> On Oct 12, 2016, at 9:05 AM, Joe Groff via swift-users 
> <swift-users@swift.org> wrote:
> It looks like LLVM does not recognize the overflow traps Swift emits on 
> arithmetic operations as dead code, so that prevents it from completely 
> eliminating the Swift loop.  That's a bug worth fixing in Swift, but unlikely 
> to make a major difference in real, non-dead code. 

The traps aren’t dead if they are reachable. Eventually we will teach LLVM 
about less-strict trap semantics so they can be speculated and reordered. But 
for now we can do that sort of thing in SIL in the most important cases.

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