[llvm] [VPlan] Add vputils::reconstructSSA (PR #212209)

[Luke Lau via llvm-commits]

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@@ -1143,3 +1143,41 @@ void vputils::detail::pullOutPermutationsImpl(
     }
   }
 }
+
+/// Implements the algorithm described in "Simple and Efficient Construction of
+/// Static Single Assignment Form" by Braun et al.
+static VPValue *reconstructSSAImpl(VPBasicBlock *VPBB,
+                                   DenseMap<VPBasicBlock *, VPValue *> &Defs) {
+  if (VPValue *Def = Defs.lookup(VPBB))
+    return Def;
+  assert(VPBB->getNumPredecessors() && "Not all paths have def");
+
+  if (VPBlockBase *Pred = VPBB->getSinglePredecessor())
+    return reconstructSSAImpl(cast<VPBasicBlock>(Pred), Defs);
+
+  // Multiple predecessors, create a join.
+  Type *Ty = Defs.begin()->second->getScalarType();
+  auto *Phi = new VPPhi({}, {}, DebugLoc::getUnknown(), "", Ty);
+  VPBB->insert(Phi, VPBB->getFirstNonPhi());
+  Defs[VPBB] = Phi;
+  for (auto *Pred : VPBB->predecessors())
+    Phi->addIncoming(reconstructSSAImpl(cast<VPBasicBlock>(Pred), Defs));
+
+  // Fold away trivial phis.
+  // TODO: Remove phi users which have become trivial too.
+  if (all_equal(Phi->incoming_values())) {
+    VPValue *Common = Phi->getIncomingValue(0);
+    Phi->replaceAllUsesWith(Common);
+    Phi->eraseFromParent();
+    Defs[VPBB] = Common;
+    return Common;
+  }
+
+  return Phi;
+}
+
+VPValue *vputils::reconstructSSA(DenseMap<VPBasicBlock *, VPValue *> Defs,
+                                 VPBasicBlock *VPBB) {
+  assert(!Defs.empty() && "Defs shouldn't be empty");
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lukel97 wrote:

I could imagine future users of reconstructSSA might dynamically construct Defs, so it might be valid for it to end up passing just one def at runtime

https://github.com/llvm/llvm-project/pull/212209