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4465550
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2010-12-16T21:29:49.600
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It does look inefficient and perhaps this is what Nathan was referring to. assuming a char is 8 bits where this code is running there are two things to do first move the whole bytes, for example if your input array is 0x00,0x00,0x12,0x34 and you shift left 8 bits then you get 0x00 0x12 0x34 0x00, there is no reason to do that in a loop 8 times one bit at a time. so start by shifting the whole chars in the array by (displacement>>3) locations and pad the holes created with zeros some sort of for(ra=(displacement>>3);ra>3)] = array[ra]; for(ra-=(displacement>>3);ra>(7-(displacement&7))). a good compiler will precompute (displacement>>3), displacement&7, 7-(displacement&7) and a good processor will have enough registers to keep all of those values. you might help the compiler by making separate variables for each of those items, but depending on the compiler and how you are using it it could make it worse too. The bottom line though is time the code. perform a thousand 1 bit shifts then a thousand 2 bit shifts, etc time the whole thing, then try a different algorithm and time it the same way and see if the optimizations make a difference, make it better or worse. If you know ahead of time this code will only ever be used for single or less than 8 bit shifts adjust the timing test accordingly. your use of the carry flag implies that you are aware that many processors have instructions specifically for chaining infinitely long shifts using the standard register length (for single bit at a time) rotate through carry basically. Which the C language does not support directly. for chaining single bit shifts you could consider assembler and likely outperform the C code. at least the single bit shifts are faster than C code can do. A hybrid of moving the bytes then if the number of bits to shift (displacement&7) is maybe less than 4 use the assembler else use a C loop. again the timing tests will tell you where the optimizations are. 
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1. POoptimized byte array shifter
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