[PATCH] D101273: [JumpThreading] Set KeepOneInputPHIs to be true when DeleteDeadBlocks

[guopeilin via Phabricator via llvm-commits]

guopeilin added a comment.

https://bugs.llvm.org/show_bug.cgi?id=50119  
The bug is that we will be trapped into an infinite loop when we doing jump threading pass, and finally exhaust the stack.
The Jump Threading call function getValueFromCondition() of LazyValueInfo, and this function will recursively call itself in a way shown in the fowllowing:

  BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond);
  Value *BL = BO->getOperand(0);
  Value *BR = BO->getOperand(1);
  
  if (BL == Cond || BR == Cond)
    return ValueLatticeElement::getOverdefined();
  
  return intersect(getValueFromCondition(Val, BL, isTrueDest, Visited),
                   getValueFromCondition(Val, BR, isTrueDest, Visited));

At some time, let's consume that the condition is `%spec.select44.i.us.1 = or i1 undef, %.b31.i.us.1`, its second operand is `%.b31.i.us.1`.
And, `%.b31.i.us.1`'s second is `%spec.select44.i.us.1` again.
So we will be trapped here, with `%.b31.i.us.1` and `%spec.select44.i.us.1` appear alternately.
I guess the root reason is that we generate invalid IRs that use do not dominated by its def.

In D101273#2715865 <https://reviews.llvm.org/D101273#2715865>, @lattner wrote:

> I *think* that the current behavior is ok.  Dominance is not defined in unreachable code, so cyclic references etc are allowed (by the verifier etc).  What is the problem that this is causing?




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