[SCEV] Collect and merge loop guards through PHI nodes with multiple incoming values

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -1316,6 +1316,25 @@ class ScalarEvolution {
 
     LoopGuards(ScalarEvolution &SE) : SE(SE) {}
 
+    /// Recursively collect loop guards in \p Guards, starting from
+    /// block \p Block with predecessor \p Pred. The intended starting point
+    /// is to collect from a loop header and its predecessor.
+    static void
+    collectFromBlock(ScalarEvolution &SE, ScalarEvolution::LoopGuards &Guards,
+                     const BasicBlock *Block, const BasicBlock *Pred,
+                     SmallPtrSetImpl<const BasicBlock *> &VisitedBlocks,
+                     unsigned Depth = 0);
+
+    /// Collect loop guards in \p Guards, starting from PHINode \p
+    /// Phi, by calling \p collectFromBlock on the incoming blocks of
+    /// \Phi and trying to merge the found constraints into a single
+    /// combined on for \p Phi.
+    static void collectFromPHI(
+        ScalarEvolution &SE, ScalarEvolution::LoopGuards &Guards,
+        const PHINode &Phi, SmallPtrSetImpl<const BasicBlock *> &VisitedBlocks,
+        SmallDenseMap<const BasicBlock *, LoopGuards> &IncomingGuards,
+        unsigned Depth);
+
   public:
     /// Collect rewrite map for loop guards for loop \p L, together with flags
     /// indicating if NUW and NSW can be preserved during rewriting.

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -222,6 +222,10 @@ static cl::opt<unsigned> RangeIterThreshold(
     cl::desc("Threshold for switching to iteratively computing SCEV ranges"),
     cl::init(32));
 
+static cl::opt<unsigned> MaxLoopGuardCollectionDepth(
+    "scalar-evolution-max-loop-guard-collection-depth", cl::Hidden,
+    cl::desc("Maximum depth for recrusive loop guard collection"), cl::init(1));
+
 static cl::opt<bool>
 ClassifyExpressions("scalar-evolution-classify-expressions",
     cl::Hidden, cl::init(true),
@@ -10648,7 +10652,7 @@ ScalarEvolution::getPredecessorWithUniqueSuccessorForBB(const BasicBlock *BB)
   if (const Loop *L = LI.getLoopFor(BB))
     return {L->getLoopPredecessor(), L->getHeader()};
 
-  return {nullptr, nullptr};
+  return {nullptr, BB};
 }
 
 /// SCEV structural equivalence is usually sufficient for testing whether two
@@ -15217,7 +15221,83 @@ bool ScalarEvolution::matchURem(const SCEV *Expr, const SCEV *&LHS,
 
 ScalarEvolution::LoopGuards
 ScalarEvolution::LoopGuards::collect(const Loop *L, ScalarEvolution &SE) {
+  BasicBlock *Header = L->getHeader();
+  BasicBlock *Pred = L->getLoopPredecessor();
   LoopGuards Guards(SE);
+  SmallPtrSet<const BasicBlock *, 8> VisitedBlocks;
+  collectFromBlock(SE, Guards, Header, Pred, VisitedBlocks);
+  return Guards;
+}
+
+void ScalarEvolution::LoopGuards::collectFromPHI(
+    ScalarEvolution &SE, ScalarEvolution::LoopGuards &Guards,
+    const PHINode &Phi, SmallPtrSetImpl<const BasicBlock *> &VisitedBlocks,
+    SmallDenseMap<const BasicBlock *, LoopGuards> &IncomingGuards,
+    unsigned Depth) {
+  if (!SE.isSCEVable(Phi.getType()))
+    return;
+
+  using MinMaxPattern = std::pair<const SCEVConstant *, SCEVTypes>;
+  auto GetMinMaxConst = [&](unsigned In) -> MinMaxPattern {
+    const BasicBlock *InBlock = Phi.getIncomingBlock(In);
+    if (!VisitedBlocks.insert(InBlock).second)
+      return {nullptr, scCouldNotCompute};
+    if (!IncomingGuards.contains(InBlock)) {
+      LoopGuards G(SE);
+      collectFromBlock(SE, G, Phi.getParent(), InBlock, VisitedBlocks,
+                       Depth + 1);
+      IncomingGuards.try_emplace(InBlock, std::move(G));
+    }
+    const LoopGuards &G = IncomingGuards.at(InBlock);
+    auto S = G.RewriteMap.find(SE.getSCEV(Phi.getIncomingValue(In)));
+    if (S == G.RewriteMap.end())
+      return {nullptr, scCouldNotCompute};
+    auto *SM = dyn_cast_if_present<SCEVMinMaxExpr>(S->second);
+    if (!SM)
+      return {nullptr, scCouldNotCompute};
+    if (const SCEVConstant *C0 = dyn_cast<SCEVConstant>(SM->getOperand(0)))
+      return {C0, SM->getSCEVType()};
+    if (const SCEVConstant *C1 = dyn_cast<SCEVConstant>(SM->getOperand(1)))
+      return {C1, SM->getSCEVType()};
+    return {nullptr, scCouldNotCompute};
+  };
+  auto MergeMinMaxConst = [](MinMaxPattern P1,
+                             MinMaxPattern P2) -> MinMaxPattern {
+    auto [C1, T1] = P1;
+    auto [C2, T2] = P2;
+    if (!C1 || !C2 || T1 != T2)
+      return {nullptr, scCouldNotCompute};
+    switch (T1) {
+    case scUMaxExpr:
+      return {C1->getAPInt().ult(C2->getAPInt()) ? C1 : C2, T1};
+    case scSMaxExpr:
+      return {C1->getAPInt().slt(C2->getAPInt()) ? C1 : C2, T1};
+    case scUMinExpr:
+      return {C1->getAPInt().ugt(C2->getAPInt()) ? C1 : C2, T1};
+    case scSMinExpr:
+      return {C1->getAPInt().sgt(C2->getAPInt()) ? C1 : C2, T1};
+    default:
+      llvm_unreachable("Trying to merge non-MinMaxExpr SCEVs.");
+    }
+  };
+  auto P = GetMinMaxConst(0);
+  for (unsigned int In = 1; In < Phi.getNumIncomingValues(); In++) {
+    if (!P.first)
+      break;
+    P = MergeMinMaxConst(P, GetMinMaxConst(In));
+  }
+  if (P.first) {
+    const SCEV *LHS = SE.getSCEV(const_cast<PHINode *>(&Phi));
+    SmallVector<const SCEV *, 2> Ops({P.first, LHS});
+    const SCEV *RHS = SE.getMinMaxExpr(P.second, Ops);
+    Guards.RewriteMap.insert({LHS, RHS});
+  }
+}
+
+void ScalarEvolution::LoopGuards::collectFromBlock(
+    ScalarEvolution &SE, ScalarEvolution::LoopGuards &Guards,
+    const BasicBlock *Block, const BasicBlock *Pred,
+    SmallPtrSetImpl<const BasicBlock *> &VisitedBlocks, unsigned Depth) {
   SmallVector<const SCEV *> ExprsToRewrite;
   auto CollectCondition = [&](ICmpInst::Predicate Predicate, const SCEV *LHS,
                               const SCEV *RHS,
@@ -15556,14 +15636,13 @@ ScalarEvolution::LoopGuards::collect(const Loop *L, ScalarEvolution &SE) {
     }
   };
 
-  BasicBlock *Header = L->getHeader();
   SmallVector<PointerIntPair<Value *, 1, bool>> Terms;
   // First, collect information from assumptions dominating the loop.
   for (auto &AssumeVH : SE.AC.assumptions()) {
     if (!AssumeVH)
       continue;
     auto *AssumeI = cast<CallInst>(AssumeVH);
-    if (!SE.DT.dominates(AssumeI, Header))
+    if (!SE.DT.dominates(AssumeI, Block))
       continue;
     Terms.emplace_back(AssumeI->getOperand(0), true);
   }
@@ -15574,27 +15653,43 @@ ScalarEvolution::LoopGuards::collect(const Loop *L, ScalarEvolution &SE) {
   if (GuardDecl)
     for (const auto *GU : GuardDecl->users())
       if (const auto *Guard = dyn_cast<IntrinsicInst>(GU))
-        if (Guard->getFunction() == Header->getParent() &&
-            SE.DT.dominates(Guard, Header))
+        if (Guard->getFunction() == Block->getParent() &&
+            SE.DT.dominates(Guard, Block))
           Terms.emplace_back(Guard->getArgOperand(0), true);
 
   // Third, collect conditions from dominating branches. Starting at the loop
   // predecessor, climb up the predecessor chain, as long as there are
   // predecessors that can be found that have unique successors leading to the
   // original header.
   // TODO: share this logic with isLoopEntryGuardedByCond.
-  for (std::pair<const BasicBlock *, const BasicBlock *> Pair(
-           L->getLoopPredecessor(), Header);
-       Pair.first;
+  std::pair<const BasicBlock *, const BasicBlock *> Pair(Pred, Block);
+  for (; Pair.first;
        Pair = SE.getPredecessorWithUniqueSuccessorForBB(Pair.first)) {
-
+    VisitedBlocks.insert(Pair.second);
     const BranchInst *LoopEntryPredicate =
         dyn_cast<BranchInst>(Pair.first->getTerminator());
     if (!LoopEntryPredicate || LoopEntryPredicate->isUnconditional())
       continue;
 
     Terms.emplace_back(LoopEntryPredicate->getCondition(),
                        LoopEntryPredicate->getSuccessor(0) == Pair.second);
+
+    // If we are recursively collecting guards stop after 2
+    // predecessors to limit compile-time impact for now.
+    if (Depth > 0 && Terms.size() == 2)
+      break;
+  }
+  // Finally, if we stopped climbing the predecessor chain because
+  // there wasn't a unique one to continue, try to collect conditions
+  // for PHINodes by recursively following all of their incoming
+  // blocks and try to merge the found conditions to build a new one
+  // for the Phi.
+  if (Pair.second->hasNPredecessorsOrMore(2) &&
+      Depth < MaxLoopGuardCollectionDepth) {
+    SmallDenseMap<const BasicBlock *, LoopGuards> IncomingGuards;
+    for (auto &Phi : Pair.second->phis()) {
+      collectFromPHI(SE, Guards, Phi, VisitedBlocks, IncomingGuards, Depth);
+    }
   }
 
   // Now apply the information from the collected conditions to
@@ -15651,7 +15746,6 @@ ScalarEvolution::LoopGuards::collect(const Loop *L, ScalarEvolution &SE) {
       Guards.RewriteMap.insert({Expr, Guards.rewrite(RewriteTo)});
     }
   }
-  return Guards;
 }
 
 const SCEV *ScalarEvolution::LoopGuards::rewrite(const SCEV *Expr) const {