Move replaceCongruentIVs into SCEVExapander and bias toward "expanded"

IVs.

Indvars previously chose randomly between congruent IVs. Now it will
bias the decision toward IVs that SCEVExpander likes to create. This
was not done to fix any problem, it's just a welcome side effect of
factoring code.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141633 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ScalarEvolutionExpander.cpp

@@ -1465,3 +1465,103 @@ SCEVExpander::getOrInsertCanonicalInductionVariable(const Loop *L,
 
   return V;
 }
+
+/// hoistStep - Attempt to hoist an IV increment above a potential use.
+///
+/// To successfully hoist, two criteria must be met:
+/// - IncV operands dominate InsertPos and
+/// - InsertPos dominates IncV
+///
+/// Meeting the second condition means that we don't need to check all of IncV's
+/// existing uses (it's moving up in the domtree).
+///
+/// This does not yet recursively hoist the operands, although that would
+/// not be difficult.
+///
+/// This does not require a SCEVExpander instance and could be replaced by a
+/// general code-insertion helper.
+bool SCEVExpander::hoistStep(Instruction *IncV, Instruction *InsertPos,
+                             const DominatorTree *DT) {
+  if (DT->dominates(IncV, InsertPos))
+    return true;
+
+  if (!DT->dominates(InsertPos->getParent(), IncV->getParent()))
+    return false;
+
+  if (IncV->mayHaveSideEffects())
+    return false;
+
+  // Attempt to hoist IncV
+  for (User::op_iterator OI = IncV->op_begin(), OE = IncV->op_end();
+       OI != OE; ++OI) {
+    Instruction *OInst = dyn_cast<Instruction>(OI);
+    if (OInst && !DT->dominates(OInst, InsertPos))
+      return false;
+  }
+  IncV->moveBefore(InsertPos);
+  return true;
+}
+
+/// replaceCongruentIVs - Check for congruent phis in this loop header and
+/// replace them with their most canonical representative. Return the number of
+/// phis eliminated.
+///
+/// This does not depend on any SCEVExpander state but should be used in
+/// the same context that SCEVExpander is used.
+unsigned SCEVExpander::replaceCongruentIVs(Loop *L, const DominatorTree *DT,
+                                           SmallVectorImpl<WeakVH> &DeadInsts) {
+  unsigned NumElim = 0;
+  DenseMap<const SCEV *, PHINode *> ExprToIVMap;
+  for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
+    PHINode *Phi = cast<PHINode>(I);
+    if (!SE.isSCEVable(Phi->getType()))
+      continue;
+
+    PHINode *&OrigPhiRef = ExprToIVMap[SE.getSCEV(Phi)];
+    if (!OrigPhiRef) {
+      OrigPhiRef = Phi;
+      continue;
+    }
+
+    // If one phi derives from the other via GEPs, types may differ.
+    // We could consider adding a bitcast here to handle it.
+    if (OrigPhiRef->getType() != Phi->getType())
+      continue;
+
+    if (BasicBlock *LatchBlock = L->getLoopLatch()) {
+      Instruction *OrigInc =
+        cast<Instruction>(OrigPhiRef->getIncomingValueForBlock(LatchBlock));
+      Instruction *IsomorphicInc =
+        cast<Instruction>(Phi->getIncomingValueForBlock(LatchBlock));
+
+      // If this phi is more canonical, swap it with the original.
+      if (!isExpandedAddRecExprPHI(OrigPhiRef, OrigInc, L,
+                                   OrigPhiRef->getType())
+          && isExpandedAddRecExprPHI(Phi, IsomorphicInc, L, Phi->getType())) {
+        std::swap(OrigPhiRef, Phi);
+        std::swap(OrigInc, IsomorphicInc);
+      }
+      // Replacing the congruent phi is sufficient because acyclic redundancy
+      // elimination, CSE/GVN, should handle the rest. However, once SCEV proves
+      // that a phi is congruent, it's often the head of an IV user cycle that
+      // is isomorphic with the original phi. So it's worth eagerly cleaning up
+      // the common case of a single IV increment.
+      if (OrigInc != IsomorphicInc &&
+          OrigInc->getType() == IsomorphicInc->getType() &&
+          SE.getSCEV(OrigInc) == SE.getSCEV(IsomorphicInc) &&
+          hoistStep(OrigInc, IsomorphicInc, DT)) {
+        DEBUG_WITH_TYPE(DebugType, dbgs()
+                        << "INDVARS: Eliminated congruent iv.inc: "
+                        << *IsomorphicInc << '\n');
+        IsomorphicInc->replaceAllUsesWith(OrigInc);
+        DeadInsts.push_back(IsomorphicInc);
+      }
+    }
+    DEBUG_WITH_TYPE(DebugType, dbgs()
+                    << "INDVARS: Eliminated congruent iv: " << *Phi << '\n');
+    ++NumElim;
+    Phi->replaceAllUsesWith(OrigPhiRef);
+    DeadInsts.push_back(Phi);
+  }
+  return NumElim;
+}