[PATCH] Scoped NoAlias Metadata

[hfinkel at anl.gov]

  I think that this is a good point, and we certainly could think of modeling this with some llvm.noalias.start/llvm.noalias.end intrinsics. I don't think this buys us anything, however:

  The issue is that that noalias pointers, after inlining, don't not alias with all other pointers in the function, only those which came from the same original function under the same control dependencies. Consequently, we'd still need some way of specifying with which other pointers the noalias pointers don't alias (it is not generally all of them). We could add some additional intrinsics for tagging these other pointers, and some metadata to tie them together. This looks much like the current solution, but:
    - Involves searching for these intrinsics during AA queries (which seems like it could get expensive)
    - Seems as though it will prevent some useful instruction combining, reordering, etc. without either removing the AA information, or some complicated gymnastics (and just getting this right in all of the relevant passes seems like a large auditing problem).

  Done this way does involve more metadata, but I think it is much easier to ensure correctness, yields little overhead in AA, and because passes should drop metadata they don't understand, easier to get right by default.

http://llvm-reviews.chandlerc.com/D2194