[PATCH] D103182: [SCEV] Generalize getSmallConstantTripCount(L) for multiple exit loops

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added a reviewer: nikic.
Herald added subscribers: bollu, hiraditya, mcrosier.
reames requested review of this revision.
Herald added a project: LLVM.

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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D103182

Files:
  llvm/include/llvm/Analysis/ScalarEvolution.h
  llvm/lib/Analysis/ScalarEvolution.cpp


Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -6942,11 +6942,8 @@
 }
 
 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
Index: llvm/include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- llvm/include/llvm/Analysis/ScalarEvolution.h
+++ llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -705,17 +705,18 @@
   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.
+  /// 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.


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