Consequences of casting away immutable from pointers

[Jonathan M Davis]

On Friday, January 05, 2018 04:16:48 Adam D. Ruppe via Digitalmars-d-learn 
wrote:
> On Friday, 5 January 2018 at 04:10:54 UTC, Steven Schveighoffer
>
> wrote:
> > The compiler assumes x is going to be 5 forever, so instead of
> > loading the value at that address, it just loads 5 into a
> > register (or maybe it just folds x == 5 into true).
>
> I was curious what dmd did, and the disassembly indeed shows it
> just loads 5 into the register and leaves it there - assuming
> since it is immutable, it will never change through any pointer
> and thus never reloads it from memory at any time.
>
> Interestingly, dmd -O just stubs out the whole function. I guess
> it assumes all the defined behavior actually accomplishes nothing
> and it is free to optimize out undefined behavior... thus the
> function needs no code. Similarly,  if the last assert is changed
> to x != 5, dmd -O doesn't even actually do a comparison (the
> value 5 never appears in the generated code!), it just outputs
> the direct call to assertion failure.

Well, it's certainly nice to see some evidence that the compiler really is
taking advantage of the guarantees that immutable is supposed to provide.

- Jonathan M Davis