Break up getProfitableChainIncrement().

The required checks are moved to ChainInstruction() itself and the
policy decisions are moved to IVChain::isProfitableInc().

Also cache the ExprBase in IVChain to avoid frequent recomputations.

No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@155676 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/LoopStrengthReduce.cpp

@@ -1441,6 +1441,12 @@ struct IVInc {
 // We typically add the head of a chain without finding subsequent links.
 struct IVChain {
   SmallVector<IVInc,1> Incs;
+  const SCEV *ExprBase;
+
+  IVChain() : ExprBase(0) {}
+
+  IVChain(const IVInc &Head, const SCEV *Base)
+    : Incs(1, Head), ExprBase(Base) {}
 
   typedef SmallVectorImpl<IVInc>::const_iterator const_iterator;
 
@@ -1461,6 +1467,12 @@ struct IVChain {
 
   // tailUserInst - Returns the last UserInst in the chain.
   Instruction *tailUserInst() const { return Incs.back().UserInst; }
+
+  // isProfitableIncrement - Returns true if IncExpr can be profitably added to
+  // this chain.
+  bool isProfitableIncrement(const SCEV *OperExpr,
+                             const SCEV *IncExpr,
+                             ScalarEvolution&);
 };
 
 /// ChainUsers - Helper for CollectChains to track multiple IV increment uses.
@@ -2341,41 +2353,23 @@ static const SCEV *getExprBase(const SCEV *S) {
 /// increment will be an offset relative to the same base. We allow such offsets
 /// to potentially be used as chain increment as long as it's not obviously
 /// expensive to expand using real instructions.
-static const SCEV *
-getProfitableChainIncrement(Value *NextIV, Value *PrevIV,
-                            const IVChain &Chain, Loop *L,
-                            ScalarEvolution &SE, const TargetLowering *TLI) {
-  // Prune the solution space aggressively by checking that both IV operands
-  // are expressions that operate on the same unscaled SCEVUnknown. This
-  // "base" will be canceled by the subsequent getMinusSCEV call. Checking first
-  // avoids creating extra SCEV expressions.
-  const SCEV *OperExpr = SE.getSCEV(NextIV);
-  const SCEV *PrevExpr = SE.getSCEV(PrevIV);
-  if (getExprBase(OperExpr) != getExprBase(PrevExpr) && !StressIVChain)
-    return 0;
-
-  const SCEV *IncExpr = SE.getMinusSCEV(OperExpr, PrevExpr);
-  if (!SE.isLoopInvariant(IncExpr, L))
-    return 0;
-
-  // We are not able to expand an increment unless it is loop invariant,
-  // however, the following checks are purely for profitability.
+bool IVChain::isProfitableIncrement(const SCEV *OperExpr,
+                                    const SCEV *IncExpr,
+                                    ScalarEvolution &SE) {
+  // Aggressively form chains when -stress-ivchain.
   if (StressIVChain)
-    return IncExpr;
+    return true;
 
   // Do not replace a constant offset from IV head with a nonconstant IV
   // increment.
   if (!isa<SCEVConstant>(IncExpr)) {
-    const SCEV *HeadExpr = SE.getSCEV(getWideOperand(Chain.Incs[0].IVOperand));
+    const SCEV *HeadExpr = SE.getSCEV(getWideOperand(Incs[0].IVOperand));
     if (isa<SCEVConstant>(SE.getMinusSCEV(OperExpr, HeadExpr)))
       return 0;
   }
 
   SmallPtrSet<const SCEV*, 8> Processed;
-  if (isHighCostExpansion(IncExpr, Processed, SE))
-    return 0;
-
-  return IncExpr;
+  return !isHighCostExpansion(IncExpr, Processed, SE);
 }
 
 /// Return true if the number of registers needed for the chain is estimated to
@@ -2469,25 +2463,39 @@ void LSRInstance::ChainInstruction(Instruction *UserInst, Instruction *IVOper,
                                    SmallVectorImpl<ChainUsers> &ChainUsersVec) {
   // When IVs are used as types of varying widths, they are generally converted
   // to a wider type with some uses remaining narrow under a (free) trunc.
-  Value *NextIV = getWideOperand(IVOper);
+  Value *const NextIV = getWideOperand(IVOper);
+  const SCEV *const OperExpr = SE.getSCEV(NextIV);
+  const SCEV *const OperExprBase = getExprBase(OperExpr);
 
   // Visit all existing chains. Check if its IVOper can be computed as a
   // profitable loop invariant increment from the last link in the Chain.
   unsigned ChainIdx = 0, NChains = IVChainVec.size();
   const SCEV *LastIncExpr = 0;
   for (; ChainIdx < NChains; ++ChainIdx) {
-    Value *PrevIV = getWideOperand(IVChainVec[ChainIdx].Incs.back().IVOperand);
+    IVChain &Chain = IVChainVec[ChainIdx];
+
+    // Prune the solution space aggressively by checking that both IV operands
+    // are expressions that operate on the same unscaled SCEVUnknown. This
+    // "base" will be canceled by the subsequent getMinusSCEV call. Checking
+    // first avoids creating extra SCEV expressions.
+    if (!StressIVChain && Chain.ExprBase != OperExprBase)
+      continue;
+
+    Value *PrevIV = getWideOperand(Chain.Incs.back().IVOperand);
     if (!isCompatibleIVType(PrevIV, NextIV))
       continue;
 
     // A phi node terminates a chain.
-    if (isa<PHINode>(UserInst)
-        && isa<PHINode>(IVChainVec[ChainIdx].tailUserInst()))
+    if (isa<PHINode>(UserInst) && isa<PHINode>(Chain.tailUserInst()))
       continue;
 
-    if (const SCEV *IncExpr =
-        getProfitableChainIncrement(NextIV, PrevIV, IVChainVec[ChainIdx],
-                                    L, SE, TLI)) {
+    // The increment must be loop-invariant so it can be kept in a register.
+    const SCEV *PrevExpr = SE.getSCEV(PrevIV);
+    const SCEV *IncExpr = SE.getMinusSCEV(OperExpr, PrevExpr);
+    if (!SE.isLoopInvariant(IncExpr, L))
+      continue;
+
+    if (Chain.isProfitableIncrement(OperExpr, IncExpr, SE)) {
       LastIncExpr = IncExpr;
       break;
     }
@@ -2501,24 +2509,24 @@ void LSRInstance::ChainInstruction(Instruction *UserInst, Instruction *IVOper,
       DEBUG(dbgs() << "IV Chain Limit\n");
       return;
     }
-    LastIncExpr = SE.getSCEV(NextIV);
+    LastIncExpr = OperExpr;
     // IVUsers may have skipped over sign/zero extensions. We don't currently
     // attempt to form chains involving extensions unless they can be hoisted
     // into this loop's AddRec.
     if (!isa<SCEVAddRecExpr>(LastIncExpr))
       return;
     ++NChains;
-    IVChainVec.resize(NChains);
+    IVChainVec.push_back(IVChain(IVInc(UserInst, IVOper, LastIncExpr),
+                                 OperExprBase));
     ChainUsersVec.resize(NChains);
     DEBUG(dbgs() << "IV Chain#" << ChainIdx << " Head: (" << *UserInst
                  << ") IV=" << *LastIncExpr << "\n");
-  }
-  else
+  } else {
     DEBUG(dbgs() << "IV Chain#" << ChainIdx << "  Inc: (" << *UserInst
                  << ") IV+" << *LastIncExpr << "\n");
-
-  // Add this IV user to the end of the chain.
-  IVChainVec[ChainIdx].add(IVInc(UserInst, IVOper, LastIncExpr));
+    // Add this IV user to the end of the chain.
+    IVChainVec[ChainIdx].add(IVInc(UserInst, IVOper, LastIncExpr));
+  }
 
   SmallPtrSet<Instruction*,4> &NearUsers = ChainUsersVec[ChainIdx].NearUsers;
   // This chain's NearUsers become FarUsers.