John Carmack on mutable variables

I had enormous fun optimizng C++ via self-modifying assembly to squeeze the utmost performance of some critical algorithms, and now this drive towards immutable everything feels like cutting my hands and legs off and forcing me to do subpar engineering.

Same here, I grew up in a world where you had a handful of registers and a bunch of memory locations, and those were your variables.

                clc
                lda value
                adc #1
                sta value
                lda value+1
                adc #0
                sta value+1

    value       .byte 0,0

These constraints are pretty much built into my concept of programming and it would take great effort to break out of it. It feels nice doing:

    x = 20
    x = x + func(y)
    x = x / func(z)

And it would feel weirdly wasteful to do:

    x = 20
    x1 = x + func(y)
    x2 = x1 / func(z)

Like, I know that variables are no longer a scarce resource, but my brain still wants to conserve them.

To me this also seem to be wasteful, even if not for the computer, but it wastes my amount of working state I can keep in my head, which is very limited.