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

[Florian Hahn via llvm-commits]

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@@ -1366,3 +1366,40 @@ void vputils::detail::pullOutPermutationsImpl(
     }
   }
 }
+
+// Implements the algorithm described in "Simple and Efficient Construction of
+// Static Single Assignment Form" by Braun et al.
+VPValue *vputils::reconstructSSA(VPBasicBlock *VPBB,
+                                 DenseMap<VPBasicBlock *, VPValue *> &Defs) {
+  assert(!Defs.empty() && "Defs shouldn't be empty");
+  assert(VPBB->getPlan() && "VPBB isn't reachable from entry");
+  if (VPValue *Def = Defs.lookup(VPBB))
+    return Def;
+  // If the entry block is reached and there's still no def, then Defs is
+  // missing a definition that covers this path.
+  assert(VPBB->getNumPredecessors() && "Not all paths have def");
+
+  if (VPBlockBase *Pred = VPBB->getSinglePredecessor())
+    return reconstructSSA(cast<VPBasicBlock>(Pred), Defs);
+
+  // Multiple predecessors, create a join.
+  Type *Ty = Defs.begin()->second->getScalarType();
+  auto *Phi = new VPPhi({}, VPIRFlags::getDefaultFlags(Instruction::PHI, Ty),
+                        DebugLoc::getUnknown(), "", Ty);
+  VPBB->insert(Phi, VPBB->getFirstNonPhi());
+  Defs[VPBB] = Phi;
+  for (auto *Pred : VPBB->predecessors())
+    Phi->addIncoming(reconstructSSA(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);
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fhahn wrote:

I guess there's no easy way to clean up after the fact instead of having to iterate over fhe full map? If not that's fin

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