> On Mar 17, 2016, at 10:34 PM, Patrick Pijnappel via swift-evolution > <[email protected]> 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 [email protected] <mailto:[email protected]> Runtime Wrangler
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