Re: [Valgrind-developers] Iop_Leftxx semantics

[Eliot M. <mo...@cs...>]

On 7/13/2025 4:52 PM, Florian Krohm wrote:
> What is the semantics of Iop_Left32 and friends?
> 
> In libvex_ir.h it says:  \x -> x | -x
> which is not descriptive at all.
> I could simply copy that formula over and use in the constant folding checker.
> But by doing so it is impossible to find any bugs - be it in VEX's fold_expr or in the checker program. So I want to 
> write my own implementation of that IROp.
> Can't do without a spec...
> 
> I took a guess and thought that "left" perhaps meant to left-propagate the left-most "1" bit. E.g. 00000101 --> 11111101
> But that's not it.. In fact for every odd integer the result has all bits set.

This appears to take the least significant 1 bit and propagate it to the left.

Seems related to x & -x, which isolates the least significant 1 bit.

Regards - Eliot Moss