Author: J. Wesley Cleveland
Date: 16:43:55 11/08/02
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On November 08, 2002 at 13:46:10, Ron Murawski wrote: >On November 08, 2002 at 12:50:41, J. Wesley Cleveland wrote: > >>On November 08, 2002 at 10:50:34, Ron Murawski wrote: >> >>[snip] >>> >>>I'm investigating power-of-two size vs prime size using single-probes. It's not >>>at all apparent to me whether the smaller table used for a power-of-two size >>>might slow down the engine more than the expensive mod instruction on a larger >>>table would. In other words, if there are 36K slots available, power-of-two >>>would only use 32K slots, whereas the prime size would use almost all of the >>>36K. >> >>You can use multiply instead of mod, e.g. with 32 bit keys: >> >>table_index = (key*number_of_table_slots) >> 32; > >I use a 64-bit key, so that wouldn't work so well for me. It seems to me that: > >table_index = ((key>>32)*number_of_table_slots) >> 32; > >should work, but it is somewhat complex and the entire expression will be >calculated in 64-bit math. I wonder if a single mod might not be cheaper than >the two 32 bit shifts. It would depend on the compiler's optimizations. > >This might be faster but is somewhat mysterious: > >table_index = ((key&4294967295)*number_of_table_slots) & 4294967295; > >where 4294967295 = 2^32 - 1 > >Ron MSVC optimizes out the shifts, but is not clever enough to do the multiplication inline. In assembler it is one load and one multiply.
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