[llvm] [VPlan] Add initial CFG simplification, removing BranchOnCond true. (PR #106748)

[Florian Hahn via llvm-commits]

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@@ -2817,6 +2817,7 @@ BasicBlock *InnerLoopVectorizer::createVectorizedLoopSkeleton(
   // faster.
   emitMemRuntimeChecks(LoopScalarPreHeader);
 
+  replaceVPBBWithIRVPBB(Plan.getScalarPreheader(), LoopScalarPreHeader);
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fhahn wrote:

Thanks, I added a note at the original place.

We could store the plan in all blocks w/o a parent region, there is already a field in all blocks to do so?

> BTW, would be good to clarify that VPlan::getPlanEntry() avoids going into an infinite loop, if invoked on flat region-less cyclic CFG, based on visiting operands in order, and relying on the operand associated with the preheader block to appear (first) before that of the latch when visiting header phis.
getPlanEntry at the moment uses a SmallSetVector for its worklist, which naturally avoids infinite cycles.

https://github.com/llvm/llvm-project/pull/106748