CGP: Break use-def graph loops in optimizeMemoryInst

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1

Dear Community,

I’m trying to implement optimization described in PR26223 and meet the following bail out condition in CodeGenPrepare::optimizeMemoryInst.

// Break use-def graph loops.

if (!Visited.insert(V).second) {

Consensus = nullptr;

break;

}

So while traversing thorough phi nodes from memory instruction to find addr mode we bail out if we meet some instruction twice.

Does anybody know/remember what is the reason for this checks.

From the first glance it seems that it will be ok if we just mark it as visited and continue processing worklist.

I did this change and make check passed.

So I seek for a help to explain this decision.

Thank you in advance,

Serguei.

2

My speculation here is that we originally didn't have the check for cycles and someone (maybe even the original author) added it to prevent stack overflow due to infinite recursion. I do not see a strong reason not to handle phi cycles in this code. Curious if anyone else disagrees? Or was around for the design of this?

Philip