[swift-evolution] [Pitch] Remove bit shift traps

[Patrick Pijnappel]

Great! Totally forgot to check those, even though I already skimmed them
before ;). Looks like a lot of great improvements in one go!

On Tue, Mar 22, 2016 at 9:52 AM, Dave Abrahams via swift-evolution <
swift-evolution at swift.org> wrote:

>
> on Thu Mar 17 2016, Patrick Pijnappel <swift-evolution at swift.org> wrote:
>
> > Currently, bit shifting with an amount greater than or equal to the size
> of
> > the type traps:
> >
> > func foo(x: Int32) {
> >   let y = x << 32 // Runtime trap (for any << or >> with amount >= 32)
> > }
> >
> > I propose to make this not trap, and just end up with 0 (or ~0 in case of
> > right-shifting a negative number):
> >
> >    - Unlike the traps for integer arithmetic and casts, it is obvious
> what
> >    a bitshift past the end does as fundamentally the behavior stays the
> same.
> >    - If the intention is to make it analogous with
> multiplication/division
> >    by 2**n, the checks don't really change anything. Right shift are
> still
> >    identical to divisions by 2**n. Left shifts are like multiplication
> by 2**n
> >    but with different overflow behavior, which is already the case with
> the
> >    current rules (e.g. Int.max << 1 doesn't trap)
> >    - It could lead to bugs where users expect this to work, e.g. the
> >    following crashes when the entire buffer is consumed: buffer = buffer
> <<
> >    bitsConsumed
> >    - Bitshift are often used in performance-sensitive code, and with the
> >    current behavior any non-constant bit shift introduces a branch.
>
> This is addressed in
>
> https://github.com/apple/swift/blob/master/test/Prototypes/Integers.swift.gyb
> which we intend to propose very soon.
>
> (negative shift amounts work too).
>
>
> https://github.com/apple/swift/blob/master/test/Prototypes/Integers.swift.gyb#L1628
>
> For users who want to be sure they're not paying for any checks, there
> is a masking bitshift (&<<, &>>, &<<=, &>>=).
>
> --
> -Dave
>
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