Re: Fix PR 65177: diamonds are not valid execution threads for jump threading

[Jeff Law <law at redhat dot com>]

On 03/27/2015 02:53 AM, Richard Biener wrote:
> Yeah, and refactoring copy_bbs so that the actual edge duplication happens
> in another function (thus we can have two of them).
Exactly.


> I was also playing with the idea to support value-numbering the stmts
> on-the-fly as we copy them and use this for cost estimation of copies
> (ok, well - basically do the copy and then either throw it away again
> or accept it).
> I thought about applying this to loop unrolling, but obviously this also applies
> to any block duplication we do during jump threading.
It'd definitely be useful in threading.  It's often the case that PHIs 
in the duplicates are degenerates and we can/should cprop them away and 
perform resultant simplifications that propagation enables.

If we could do that integrated with the actual duplication, then we'd be 
able to drop the phi-only cprop pass which exists solely to clean up 
that stuff.

jeff