I got an idea while reading this nice blending trick:
http://stackoverflow.com/questions/20681590/how-does-this-color-blending-trick-work

We could do a similar thing when adding components in BitBlt
partitionedAdd: word1 to: word2 nBits: nBits nPartitions: nParts

For a simple example, say I have 2 parts of 4 bits

I will start reasonning with extended precision (thus on 16 bits here)

"The sum with carry is" sumWithCarry:=word1+word2.

"But carry must not be propagated past each component."

"So we must care of what happens at" carryOverflowMask := 2r100010000.

"The sum without any carry is" sumWithoutCarry:=word1 bitXor: word2.

"If the sum without carry differ from sum with carry then an overflow occured."

"We can thus detect presence of a carry overflow:"

carryOverflow := (sumWithCarry bitXor: sumWithoutCarry) bitAnd: carryOverflowMask.

"If an undue carry occured, we just removet it:"

sumWithoutUndueCarry := sumWithCarry - carryOverflow.

"But in this case, previous component did overflow, we must saturate it at 2r1111, that is:" componentMask:=1<<nBits-1.

"we just have to multiply each carryOverflow bit with this componentMask:"

result := sumWithoutUndueCarry bitOr: carryOverflow >> nBit * componentMask.

-----------

"Generalization: note that 2r00010001 * componentMask = 2r11111111"

"Correlatively, for arbitrary nBits and nParts parameters:"

carryOverflowMask := 1<<(nParts*nBits)-1//componentMask<<nBits.

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In BitBlt, we want to operate on 32bits words.

We can use #usqLong (64 bits at least) all the way, and obtain branch free replacement for above method
  
    <var: #word1 type: 'unsigned int'>
    <var: #word2 type: 'unsigned int'>
    <var: #one type: #usqLong>
    <var: #componentMask type: #usqLong>
    <var: #carryOverflowMask type: #usqLong>
    <var: #carryOverflow type: #usqLong>
    <var: #sum type: #usqLong>
    one := 1.
    componentMask := one<<nBits-1.
    carryOverflowMask := one<<(nBits*nParts)-1//componentMask<<nBits.
    sum := word1.
    sum := sum + word2.
    carryOverflow := ((word1 bitXor: word2) bitXor: sum) bitAnd: carryOverflowMask.
    ^sum-carryOverflow bitOr: carryOverflow>>nBits * componentMask

-----------

But maybe we can do better and express all with native unsigned int operations.

We must then look if bits at 2r10001000 could produce a carry.

We have a carry in next bit if at least 2 out of the 3 are true among word1 word2 carry...

That is (word1 bitAnd: word2) bitOr: ((word1 bitOr: word2) bitAnd: carry).

    <var: #word1 type: 'unsigned int'>
    <var: #word2 type: 'unsigned int'>
    <var: #one type: #usqLong> "because we cannot shift <<32 a usqInt in C..."
    <var: #componentMask type: 'unsigned int'>
    <var: #carryOverflowMask type: 'unsigned int'>
    <var: #carryOverflow type: 'unsigned int'>
    <var: #carry type: 'unsigned int'>
    <var: #sum type: 'unsigned int'>
    one := 1.
    componentMask := one<<nBits-1.
    carryOverflowMask := one<<(nBits*nParts)-1.
    carryOverflowMask := carryOverflowMask//componentMask<<(nBits-1).

    sum := word1.
    sum := sum + word2.

    carry := (word1 bitXor: word2) bitXor: sum.

    carryOverflow := ((word1 bitAnd: word2) bitOr: ((word1 bitOr: word2) bitAnd: carry)) bitAnd: carryOverflowMask.
    ^sum-(carryOverflow<<1) bitOr: carryOverflow>>(nBits-1) * componentMask

Maybe some good soul may help reducing # ops further, but we already have a branch free sum.

I did no performance test.

Does one have good BitBlt benchmark?