[LLVMdev] Jump Theading/GVN bug - moving discussion to llvm-dev

[Chandler Carruth]

On Mon, Feb 23, 2015 at 8:32 PM, Chandler Carruth <chandlerc at google.com>
wrote:

> Handling unreachable code is annoying. I agree. However, its important to
> characterize how annoying. Most code is not unreachable, and we're
> (obviously) fine just bailing on such crazy code paths. So in practice the
> common cost is keeping a local set to detect cycles in the graph where we
> aren't expecting them. We are essentially always traversing a linked list
> representing this graph. Because these linked list nodes don't have any
> particular reason to have high locality, we have a cache-hostile traversal
> where we have to do primarily local and cache friendly book-keeping at each
> step to catch duplication. I suspect that this has a very small (but
> non-zero) impact on compile time.


The other thing I meant to include here but forgot...

Another reason why handling these cases has a tendency to be less expensive
than you might expect is that we very often already have a set-like
structure in place to remove *duplication*, and it is often very easy to
re-use it to avoid cycles. So we won't be able to completely remove many of
the sets you might expect to remove, they would just be narrower in scope
due to not needing to handle cycles. My strong suspicion is that that
narrower scope does not in fact translate to a significant efficiency gain.
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