[SCEV] Generalize getSmallConstantTripCount(L) for multiple exit loops

[reames]

Summary

This came up in review for another patch, see https://reviews.llvm.org/D102982#2782407 for full context.

[nikic]

LGTM, thanks!

llvm/include/llvm/Analysis/ScalarEvolution.h

public:
 
   /// Convert from an "exit count" (i.e. "backedge taken count") to a "trip
   /// count".  A "trip count" is the number of times the header of the loop
   /// will execute if an exit is taken after the specified number of backedges
   /// have been taken.  (e.g. TripCount = ExitCount + 1)  A zero result
   /// must be interpreted as a loop having an unknown trip count.
   const SCEV *getTripCountFromExitCount(const SCEV *ExitCount);
 
-  /// 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.
+  /// Returns the exact trip count of the loop if we can compute it, and
+  /// the result is a small constant.  '0' is used to represent an unknown
+  /// or non-constant trip count.  Note that a trip count is simply one more
+  /// than the backedge taken count for the loop.
   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
+  /// Return the exact trip count for this loop if we exit through ExitingBlock.
+  /// '0' is used to represent an unknown or non-constant trip count.  Note
+  /// that a trip count is simply one more than the backedge taken count for
+  /// the same exit.
+  /// This "trip count" assumes that control exits via ExitingBlock. More
+  /// precisely, it is the number of times that control will 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(const Loop *L,
                                      const BasicBlock *ExitingBlock);
 
   /// Returns the upper bound of the loop trip count as a normal unsigned
   /// value.

llvm/lib/Analysis/ScalarEvolution.cpp

static unsigned getConstantTripCount(const SCEVConstant *ExitCount) {
   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(const Loop *L) {
-  if (BasicBlock *ExitingBB = L->getExitingBlock())
-    return getSmallConstantTripCount(L, ExitingBB);
-
-  // No trip count information for multiple exits.
-  return 0;
+  auto *ExitCount = dyn_cast<SCEVConstant>(getBackedgeTakenCount(L, Exact));
+  return getConstantTripCount(ExitCount);
 }
 
 unsigned
 ScalarEvolution::getSmallConstantTripCount(const Loop *L,
                                            const BasicBlock *ExitingBlock) {
   assert(ExitingBlock && "Must pass a non-null exiting block!");
   assert(L->isLoopExiting(ExitingBlock) &&
          "Exiting block must actually branch out of the loop!");