Difference between signal[idx] and 1'(signal >> idx); how to convert between them

[gussmith23]

Hi all!

I have a design that looks like

assign out = mem[idx];

which, under the hood, uses a $shiftx cell. I'm then writing out to a custom backend, and I really don't want to have to implement $shiftx in my backend. Thus I would really like to convert it to the simpler

assign out = 1'(mem >> idx);

I suspect this transformation may not actually be equivalent in the eyes of Yosys, however, and even if it is, I don't know how to implement it!

I have a few questions:

• is this conversion possible?
• If it's not possible, is there any other way to "get rid of" $shiftxs and convert them into something simpler?

Edit: I'm realizing I can also just implement $shiftx in my backend for very specific cases of $shiftx; for example, the case where both operands are unsigned and it just reduces to a $shr. This does not handle the fact that $shiftx shifts in X values, however.

[whitequark]

Are you ignoring X values everywhere else? If you do you can ignore them in $shiftx too.

[gussmith23]

I am, yeah!