How to use bitwise operations to assign specific bits of a byte according to two other bytes? (bit blend according to a mask)

The standard bithack for this is "Merge bits from two values according to a mask" - I added your variable names for the inputs to the existing comments from Sean Anderson's collection of bithacks.

unsigned int a;    // (ByteToAlter)    value to merge in non-masked bits
unsigned int b;    // (ValuesToAssign) value to merge in masked bits
unsigned int mask; // (BitsToAssign)   1 where bits from b should be selected; 0 where from a.

unsigned int r;    // result of (a & ~mask) | (b & mask) goes here

r = a ^ ((a ^ b) & mask); 

As the bithack comments note, the straight-forward way is (a & ~mask) | (b & mask) - clear the bits you're not keeping in each input, then OR them together.

How a ^ ((a ^ b) & mask) works:

bitdiff = a^b is the bitwise difference between those inputs.
It has a 1 where they differ, and a 0 where they're the same. (By definition of XOR).

a ^ bitdiff would flip every bit in a that's different from b. In fact, b == a ^ bitdiff.
One way to show that's true is that XOR is associative, so a ^ (a^b) = (a ^ a) ^ b.
And x^x = 0, just like x-x = 0.
0 ^ x = x, so (a^a) ^ b = 0 ^ b = b.

But if we mask the bitdiff to only set bits of a to bits from b at certain positions, we've achieved our goal: bitwise blend according to a mask. blend = a ^ (bitdiff & mask);

Special cases of the (a & ~mask) | (b & mask) simple version

If your inputs are arranged so ValuesToAssign only has any 1 bits at positions selected by the mask, you can optimize away the b & mask part, leaving just (a & ~mask) | b. (Eraklon's answer). Clear the unselected bits, then OR in the new values to set any bits that should be set.

A further special case is when ValuesToAssign == BitsToAssign, i.e. the modification only ever sets bits, never clearing them. That's what OR does, so of course this case optimizes to a | b, with no need to clear bits in a before ORing.

Efficiency:

r = a ^ ((a ^ b) & mask); is only 3 boolean operations,
vs. 4 for (a & ~mask) | (b & mask) if all three inputs are runtime-variables. (One bitwise NOT, two AND, plus an OR).

If mask is a constant, then constant-propagation into ~mask makes it a constant, and most machines can do AND-immediate with at least an 8-bit AND mask. So you'd still only need 3 instruction: two ANDs (with inverse constants) and an OR.

Some machines (like x86 with BMI1) even have an andn instruction that does x & ~y, allowing the (a & ~mask) | (b & mask) to be done with 3 instructions.

For latency, (a & ~mask) | (b & mask) has more instruction-level parallelism. If mask and ~mask are ready ahead of a and b, there are only two parallel AND operations, and an OR, from a and b inputs being ready to the output being ready. On a normal superscalar machine (that can do two independent AND ops in the same cycle), that's only 2 cycle latency from inputs to outputs. (Again, assuming mask is ready early, or that an instruction like andn exists to do a & ~mask in a single operation).

If the critical path goes through mask (i.e. it's not ready early), and you don't have an instruction like andn to do ~ and & as one operation, the latency from mask to result is 3 operations, ~, &, and |. (Assuming the b & mask can run in parallel without slowing down any of those three).

Latencies for (a & ~mask) | (b & mask) on a modern OoO exec machine.
(Not the same thing as throughput cost)

• a -> result: 2 cycles
• b -> result: 2 cycles
• mask -> result: 3 cycles (or 2 on some machines)

But the bit-difference way doesn't have any ILP; it's a chain of 3 operations. a^b requires both of those inputs to be ready for the first step, then mask needs to be ready for the & mask step. The final a ^ ... step is using inputs that were already needed earlier. So it's only 3 operations, but they're serial.

Latencies for a ^ ((a ^ b) & mask) on a modern OoO exec machine.

• a -> result: 3 cycles
• b -> result: 3 cycles
• mask -> result: 2 cycles

Related Q&As:

• Merge bit sequences a and b according to a mask - this is called a blend in SIMD programming. IDK if anyone else uses the "bit-blend" term I like to use for this operation, but IMO that clearly describes it. x86's AVX-512 extension has a 3-input boolean operation vpternlog with a truth-table from an 8-bit immediate, and thus can do it in a single instruction.

• Swapping bits at a given point between two bytes - The same bithack idea, but applying the masked bit-difference to both inputs to exchange bits at the mask positions.

• https://catonmat.net/low-level-bit-hacks - starts gently with an intro to the operators (like ^ being bitwise XOR). Includes bithacks that use + and - (and the carry propagation effects of hitting a 1 or 0, like x & (x-1) to clear the right-most set bit.)

• https://agner.org/optimize/ for more about tuning for modern CPUs.

BitsToAssign & ValuesToAssign | ~BitsToAssign & ByteToAlter Please try this.

Explanation: Select ValueToAssign if BitsToAssign is True else select ByteToAlter

void printbin(unsigned char val)
{
    unsigned char mask = 1 << (sizeof(mask) * 8 - 1);
    while(mask)
    {
        printf("%c", val & mask ? '1' : '0');
        mask >>= 1;
    }
}

unsigned merge(unsigned ByteToAlter, unsigned ValuesToAssign, unsigned BitsToAssign)
{
    unsigned clearMask = ~BitsToAssign;

    return (ByteToAlter & clearMask) | (ValuesToAssign & BitsToAssign);
}

int main(void)
{
    printbin(merge(0b11110001, 0b01000010, 0b01101011));
    
}