[SCEV] Look at backedge dominating conditions (re-land r233447).

Summary:
This change teaches ScalarEvolution::isLoopBackedgeGuardedByCond to look
at edges within the loop body that dominate the latch.  We don't do an
exhaustive search for all possible edges, but only a quick walk up the
dom tree.

This re-lands r233447.  r233447 was reverted because it caused massive
compile-time regressions.  This change has a fix for the same issue.

llvm-svn: 233829

1 files changed, 62 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 4e713fb1218..97332b7129b 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -6698,6 +6698,65 @@ ScalarEvolution::isLoopBackedgeGuardedByCond(const Loop *L,
       return true;
   }
 
+  struct ClearWalkingBEDominatingCondsOnExit {
+    ScalarEvolution &SE;
+
+    explicit ClearWalkingBEDominatingCondsOnExit(ScalarEvolution &SE)
+        : SE(SE){};
+
+    ~ClearWalkingBEDominatingCondsOnExit() {
+      SE.WalkingBEDominatingConds = false;
+    }
+  };
+
+  // We don't want more than one activation of the following loop on the stack
+  // -- that can lead to O(n!) time complexity.
+  if (WalkingBEDominatingConds)
+    return false;
+
+  WalkingBEDominatingConds = true;
+  ClearWalkingBEDominatingCondsOnExit ClearOnExit(*this);
+
+  // If the loop is not reachable from the entry block, we risk running into an
+  // infinite loop as we walk up into the dom tree.  These loops do not matter
+  // anyway, so we just return a conservative answer when we see them.
+  if (!DT->isReachableFromEntry(L->getHeader()))
+    return false;
+
+  for (DomTreeNode *DTN = (*DT)[Latch], *HeaderDTN = (*DT)[L->getHeader()];
+       DTN != HeaderDTN;
+       DTN = DTN->getIDom()) {
+
+    assert(DTN && "should reach the loop header before reaching the root!");
+
+    BasicBlock *BB = DTN->getBlock();
+    BasicBlock *PBB = BB->getSinglePredecessor();
+    if (!PBB)
+      continue;
+
+    BranchInst *ContinuePredicate = dyn_cast<BranchInst>(PBB->getTerminator());
+    if (!ContinuePredicate || !ContinuePredicate->isConditional())
+      continue;
+
+    Value *Condition = ContinuePredicate->getCondition();
+
+    // If we have an edge `E` within the loop body that dominates the only
+    // latch, the condition guarding `E` also guards the backedge.  This
+    // reasoning works only for loops with a single latch.
+
+    BasicBlockEdge DominatingEdge(PBB, BB);
+    if (DominatingEdge.isSingleEdge()) {
+      // We're constructively (and conservatively) enumerating edges within the
+      // loop body that dominate the latch.  The dominator tree better agree
+      // with us on this:
+      assert(DT->dominates(DominatingEdge, Latch) && "should be!");
+
+      if (isImpliedCond(Pred, LHS, RHS, Condition,
+                        BB != ContinuePredicate->getSuccessor(0)))
+        return true;
+    }
+  }
+
   return false;
 }
 
@@ -7968,8 +8027,8 @@ ScalarEvolution::SCEVCallbackVH::SCEVCallbackVH(Value *V, ScalarEvolution *se)
 //===----------------------------------------------------------------------===//
 
 ScalarEvolution::ScalarEvolution()
-  : FunctionPass(ID), ValuesAtScopes(64), LoopDispositions(64),
-    BlockDispositions(64), FirstUnknown(nullptr) {
+    : FunctionPass(ID), WalkingBEDominatingConds(false), ValuesAtScopes(64),
+      LoopDispositions(64), BlockDispositions(64), FirstUnknown(nullptr) {
   initializeScalarEvolutionPass(*PassRegistry::getPassRegistry());
 }
 
@@ -8000,6 +8059,7 @@ void ScalarEvolution::releaseMemory() {
   }
 
   assert(PendingLoopPredicates.empty() && "isImpliedCond garbage");
+  assert(!WalkingBEDominatingConds && "isLoopBackedgeGuardedByCond garbage!");
 
   BackedgeTakenCounts.clear();
   ConstantEvolutionLoopExitValue.clear();