Multiply by negative one

[mcp]

IIUC, you want to express something like this in assembly:

int result = n ? -1 : 0;

In that case, using neg+sbb does the trick:

mov eax, [val]
neg eax
sbb eax, eax

This works since neg sets the carry flag if its operand is non-zero, and clears it otherwise. The sbb instruction then subtracts the carry flag from zero and thus yields -1 for non-zero inputs, and zero otherwise.

Example:
eax contains a non-zero value, say x. neg eax turns this into -x and sets the CF. sbb eax, eax computes eax = eax - eax - cf <-> eax = 0 - CF.
Same drill when eax is zero initially.

[chants]

Yes, that was the pearl of wisdom needed here. I had long forgotten this old SBB trick. From a C perspective its the casting to bool operation.

It would be nice if all the assembler bit tricks were compiled somewhere as similar to:
https://graphics.stanford.edu/~seander/bithacks.html