Author: Dann Corbit
Date: 10:42:07 07/17/03
The assembly versions will surely be faster in a test harness.
But when you poke them into actual code and turn on the maximal optimizer
settings, do the assembly high and low bit functions actually beat this C code?
I ask because it seems that the optimizer is somehow smarter when dealing with C
with some experiments that I have run.
What happens when you try it?
#ifdef _MSC_VER
typedef unsigned __int64 Bitboard;
#else
typedef unsigned long long Bitboard;
#endif
/* Returns top bit of x, or 0 if x = 0. */
/* This algorithm is due to Robert Harley */
Bitboard hi_bit(Bitboard x)
{
x |= x >> 1;
x |= x >> 2;
x |= x >> 4;
x |= x >> 8;
x |= x >> 16;
x |= x >> 32;
return x ^ x >> 1;
} /* end function top_bit */
//
// Hard to say which of these will be faster on your system.
//
Bitboard lo_bit0(Bitboard n)
{
return ((n & (n - 1)) ^ n);
}
Bitboard lo_bit1(Bitboard n)
{
return n & -n;
}
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