Fastest ways to set and get a bit

Question

I'm just trying to develop ultra-fast functions for setting and getting bits in uint32 arrays. For example, you can say "set bit 1035 to 1". Then, the uint32 indexed with 1035 / 32 is used with the bitposition 1035 % 32. I especially don't like the branching in the setbit function.

Here is my approach:

void SetBit(uint32* data, const uint32 bitpos, const bool newval)
{
   if (newval)
   {
      //Set On
      data[bitpos >> 5u] |= (1u << (31u - (bitpos & 31u)));
      return;
   }
   else
   {
      //Set Off
      data[bitpos >> 5u] &= ~(1u << (31u - (bitpos & 31u)));
      return;
   }
}

and

bool GetBit(const uint32* data, const uint32 bitpos)
{
   return (data[bitpos >> 5u] >> (31u - (bitpos & 31u))) & 1u;
}

Thank you!

What architecture? What language? What does the compiler output now? You might find that it's already pretty fast. — Carl Norum, Dec 07 '10 at 19:44
It is x86 (32 bit). Indeed, it is already quite fast, but i think - especially in the setbit function, I could still be faster... — vls, Dec 07 '10 at 19:49
Your SetBit is really a FlipBit, it seems, for that you could probably just use xor (typically the ~= operator). — Bjarke Freund-Hansen, Jan 29 '11 at 21:12
It's not a FlipBit because you can set the bit to a fixed defined state independent of the previous state. — vls, Jan 31 '11 at 15:45

score 1 · Answer 1 · answered Feb 07 '12 at 21:45

First, I would drop the 31u - ... from all expressions: all it does is reordering the bits in your private representation of the bit set, so you can flip this order without anyone noticing.

Second, you can get rid of the branch by using a clever bit hack:

void SetBit(uint32* data, const uint32 bitpos, const bool f)
{
    uint32 &w = data[bitpos >> 5u];
    uint32 m = 1u << (bitpos & 31u);
    w = (w & ~m) | (-f & m);
}

Third, you can simplify your getter by letting the compiler do the conversion:

bool GetBit(const uint32* data, const uint32 bitpos)
{
    return data[bitpos >> 5u] & (1u << (bitpos & 31u));
}

Fastest ways to set and get a bit

1 Answers1