[PATCH] D71974: [Attributor][WIP] Connect AAIsDead with AAUndefinedBehavior

[Stefanos Baziotis via Phabricator via llvm-commits]

baziotis updated this revision to Diff 245764.
baziotis added a comment.

- Rebased with AAUB not manifesting

In this diff, note that I have commented the manifesting in AAUB, so that, for now, we can test UB blocks are deleted because of AAIsDead.
Btw, may I throw the idea that this be done in the end ? That is, blocks be deleted only because of AAIsDead.

Anyway, here are some examples:

  define i32 @cond_br_on_undef_interproc(i1 %cond) {
    br i1 %cond, label %t, label %e
  t:
    %a = load i32, i32* null
    ret i32 1
  e:
    ret i32 2
  }

In here, happens what we expect. The entry block is marked live initially and then in `AAIsDeadFunction::updateImpl()` the block with the UB is removed from alive blocks.
When the manifest phase in `Attributor::run()` comes, `AAIsDeadFunction` has 2/3 live blocks, so it is considered live (the AA) and its manifest ultimately deletes the block.

  define i32 @example(i1* %alloc) {
    %cond = load i1, i1* %alloc
    br i1 %cond, label %t, label %e
  t:
    ret i32 1
  e:
    ret i32 2
  }

Here, `isAssumedToCauseUB()` is overoptimistic and that means that the branch is assumed UB and its block is removed in `AAIsDeadFunction::updateImpl()`.
The catch is that although assumed information is used, AAUB never corrects that since it looks at live instructions for UB.
That's a problem in and of itself but that was happening before. The weird thing is that in the manifest stage of `run`, `AAIsDeadFunction` is considered
dead because the entry point of the function is considered dead. Shouldn't that mean that the function has to be deleted?


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D71974/new/

https://reviews.llvm.org/D71974

Files:
  llvm/include/llvm/Transforms/IPO/Attributor.h
  llvm/lib/Transforms/IPO/Attributor.cpp

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