[llvm-dev] Bug in TailDuplicator?

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[llvm-dev] Bug in TailDuplicator?

Jonathan Smith via llvm-dev

I think there might be a bug in the tail duplicator (called from
MachineBlockPlacement in my case), when duplicating a block that
contains an implicit fall-through.
Suppose you have the following blocks

      Predecessors according to CFG: BB#2
        conditional_branch <BB#3>
      < implicit fall-through to BB#2 >
      Successors according to CFG: BB#2 BB#3

      Predecessors according to CFG: BB#1 BB#0
        unconditional_branch <BB#1>
      Successors according to CFG: BB#1

      Predecessors according to CFG: BB#1

It duplicates BB#1 into BB#2 but misses out the branch that goes back up:

      Predecessors according to CFG: BB#1 BB#0
        <BB#2 instructions>
        <BB#1 instructions>
        conditional_branch <BB#3>
      Successors according to CFG: BB#3

So this is not a loop any more.

Instead, a branch needs to be inserted to replace the implicit
fall-through in BB#1.

I have attached a patch for this.

(Incidentally, I see a duplication of the statement
"TII->removeBranch(*PrevBB);" if run after register allocation. I've
removed this call in the patch as well.)

Unfortunately, I am working on a proprietary target, so cannot provide a
test case. I have, however, attached a "pseudo mir" which should be
easily adaptable for a public target.

Maybe this is not a bug, but my function at this stage is not canonical?
Also, I am not on tip (though I checked the TailDuplicator tip), so it
is possible this is fixed elsewhere.


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tailDuplicatorPatch.diff (1K) Download Attachment
tail_duplicator_bug.pseudo_mir (1K) Download Attachment