[SCEV] Remove computeLoadConstantCompareExitLimit() (NFCI)

[nikic]

Summary

The functionality of this method is already covered by computeExitCountExhaustively() in a more general fashion. It was added at a time when exhaustive exit count calculation did not support constant folding loads yet. I double checked that dropping this code causes no binary changes in test-suite.

[reames]

LGTM, and thanks for the cleanup. Less complicated code is always good.

llvm/include/llvm/Analysis/ScalarEvolution.h

private:
   /// Compute the number of times the backedge of the specified loop will
   /// execute if its exit condition were a switch with a single exiting case
   /// to ExitingBB.
   ExitLimit computeExitLimitFromSingleExitSwitch(const Loop *L,
                                                  SwitchInst *Switch,
                                                  BasicBlock *ExitingBB,
                                                  bool IsSubExpr);
 
-  /// Given an exit condition of 'icmp op load X, cst', try to see if we can
-  /// compute the backedge-taken count.
-  ExitLimit computeLoadConstantCompareExitLimit(LoadInst *LI, Constant *RHS,
-                                                const Loop *L,
-                                                ICmpInst::Predicate p);
-
   /// Compute the exit limit of a loop that is controlled by a
   /// "(IV >> 1) != 0" type comparison.  We cannot compute the exact trip
   /// count in these cases (since SCEV has no way of expressing them), but we
   /// can still sometimes compute an upper bound.
   ///
   /// Return an ExitLimit for a loop whose backedge is guarded by `LHS Pred
   /// RHS`.
   ExitLimit computeShiftCompareExitLimit(Value *LHS, Value *RHS, const Loop *L,

llvm/lib/Analysis/ScalarEvolution.cpp

 #include <utility>
 #include <vector>
 
 using namespace llvm;
 using namespace PatternMatch;
 
 #define DEBUG_TYPE "scalar-evolution"
 
-STATISTIC(NumArrayLenItCounts,
-          "Number of trip counts computed with array length");
 STATISTIC(NumTripCountsComputed,
           "Number of loops with predictable loop counts");
 STATISTIC(NumTripCountsNotComputed,
           "Number of loops without predictable loop counts");
 STATISTIC(NumBruteForceTripCountsComputed,
           "Number of loops with trip counts computed by force");
 
 static cl::opt<unsigned>
ScalarEvolution::computeExitLimitFromICmp(const Loop *L,
   // If the condition was exit on true, convert the condition to exit on false
   ICmpInst::Predicate Pred;
   if (!ExitIfTrue)
     Pred = ExitCond->getPredicate();
   else
     Pred = ExitCond->getInversePredicate();
   const ICmpInst::Predicate OriginalPred = Pred;
 
-  // Handle common loops like: for (X = "string"; *X; ++X)
-  if (LoadInst *LI = dyn_cast<LoadInst>(ExitCond->getOperand(0)))
-    if (Constant *RHS = dyn_cast<Constant>(ExitCond->getOperand(1))) {
-      ExitLimit ItCnt =
-        computeLoadConstantCompareExitLimit(LI, RHS, L, Pred);
-      if (ItCnt.hasAnyInfo())
-        return ItCnt;
-    }
-
   const SCEV *LHS = getSCEV(ExitCond->getOperand(0));
   const SCEV *RHS = getSCEV(ExitCond->getOperand(1));
 
   // Try to evaluate any dependencies out of the loop.
   LHS = getSCEVAtScope(LHS, L);
   RHS = getSCEVAtScope(RHS, L);
 
   // At this point, we would like to compute how many iterations of the
EvaluateConstantChrecAtConstant(const SCEVAddRecExpr *AddRec, ConstantInt *C,
                                 ScalarEvolution &SE) {
   const SCEV *InVal = SE.getConstant(C);
   const SCEV *Val = AddRec->evaluateAtIteration(InVal, SE);
   assert(isa<SCEVConstant>(Val) &&
          "Evaluation of SCEV at constant didn't fold correctly?");
   return cast<SCEVConstant>(Val)->getValue();
 }
 
-/// Given an exit condition of 'icmp op load X, cst', try to see if we can
-/// compute the backedge execution count.
-ScalarEvolution::ExitLimit
-ScalarEvolution::computeLoadConstantCompareExitLimit(
-  LoadInst *LI,
-  Constant *RHS,
-  const Loop *L,
-  ICmpInst::Predicate predicate) {
-  if (LI->isVolatile()) return getCouldNotCompute();
-
-  // Check to see if the loaded pointer is a getelementptr of a global.
-  // TODO: Use SCEV instead of manually grubbing with GEPs.
-  GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(LI->getOperand(0));
-  if (!GEP) return getCouldNotCompute();
-
-  // Make sure that it is really a constant global we are gepping, with an
-  // initializer, and make sure the first IDX is really 0.
-  GlobalVariable *GV = dyn_cast<GlobalVariable>(GEP->getOperand(0));
-  if (!GV || !GV->isConstant() || !GV->hasDefinitiveInitializer() ||
-      GEP->getNumOperands() < 3 || !isa<Constant>(GEP->getOperand(1)) ||
-      !cast<Constant>(GEP->getOperand(1))->isNullValue())
-    return getCouldNotCompute();
-
-  // Okay, we allow one non-constant index into the GEP instruction.
-  Value *VarIdx = nullptr;
-  std::vector<Constant*> Indexes;
-  unsigned VarIdxNum = 0;
-  for (unsigned i = 2, e = GEP->getNumOperands(); i != e; ++i)
-    if (ConstantInt *CI = dyn_cast<ConstantInt>(GEP->getOperand(i))) {
-      Indexes.push_back(CI);
-    } else if (!isa<ConstantInt>(GEP->getOperand(i))) {
-      if (VarIdx) return getCouldNotCompute();  // Multiple non-constant idx's.
-      VarIdx = GEP->getOperand(i);
-      VarIdxNum = i-2;
-      Indexes.push_back(nullptr);
-    }
-
-  // Loop-invariant loads may be a byproduct of loop optimization. Skip them.
-  if (!VarIdx)
-    return getCouldNotCompute();
-
-  // Okay, we know we have a (load (gep GV, 0, X)) comparison with a constant.
-  // Check to see if X is a loop variant variable value now.
-  const SCEV *Idx = getSCEV(VarIdx);
-  Idx = getSCEVAtScope(Idx, L);
-
-  // We can only recognize very limited forms of loop index expressions, in
-  // particular, only affine AddRec's like {C1,+,C2}<L>.
-  const SCEVAddRecExpr *IdxExpr = dyn_cast<SCEVAddRecExpr>(Idx);
-  if (!IdxExpr || IdxExpr->getLoop() != L || !IdxExpr->isAffine() ||
-      isLoopInvariant(IdxExpr, L) ||
-      !isa<SCEVConstant>(IdxExpr->getOperand(0)) ||
-      !isa<SCEVConstant>(IdxExpr->getOperand(1)))
-    return getCouldNotCompute();
-
-  unsigned MaxSteps = MaxBruteForceIterations;
-  for (unsigned IterationNum = 0; IterationNum != MaxSteps; ++IterationNum) {
-    ConstantInt *ItCst = ConstantInt::get(
-                           cast<IntegerType>(IdxExpr->getType()), IterationNum);
-    ConstantInt *Val = EvaluateConstantChrecAtConstant(IdxExpr, ItCst, *this);
-
-    // Form the GEP offset.
-    Indexes[VarIdxNum] = Val;
-
-    Constant *Result = ConstantFoldLoadThroughGEPIndices(GV->getInitializer(),
-                                                         Indexes);
-    if (!Result) break;  // Cannot compute!
-
-    // Evaluate the condition for this iteration.
-    Result = ConstantExpr::getICmp(predicate, Result, RHS);
-    if (!isa<ConstantInt>(Result)) break;  // Couldn't decide for sure
-    if (cast<ConstantInt>(Result)->getValue().isMinValue()) {
-      ++NumArrayLenItCounts;
-      return getConstant(ItCst);   // Found terminating iteration!
-    }
-  }
-  return getCouldNotCompute();
-}
-
 ScalarEvolution::ExitLimit ScalarEvolution::computeShiftCompareExitLimit(
     Value *LHS, Value *RHSV, const Loop *L, ICmpInst::Predicate Pred) {
   ConstantInt *RHS = dyn_cast<ConstantInt>(RHSV);
   if (!RHS)
     return getCouldNotCompute();
 
   const BasicBlock *Latch = L->getLoopLatch();
   if (!Latch)