[SCEV] Use const Loop *L instead of Loop *L

[huihuiz]

Summary

Use const pointer in the trip count and trip multiple calculations.

[efriedma]

https://reviews.llvm.org/rL298161

include/llvm/Analysis/ScalarEvolution.h

public:
   bool isLoopBackedgeGuardedByCond(const Loop *L, ICmpInst::Predicate Pred,
                                    const SCEV *LHS, const SCEV *RHS);
 
   /// Returns the maximum trip count of the loop if it is a single-exit
   /// loop and we can compute a small maximum for that loop.
   ///
   /// Implemented in terms of the \c getSmallConstantTripCount overload with
   /// the single exiting block passed to it. See that routine for details.
-  unsigned getSmallConstantTripCount(Loop *L);
+  unsigned getSmallConstantTripCount(const Loop *L);
 
   /// Returns the maximum trip count of this loop as a normal unsigned
   /// value. Returns 0 if the trip count is unknown or not constant. This
   /// "trip count" assumes that control exits via ExitingBlock. More
   /// precisely, it is the number of times that control may reach ExitingBlock
   /// before taking the branch. For loops with multiple exits, it may not be
   /// the number times that the loop header executes if the loop exits
   /// prematurely via another branch.
-  unsigned getSmallConstantTripCount(Loop *L, BasicBlock *ExitingBlock);
+  unsigned getSmallConstantTripCount(const Loop *L, BasicBlock *ExitingBlock);
 
   /// Returns the upper bound of the loop trip count as a normal unsigned
   /// value.
   /// Returns 0 if the trip count is unknown or not constant.
-  unsigned getSmallConstantMaxTripCount(Loop *L);
+  unsigned getSmallConstantMaxTripCount(const Loop *L);
 
   /// Returns the largest constant divisor of the trip count of the
   /// loop if it is a single-exit loop and we can compute a small maximum for
   /// that loop.
   ///
   /// Implemented in terms of the \c getSmallConstantTripMultiple overload with
   /// the single exiting block passed to it. See that routine for details.
-  unsigned getSmallConstantTripMultiple(Loop *L);
+  unsigned getSmallConstantTripMultiple(const Loop *L);
 
   /// Returns the largest constant divisor of the trip count of this loop as a
   /// normal unsigned value, if possible. This means that the actual trip
   /// count is always a multiple of the returned value (don't forget the trip
   /// count could very well be zero as well!). As explained in the comments
   /// for getSmallConstantTripCount, this assumes that control exits the loop
   /// via ExitingBlock.
-  unsigned getSmallConstantTripMultiple(Loop *L, BasicBlock *ExitingBlock);
+  unsigned getSmallConstantTripMultiple(const Loop *L,
+                                        BasicBlock *ExitingBlock);
 
   /// Get the expression for the number of loop iterations for which this loop
   /// is guaranteed not to exit via ExitingBlock. Otherwise return
   /// SCEVCouldNotCompute.
-  const SCEV *getExitCount(Loop *L, BasicBlock *ExitingBlock);
+  const SCEV *getExitCount(const Loop *L, BasicBlock *ExitingBlock);
 
   /// If the specified loop has a predictable backedge-taken count, return it,
   /// otherwise return a SCEVCouldNotCompute object. The backedge-taken count
   /// is the number of times the loop header will be branched to from within
   /// the loop. This is one less than the trip count of the loop, since it
   /// doesn't count the first iteration, when the header is branched to from
   /// outside the loop.
   ///

lib/Analysis/ScalarEvolution.cpp

static unsigned getConstantTripCount(const SCEVConstant *ExitCount) {
   // Guard against huge trip counts.
   if (ExitConst->getValue().getActiveBits() > 32)
     return 0;
 
   // In case of integer overflow, this returns 0, which is correct.
   return ((unsigned)ExitConst->getZExtValue()) + 1;
 }
 
-unsigned ScalarEvolution::getSmallConstantTripCount(Loop *L) {
+unsigned ScalarEvolution::getSmallConstantTripCount(const Loop *L) {
   if (BasicBlock *ExitingBB = L->getExitingBlock())
     return getSmallConstantTripCount(L, ExitingBB);
 
   // No trip count information for multiple exits.
   return 0;
 }
 
-unsigned ScalarEvolution::getSmallConstantTripCount(Loop *L,
+unsigned ScalarEvolution::getSmallConstantTripCount(const Loop *L,
                                                     BasicBlock *ExitingBlock) {
   assert(ExitingBlock && "Must pass a non-null exiting block!");
   assert(L->isLoopExiting(ExitingBlock) &&
          "Exiting block must actually branch out of the loop!");
   const SCEVConstant *ExitCount =
       dyn_cast<SCEVConstant>(getExitCount(L, ExitingBlock));
   return getConstantTripCount(ExitCount);
 }
 
-unsigned ScalarEvolution::getSmallConstantMaxTripCount(Loop *L) {
+unsigned ScalarEvolution::getSmallConstantMaxTripCount(const Loop *L) {
   const auto *MaxExitCount =
       dyn_cast<SCEVConstant>(getMaxBackedgeTakenCount(L));
   return getConstantTripCount(MaxExitCount);
 }
 
-unsigned ScalarEvolution::getSmallConstantTripMultiple(Loop *L) {
+unsigned ScalarEvolution::getSmallConstantTripMultiple(const Loop *L) {
   if (BasicBlock *ExitingBB = L->getExitingBlock())
     return getSmallConstantTripMultiple(L, ExitingBB);
 
   // No trip multiple information for multiple exits.
   return 0;
 }
 
 /// Returns the largest constant divisor of the trip count of this loop as a
 /// normal unsigned value, if possible. This means that the actual trip count is
 /// always a multiple of the returned value (don't forget the trip count could
 /// very well be zero as well!).
 ///
 /// Returns 1 if the trip count is unknown or not guaranteed to be the
 /// multiple of a constant (which is also the case if the trip count is simply
 /// constant, use getSmallConstantTripCount for that case), Will also return 1
 /// if the trip count is very large (>= 2^32).
 ///
 /// As explained in the comments for getSmallConstantTripCount, this assumes
 /// that control exits the loop via ExitingBlock.
 unsigned
-ScalarEvolution::getSmallConstantTripMultiple(Loop *L,
+ScalarEvolution::getSmallConstantTripMultiple(const Loop *L,
                                               BasicBlock *ExitingBlock) {
   assert(ExitingBlock && "Must pass a non-null exiting block!");
   assert(L->isLoopExiting(ExitingBlock) &&
          "Exiting block must actually branch out of the loop!");
   const SCEV *ExitCount = getExitCount(L, ExitingBlock);
   if (ExitCount == getCouldNotCompute())
     return 1;
 
if (!Result || Result->getValue().getActiveBits() > 32 ||
     return 1;
 
   return (unsigned)Result->getZExtValue();
 }
 
 /// Get the expression for the number of loop iterations for which this loop is
 /// guaranteed not to exit via ExitingBlock. Otherwise return
 /// SCEVCouldNotCompute.
-const SCEV *ScalarEvolution::getExitCount(Loop *L, BasicBlock *ExitingBlock) {
+const SCEV *ScalarEvolution::getExitCount(const Loop *L,
+                                          BasicBlock *ExitingBlock) {
   return getBackedgeTakenInfo(L).getExact(ExitingBlock, this);
 }
 
 const SCEV *
 ScalarEvolution::getPredicatedBackedgeTakenCount(const Loop *L,
                                                  SCEVUnionPredicate &Preds) {
   return getPredicatedBackedgeTakenInfo(L).getExact(this, &Preds);
 }