[llvm-dev] Possible bug in adjusting PHINode from removePredecessor?

[David Majnemer via llvm-dev]

I suspect such a change will only paper over your actual problem.

On Wed, Aug 12, 2015 at 2:39 PM, Hariharan Sandanagobalane via llvm-commits
<llvm-commits at lists.llvm.org> wrote:

> The following change fixes this issue in trunk. Does this fix look good?
>
> Thanks
> Hari
>
> $ svn diff PruneEH.cpp
> Index: PruneEH.cpp
> ===================================================================
> --- PruneEH.cpp (revision 243617)
> +++ PruneEH.cpp (working copy)
> @@ -268,7 +268,7 @@
>    std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
>
>    for (unsigned i = 0, e = Succs.size(); i != e; ++i)
> -    Succs[i]->removePredecessor(BB);
> +    Succs[i]->removePredecessor(BB, true);
>
>    BB->eraseFromParent();
>  }
>
> On Mon, Aug 10, 2015 at 1:24 PM, Hariharan Sandanagobalane
> <hariharan.gcc at gmail.com> wrote:
> > Hi,
> > Simple description of the problem below. I have code coming into
> > pruneEH as follows
> > fn a {
> > entry:
> > call fn b
> > ...
> >
> > for_cond:
> > %i = phi [1, entry] [%x, for_body]
> > cmp $i with someval
> > cond-br for_body or for_exit
> >
> > for_body:
> > ...
> > $x = $i + 1
> > branch for_cond
> >
> > for_exit
> > ...
> > }
> >
> > PruneEH determines that the call to fn-b won't return. The code is
> > modified thus.
> >
> > fn a {
> > entry:
> > call fn b
> > unreachable insn /* Instructions after call to fn-b replaced with
> > unreachable insn */
> >
> > for_cond: /* No path from entry block */
> > %i = phi [%x, for_body]
> > cmp $i with someval
> > cond-br for_body or for_exit
> >
> > for_body:
> > ...
> > $x = $i + 1
> > branch for_cond
> >
> > for_exit
> > ...
> > }
> >
> > which then becomes
> >
> > fn a {
> > entry:
> > call fn b
> > unreachable insn /* Instructions after call to fn-b replaced with
> > unreachable insn */
> >
> > for_cond: /* No path from entry block */
> > cmp $x with someval
> > cond-br for_body or for_exit
> >
> > for_body:
> > ...
> > $x = $x + 1
> > branch for_cond
> >
> > for_exit
> > ...
> > }
> >
> > The instruction
> > $x = $x + 1
> > is obviously illegal in SSA form, which shows up as an infinite loop
> > in value numbering.
> >
> > The source of the problem exists in BasicBlock::removePredecessor
> > function in BasicBlock.cpp. The comment in that function describes
> > this exact scenario
> >
> >  // If there are exactly two predecessors, then we want to nuke the PHI
> nodes
> >  // altogether. However, we cannot do this, if this in this case:
> >  //
> >  // Loop:
> >  //   %x = phi [X, Loop]
> >  //   %x2 = add %x, 1        ;; This would become %x2 = add %x2, 1
> >  //   br Loop                ;; %x2 does not dominate all uses
> >  //
> >  // This is because the PHI node input is actually taken from the
> predecessor
> >  // basic block. The only case this can happen is with a self loop, so we
> >  // check for this case explicitly now.
> >
> > but goes on to cause the same issue. There are 2 potential problems in
> > this function.
> >
> > 1. The comment above describes a self-loop block. The same problem can
> > occur in loops with more than 1 block, as our example shows. In
> > general, this can happen when the predecessor being removed does not
> > belong to the same loop level as the basic block containing the
> > PhiNode.
> > 2. The version which introduced this comment r2694 did implement the
> > self-loop case okay. A subsequent change - revision 22664 - broke
> > this.
> >
> > The revision 22664 dates back to 2005, so this issue probably has been
> > around for 10 years. I am not sure why nobody else has seen a problem
> > here.
> >
> > I saw this issue in a large testcase. I will try to get a small repro
> > to illustrate the issue.
> >
> > Regards
> > Hari
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