On Jun 22, 2014, at 9:35 PM, 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.
It gets easier to optimize if you adopt a less precise model of exceptions. For
example, you could pick a model where you preserve control dependence and
externally visible side effects, but allow reordering in other cases. This does
get tricky if destructors themselves have externally visible side effects that
are dependent on intervening stores that can be elided.
This probably requires whole-program compilation with some knowledge of
externally visible side effects, or more restrictions placed on destructors
than there are currently. It also is hard to make work with unsafe code, since
unsafe code might require exact placement of unwinding for memory safety in
destructors.
Cameron
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