On 23/06/14 14:46, comex wrote:
On Mon, Jun 23, 2014 at 12:35 AM, Daniel Micay <danielmi...@gmail.com> wrote:
An operation that can unwind isn't pure. It impedes code motion such as
hoisting operations out of a loop, which is very important for easing
the performance issues caused by indexing bounds checks. LLVM doesn't
model the `nounwind` effect on functions simply for fun.
No it doesn't! Or maybe it does today, but an unwindable operation is
guaranteed to be repeatable without consequence, which I'd like to
think can account for most cases where operations are hoisted out of
loops (again, could be wrong :), and not to modify any memory (unless
it traps, but in Rust at that point you are guaranteed to be exiting
the function immediately).
I would think that something simple like
let mut sum = 0;
for x in some_int_array.iter() {
sum += x;
}
would be very hard to vectorise with unwinding integer operations.
Huon
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