[PATCH] D138699: [SCEV] Introduce field for storing SymbolicMaxNotTaken

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
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Currently, it is initialized with either exact (if available) or
with constant max exit count. In the future, this can be improved.

Hypothetically this is not an NFC (it is possible that exact is not
known and max is known for a particular exit), but for how we use
it now it should be an NFC.


https://reviews.llvm.org/D138699

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
@@ -8559,8 +8559,11 @@
 
 const SCEV *ScalarEvolution::BackedgeTakenInfo::getSymbolicMax(
     const BasicBlock *ExitingBlock, ScalarEvolution *SE) const {
-  // FIXME: Need to implement this. Return exact for now.
-  return getExact(ExitingBlock, SE);
+  for (const auto &ENT : ExitNotTaken)
+    if (ENT.ExitingBlock == ExitingBlock && ENT.hasAlwaysTruePredicate())
+      return ENT.SymbolicMaxNotTaken;
+
+  return SE->getCouldNotCompute();
 }
 
 /// getConstantMax - Get the constant max backedge taken count for the loop.
@@ -8610,6 +8613,13 @@
   if (ConstantMaxNotTaken->isZero())
     ExactNotTaken = ConstantMaxNotTaken;
 
+  // FIXME: For now, SymbolicMaxNotTaken is either exact (if available) or
+  // constant max. In the future, we are planning to make it more powerful.
+  if (isa<SCEVCouldNotCompute>(ExactNotTaken))
+    SymbolicMaxNotTaken = ConstantMaxNotTaken;
+  else
+    SymbolicMaxNotTaken = ExactNotTaken;
+
   assert((isa<SCEVCouldNotCompute>(ExactNotTaken) ||
           !isa<SCEVCouldNotCompute>(ConstantMaxNotTaken)) &&
          "Exact is not allowed to be less precise than Max");
@@ -8646,7 +8656,8 @@
         BasicBlock *ExitBB = EEI.first;
         const ExitLimit &EL = EEI.second;
         return ExitNotTakenInfo(ExitBB, EL.ExactNotTaken,
-                                EL.ConstantMaxNotTaken, EL.Predicates);
+                                EL.ConstantMaxNotTaken, EL.SymbolicMaxNotTaken,
+                                EL.Predicates);
   });
   assert((isa<SCEVCouldNotCompute>(ConstantMax) ||
           isa<SCEVConstant>(ConstantMax)) &&
@@ -14751,9 +14762,6 @@
   for (BasicBlock *ExitingBB : ExitingBlocks) {
     const SCEV *ExitCount =
         getExitCount(L, ExitingBB, ScalarEvolution::SymbolicMaximum);
-    if (isa<SCEVCouldNotCompute>(ExitCount))
-      ExitCount = getExitCount(L, ExitingBB,
-                                  ScalarEvolution::ConstantMaximum);
     if (!isa<SCEVCouldNotCompute>(ExitCount)) {
       assert(DT.dominates(ExitingBB, L->getLoopLatch()) &&
              "We should only have known counts for exiting blocks that "
Index: llvm/include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- llvm/include/llvm/Analysis/ScalarEvolution.h
+++ llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -1314,6 +1314,7 @@
     const SCEV *ExactNotTaken; // The exit is not taken exactly this many times
     const SCEV *ConstantMaxNotTaken; // The exit is not taken at most this many
                                      // times
+    const SCEV *SymbolicMaxNotTaken;
 
     // Not taken either exactly ConstantMaxNotTaken or zero times
     bool MaxOrZero = false;
@@ -1360,14 +1361,16 @@
     PoisoningVH<BasicBlock> ExitingBlock;
     const SCEV *ExactNotTaken;
     const SCEV *ConstantMaxNotTaken;
+    const SCEV *SymbolicMaxNotTaken;
     SmallPtrSet<const SCEVPredicate *, 4> Predicates;
 
     explicit ExitNotTakenInfo(
         PoisoningVH<BasicBlock> ExitingBlock, const SCEV *ExactNotTaken,
-        const SCEV *ConstantMaxNotTaken,
+        const SCEV *ConstantMaxNotTaken, const SCEV *SymbolicMaxNotTaken,
         const SmallPtrSet<const SCEVPredicate *, 4> &Predicates)
         : ExitingBlock(ExitingBlock), ExactNotTaken(ExactNotTaken),
-          ConstantMaxNotTaken(ConstantMaxNotTaken), Predicates(Predicates) {}
+          ConstantMaxNotTaken(ConstantMaxNotTaken),
+          SymbolicMaxNotTaken(SymbolicMaxNotTaken), Predicates(Predicates) {}
 
     bool hasAlwaysTruePredicate() const {
       return Predicates.empty();


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