[PATCH] D29705: Fix PR 24415 (at least), by making our post-dominator tree behavior sane.

[Daniel Berlin via Phabricator via llvm-commits]

dberlin created this revision.

Currently, our post-dom tree tries to ignore and remove the effects of
infinite loops.  It fails miserably at this, because it tries to do it
ahead of time, and thus can only detect self-loops, and any other type
of infinite loop, it pretends doesn't exist at all.

This can, in a bunch of cases, lead to wrong answers and a completely
empty post-dom tree.

Wrong answer:

  declare void foo()
  define internal void @f() {
  entry:
    br i1 undef, label %bb35, label %bb3.i
  
  bb3.i:
    call void @foo()
    br label %bb3.i
  
  bb35.loopexit3:
    br label %bb35
  
  bb35:
    ret void
  }

We get:

  Inorder PostDominator Tree:
    [1]  <<exit node>> {0,7}
      [2] %bb35 {1,6}
        [3] %bb35.loopexit3 {2,3}
        [3] %entry {4,5}

This is a trivial modification of the testcase for PR 6047
Note that we pretend bb3.i doesn't exist.
We also pretend that bb35 post-dominates entry.

While it's true that it does not exit in a theoretical sense, it's not
really helpful to try to ignore the effect and pretend that bb35
post-dominates entry.  Worse, we pretend the infinite loop does
nothing (it's usually considered a side-effect), and doesn't even
exist, even when it calls a function.  Sadly, this makes it impossible
to use when you are trying to move code safely.  All compilers also
create virtual or real single exit nodes (including us), and connect
infinite loops there (which this patch does).  In fact, others have
worked around our behavior here, to the point of building their own
post-dom trees:
https://zneak.github.io/fcd/2016/02/17/structuring.html and pointing
out the region infrastructure is near-useless for them with postdom in
this state :(

Completely empty post-dom tree:

  define void @spam() #0 {
  bb:
    br label %bb1
  
  bb1:                                              ; preds = %bb1, %bb
    br label %bb1
  
  bb2:                                              ; No predecessors!
    ret void
  }

Printing analysis 'Post-Dominator Tree Construction' for function 'foo':
========================================================================

Inorder PostDominator Tree:

  [1]  <<exit node>> {0,1}

:(

(note that even if you ignore the effects of infinite loops, bb2
should be present as an exit node that post-dominates nothing).

This patch changes post-dom to properly handle infinite loops and does
root finding during calculation to prevent empty tress in such cases.

We match gcc's (and the canonical theoretical) behavior for infinite
loops (find the backedge, connect it to the exit block).

Testcases coming as soon as i finish running this on a ton of random graphs :)


https://reviews.llvm.org/D29705

Files:
  include/llvm/Support/GenericDomTree.h
  include/llvm/Support/GenericDomTreeConstruction.h

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