The paper tries to solve the problem of SELECT and RANK: SELECT is to find the \(r\)-th asserted bit in a bit vector and RANK is to find the total number of asserted bits in the \(j\)-bit prefix of the bit vector. For example, if the bit vector is 100101001010 (12-bit vector), RANK(5) is 3 for the bits 0 to 5 in the bit vector is 100101 and SELECT(4) is 8 for bits 0 to 8 is 100101001.

Implementing SELECT and RANK for arbitrarily long vector in a naive way involves loops. For example, we can use POPCOUNT to count the asserted bits in a machine word. With a bit mask, we can make RANK function for a machine word. For multiple machine words, we can simply accumulate the count for each words in a loop.

SELECT operation is harder to implement. The paper mention about a slightly different problem in section 2: the SELECT on large bit vectors. Two references are reviewed: The CS-Poppy and SDSL. The former maintains a hierarchy of bit blocks. For each block in the same level of hierarchy, the result of RANK is stored. Therefore, to find the result for SELECT, we can do a linear search on the top level for the block that held the result, then move down a level for a refined block and so on. At the lowest level, we can do linear scan of machine words. SDSL, however, store the position of every 4096-th asserted bits in a table. Upon evaluating SELECT, we can easily find the candidate range and perform linear scan for the result. In either way, some form of iteration is required.

The paper mention about PTSelect, as invented by the first author. This make use of two new machine instructions in Intel’s Haswell CPUs:

  • PDEP(\(v,x\)): Deposit bits from operand \(v\) to bits as specified by operand \(x\)
  • TZCNT(\(x\)): Returns the number of trailing zeros in \(x\)

For example, PDEP(\(v,x\)) for \(v=\)abcdefgh and \(x=\)10101101 results in bit vector \(y=\)d0e0fg0h. We can observe that the zero positions in \(x\) remains to be zero in \(y\) but the asserted positions in \(x\) will be set according to the value of \(v\), from the LSB to the MSB.

The PTSelect algorithm is then implemented as:

\[\mathrm{PTSelect}(x, r) = \mathrm{TZCNT}(\mathrm{PDEP}(2^r, x))\]

Which \(\mathrm{PTSelect}(x, r)\) find the position of the \(r\)-th asserted bits in \(x\). The function above use a left-shift on 2 to create a bit vector of only a single bit asserted and with \(r\) trailing zeros. Then with PDEP to distribute the \(r\) zero on \(x\), we reset \(r\) asserted bits from the LSB of \(x\) and keep only the \((r+1)\)-th asserted bit. Then the SELECT instruction is simply counting the trailing zero in such manipulated bit vector.

Bibliographic data

   title = "A Fast x86 Implementation of Select",
   author = "Prashant Pandey and Michael A. Bender and Rob Johnson",
   year = "2017",
   url = "arxiv:1706.00990v1",