@steve something something dual carry chains with ADCX/ADOX.
Not relevant for actual adds but does matter for muls.
No idea how you handle loops when you have to.
Presumably unroll and spill/restore carry/overflow into a designated landing pad reg around the test
@steve idly wondering whether you can do something like
mov r8d, 0
adcx r8d, r8d
adox r8d, r8d
shl r8d, 30
to save and then
add r8d, r8d
to restore. That will give you carry back, but I always forget what exactly overflow was. Carry-in into MSB != carry-out, right?
in this case carry-out from MSB is saved_CF and carry-in is saved_OF, so it would set OF to saved_OF^saved_CF.
Almost. Pity. No actual use for it but it felt cute. :)
@steve anyway, with low-ish trip count, probably something like
sub rdx, rsi
sub rdi, rsi
clc
lp:
mov rax, [rsi]
adc rax, [rdx + rsi]
mov [rdi + rsi], rax
lea rsi, [rsi + 8]
dec rcx
jnz lp
or maybe (probably not)
lea rsi, [rsi + rcx*8]
lea rdx, [rdx + rcx*8]
lea rdi, [rdi + rcx*8]
neg rcx
jz skip
clc
lp:
mov rax, [rsi + rcx*8]
adc rax, [rdx + rcx*8]
mov [rdi + rcx*8], rax
inc rcx
jnz lp
Oct 13, 2026, 1:58 AM@rygorous yeah, that feels about right. `neg rcx` was pretty standard when I cared about this, FWIW (but x86 is wider now than it was then).