www.lysator.liu.se/c/duffs-device.html. Note that the omission of
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postfix ++ from *to was intentional (though confusing). Duff's device can be
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used to implement memory copy, but the original aim was to copy values
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serially into a magic IO register.
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