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

Greg Parker gparker at apple.com
Fri Mar 18 01:05:02 CDT 2016

> On Mar 17, 2016, at 10:34 PM, Patrick Pijnappel via swift-evolution <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.

Defining large shifts to 0 or ~0 does not improve performance on some architectures. The result of the i386 and x86_64 shift instruction is undefined for shift equal to or larger than the data size. The arm64 shift instruction shifts by (shift_value % data_size). If you want large shifts to return zero on architectures like these then you still need a branch.

Greg Parker     gparker at apple.com <mailto:gparker at apple.com>     Runtime Wrangler

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