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

[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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ayalz wrote:

The original position seems (more) reasonable, being right after LoopScalarPreHeader is built. Worth leaving behind some comment why this replacement is currently done later, "for now"?

VPlan is assumed to always be connected, with all its VPBB's reachable from its entry. Can the original points maintain this connectivity, w/o storing the parental plan in all VPBBs nor passing Plan to replaceVPBBWithIRVPBB()?

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.

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