You can also write ```Julia firstbit(n::Integer) = signed(n) < 0 ``` I hope that LLVM generates the same machine code for this as for Steven's bit masks above. Using bit masks is more general since it allows you to test every bit, not just the "first" (sign) bit.
-erik On Sat, Aug 13, 2016 at 11:10 AM, Steven G. Johnson <[email protected]> wrote: > > > On Saturday, August 13, 2016 at 9:22:14 AM UTC-4, jw3126 wrote: >> >> Sorry my question was confusing. What I want is a function that behaves >> as follows on bittypes: >> >> *0*0101101 -> 0 >> *0*1110101 -> 0 >> *0*1011011 -> 0 >> *1*0011101 -> 1 >> *1*1010001 -> 1 >> *1*0110111 -> 1 >> > > Normally to compute this function you would just check whether the > bitwise-and with 10000000 is nonzero. e.g. > > firstbit(n::Union{UInt8,Int8}) = (n & 0x80) != 0 > firstbit(n::Union{UInt16,Int16}) = (n & 0x8000) != 0 > firstbit(n::Union{UInt32,Int32}) = (n & 0x80000000) != 0 > firstbit(n::Union{UInt64,Int64}) = (n & 0x8000000000000000) != 0 > firstbit(n::Union{UInt128,Int128}) = (n & 0x80000000000000000000000000000000) > != 0 > firstbit(n::Integer) = leading_ones(n) > 0 > -- Erik Schnetter <[email protected]> http://www.perimeterinstitute.ca/personal/eschnetter/
