Saturday, April 19, 2014

Computing changed bits in a range of values

So, there's a reason to do this, other than just wasting CPU cycles. I need to take in a range of 2 byte values and compute a mask on all changed bits between the lower and upper value of this range.

The underlying reason is this allows for a quick match against network packet port ranges. So below I have a little test app that computes the changed bits given an arbitrary start stop value.

This ends up producing the following output:

slioch@slioch-All-Series:~/vyatta$ ./range 32076 62199
slioch@slioch-All-Series:~/vyatta$ ./range 52076 62199
slioch@slioch-All-Series:~/vyatta$ ./range 62076 62199
slioch@slioch-All-Series:~/vyatta$ ./range 1 2
slioch@slioch-All-Series:~/vyatta$ ./range 1 3
slioch@slioch-All-Series:~/vyatta$ ./range 1 4
slioch@slioch-All-Series:~/vyatta$ ./range 1 1024
slioch@slioch-All-Series:~/vyatta$ ./range 1 1023

The "x" represents a bit position that changes, while the "-" represents a position that doesn't change. This ends up allowing for a quick comparison against a start-stop (or range) of values. Note that the representation above has the LSB (least signficant bit) on the left.

#include <string.h>
#include <strings.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdbool.h>
#include <math.h>


static void
byte_to_binary(unsigned int *x, int num)
  unsigned int mask = 0x01;
  int z;
  for (z = 0; z < num; ++z) {
    printf(((*x & mask) == mask) ? "x" : "-");
    mask = mask << 1;
      mask = 0x01;

   Calculate fixed/wildcards in ranges of numbers
   for binary representations.
int main(int argc, char **argv)
  if (argc < 3) {
    printf("need start and end\n");

  long int start = strtoul(argv[1], NULL, 10);
  long int end = strtoul(argv[2], NULL, 10);
  if (end < start) {
    printf("Error in range\n");

  //now let's see compute all the bits changed in this range
  unsigned int i, accum = 0;
  accum = start ^ end;
  int loc = 32 - __builtin_clz(accum);
  for (i = 0; i < loc; ++i)
    accum |= 0x1 << i;
  byte_to_binary(&accum, 16);


Wednesday, April 16, 2014

A little fun with L1/L2/L3 cache RAM fetch speed

I know it has been a while since I've posted. And that's pretty much a reflection of being super busy as well as not having a little nugget at the tips of my fingers to post.

So I'm back.

I wrote the little program below to test the cost of fetching values from lookup tables of various sizes. Presumably this would show up in performance differences once the lookup table exceeds the size of the various cache it could easily fit within.

So, my little experiment went as followed: