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Duff's Device and Coroutines (2008)
On Duff's Device and Coroutines Posted on Wednesday, January 30, 2008. At first glance, Duff's Device is one of the most mysterious pieces of C code you'll ever see: void send(short *to, short *from, int count) { int n=(count+7)/8; switch(count%8){ case 0: do{ *to = *from++; case 7: *to = *from++; case 6: *to = *from++; case 5: *to = *from++; case 4: *to = *from++; case 3: *to = *from++; case 2: *to = *from++; case 1: *to = *from++; }while(--n>0); } } It's an 8x-unrolled while loop interlaced with a switch statement.
In 2000, Simon Tatham wrote about a C coroutine implementation based on this idea, to make callback-based event-driven programming easier. A switch-based implementation has the advantage of being portable and relatively simple to emit using preprocessor macros, though it doesn't allow the use of other switch statements in the code. A year ago, I asked Duff if that kind of coroutine implementation is what he had in mind, and he confirmed that it was, pointing at a blog comment and adding:
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