[PATCH] [FunctionAttr] Infer nonnull attributes on returns

[Philip Reames]

Hi nlewycky, pete, hfinkel,

Teach FunctionAttr to infer the nonnull attribute on return values of functions which never return a potentially null value.  This is done both via a conservative local analysis for the function itself and a optimistic per-SCC analysis.  If no function in the SCC returns anything which could be null (other than values from other functions in the SCC), we can conclude no function returned a null pointer.  Even if some function within the SCC returns a null pointer, we may be able to locally conclude that some don't.  

This is inspired by discussion on http://reviews.llvm.org/D9132, but is mostly orthogonal to that patch except that they both relate to nonnul attributes.

http://reviews.llvm.org/D9688

Files:
  lib/Transforms/IPO/FunctionAttrs.cpp
  test/Transforms/FunctionAttrs/nonnull.ll

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