[llvm-dev] ConstantFoldTerminator doesn't delete trivially dead phi inputs

[Nicolai Hähnle via llvm-dev]

On Fri, May 15, 2020 at 7:47 PM Johannes Doerfert via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> On 5/15/20 5:57 AM, Jay Foad via llvm-dev wrote:
> > I'm looking into some missed optimizations where CodeGenPrepare seems
> > to leave trivially dead instructions lying around.
> >
> > This happens because CodeGenPrepare::runOnFunction calls
> > ConstantFoldTerminator which folds a "br i1 false" into an
> > unconditional branch and calls BasicBlock::removePredecessor which
> > calls PHINode::removeIncomingValue. Each incoming value that is
> > removed from a phi node might be dead now, but nothing deletes them.
> >
> > Is there any way this could be improved? Could
> > PHINode::removeIncomingValue call
> > RecursivelyDeleteTriviallyDeadInstructions on the value it removes, or
> > is that a layering violation?

I'd say it'd be surprising and could lead to crashes in users of
removeIncomingValue that happen to hold on to one of the now-dead
instructions for unrelated reasons.


> I think this is certainly a desirable behavior that could be opt-in
> for `ConstantFoldTerminator`.

This seems like a pretty reasonable way forward to me.

Cheers,
Nicolai


>
>
> > Thanks,
> > Jay.
> > _______________________________________________
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> > llvm-dev at lists.llvm.org
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