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In https://reviews.llvm.org/D68103 the InstCombine learned that shift-by-sext is simply a shift-by-zext. But the transform is limited to single-use sext. We can quite trivially get a case where there are two shifts by the same sext: https://godbolt.org/z/j6mO3t <- We should handle those cases. In https://reviews.llvm.org/D68103#1686130 Sanjay Patel notes that this sext is intrusive for

...le use demanded use routine would be very > powerful. Is it worth exploring the harder topic for generality? There is some support for multi-use demandedbits in backend. As for middle-end i'm not sure. For sure, having that powerful mechanism may be useful, in general. > 3) If we had an anyextend IR node, it might be reasonable to eagerly > produce the duplicate nodes, and rely on later CSE. I keep running > across cases where we have an extend where we know the high bits don't > matter, maybe it's time to represent that? > > > I implemented this proposed suggesti...

...t; powerful. Is it worth exploring the harder topic for generality? >> There is some support for multi-use demandedbits in backend. >> As for middle-end i'm not sure. >> For sure, having that powerful mechanism may be useful, in general. >> >> > 3) If we had an anyextend IR node, it might be reasonable to eagerly >> > produce the duplicate nodes, and rely on later CSE. I keep running >> > across cases where we have an extend where we know the high bits don't >> > matter, maybe it's time to represent that? >> > >> &...

...ic for generality? > > >> There is some support for multi-use demandedbits in backend. > > >> As for middle-end i'm not sure. > > >> For sure, having that powerful mechanism may be useful, in general. > > >> > > >> > 3) If we had an anyextend IR node, it might be reasonable to eagerly > > >> > produce the duplicate nodes, and rely on later CSE. I keep running > > >> > across cases where we have an extend where we know the high bits don't > > >> > matter, maybe it's time to represent...