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

[David Sherwood via llvm-commits]

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@@ -1017,9 +1017,17 @@ void VPlan::execute(VPTransformState *State) {
          "middle block has unexpected successors");
   VPBasicBlock *ScalarPhVPBB = cast<VPBasicBlock>(
       MiddleSuccs.size() == 1 ? MiddleSuccs[0] : MiddleSuccs[1]);
-  assert(!isa<VPIRBasicBlock>(ScalarPhVPBB) &&
-         "scalar preheader cannot be wrapped already");
-  replaceVPBBWithIRVPBB(ScalarPhVPBB, ScalarPh);
+  if (!isa<VPIRBasicBlock>(ScalarPhVPBB)) {
+    replaceVPBBWithIRVPBB(ScalarPhVPBB, ScalarPh);
+  } else {
+    // There is no edge to the scalar pre-header in VPlan. Phis in ScalarPh have
+    // been created during skeleton construction, so remove the incoming values
+    // from the middle block here.
+    // TODO: Remove this once phis in the scalar preheader are managed in VPlan
+    // directly.
+    for (auto &Phi : ScalarPh->phis())
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david-arm wrote:

Does this change somehow relate to the change in `simplifyCFG`? Perhaps I'm missing something, but they seem unrelated.

This seems to be relying upon the assumption that if ScalarPh is already a IRVPBB, then there is no path from the middle block to the scalar.ph block. I can't see that assumption tested or documented anywhere. Also, given that we've just extracted `ScalarPhVPBB` from a successor of `MiddleVPBB` it seems a little contradictory?

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