[PATCH] D15124: Use @llvm.invariant.start/end intrinsics to extend the meaning of basic AA's pointsToConstantMemory(), for GVN-based load elimination purposes [Local objects only]

[Daniel Berlin via llvm-commits]

>
> This works if there's only one start. If there are multiple starts, then
> we need only the set to dominate, not any individual one.
>
> invariant_start         invariant_start
>           |                             |
>           |________________ |
>                          |
>                        load
>                          |
>                         ret
>
> At least in theory, this is fine too. The "right" way to solve this might
> be to set it up as a lattice/dataflow problem on the CFG, and then iterate
> until convergence (which should happen in two iterations, aside perhaps
> from pathological cases).
>
Yes, you can't solve this sparsely without virtual phi nodes for the data,
and it's not clear to me it's worth the cost of doing that.

(I've seen compilers that do it for extra data they want to use during ssa
optimizations, but ... so far i'd say not worth it)
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