Lair Of The Multimedia Guru

February 4, 2006

Fast sign extension

You want to convert a signed n-bit integer into a m-bit interger, now if n and m are one of 8,16,32,64 then its easy, just cast to the respective type and let the compiler choose how to do it efficiently, but what if n or m are not one of these?

Bit Twiddling Hacks suggests:
C= 1 << (n – 1)
-(x & C) | x
which needs 3 operations if n is constant and 5 if not

can this be done with fewer instructions? yes
C= (-1)<<(n-1)
(x+C)^C
which needs 2 operations if n is constant and 4 if not

PS: the C= (-1)<<(n-1) could be replaced by a look up table …

Filed under: Optimization — Michael @ 23:10

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