[PATCH] D11214: [PM/AA] Remove the addEscapingUse update API that won't be easy to directly model in the new PM.

[Chandler Carruth]

chandlerc created this revision.
chandlerc added reviewers: hfinkel, Gerolf, pete.
chandlerc added a subscriber: llvm-commits.
chandlerc added a dependency: D11213: [PM/AA] Disable the core unsafe aspect of GlobalsModRef in the face of basic changes to the IR such as folding pointers through PHIs, Selects, integer casts, store/load pairs, or outlining..

This also was an incredibly brittle and expensive update API that was
never fully utilized by all the passes that claimed to preserve AA, nor
could it reasonably have been extended to all of them. Any number of
places add uses of values. If we ever wanted to reliably instrument
this, we would want a callback hook much like we have with ValueHandles,
but doing this for every use addition seems *extremely* expensive in
terms of compile time.

The only user of this update mechanism is GlobalsModRef. The idea of
using this to keep it up to date doesn't really work anyways as its
analysis requires a symmetric analysis of two different memory
locations. It would be very hard to make updates be sufficiently
rigorous to *guarantee* symmetric analysis in this way, and it pretty
certainly isn't true today.

However, folks have been using GMR with this update for a long time and
seem to not be hitting the issues. The reported issue that the update
hook fixes isn't even a problem any more as other changes to
GetUnderlyingObject worked around it, and that issue stemmed from *many*
years ago. As a consequence, a prior patch provided a flag to control
the unsafe behavior of GMR, and this patch removes the update mechanism
that has questionable compile-time tradeoffs and is causing problems
with moving to the new pass manager. Note the lack of test updates --
not one test in tree actually requires this update, even for a contrived
case.

All of this was extensively discussed on the dev list, this patch will
just enact what that discussion decides on. I'm sending it for review in
part to show what I'm planning, and in part to show the *amazing* amount
of work this avoids. Every call to the AA here is something like three
to six indirect function calls, which in the non-LTO pipeline never do
any work! =[

Depends on D11213.

http://reviews.llvm.org/D11214

Files:
  include/llvm/Analysis/AliasAnalysis.h
  lib/Analysis/AliasAnalysis.cpp
  lib/Analysis/IPA/GlobalsModRef.cpp
  lib/Analysis/NoAliasAnalysis.cpp
  lib/Transforms/Scalar/GVN.cpp
  lib/Transforms/Scalar/MergedLoadStoreMotion.cpp

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