Allow LLE/LD and the loop versioning infrastructure to use SCEV predicates

Summary:
LAA currently generates a set of SCEV predicates that must be checked by users.
In the case of Loop Distribute/Loop Load Elimination, no such predicates could have
been emitted, since we don't allow stride versioning. However, in the future there
could be SCEV predicates that will need to be checked.

This change adds support for SCEV predicate versioning in the Loop Distribute, Loop
Load Eliminate and the loop versioning infrastructure.

Reviewers: anemet

Subscribers: mssimpso, sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D14240

llvm-svn: 252467

1 files changed, 22 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopDistribute.cpp b/llvm/lib/Transforms/Scalar/LoopDistribute.cpp
index 1584f0fa3eb..67ebd2532b1 100644
--- a/llvm/lib/Transforms/Scalar/LoopDistribute.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopDistribute.cpp
@@ -55,6 +55,11 @@ static cl::opt<bool> DistributeNonIfConvertible(
              "if-convertible by the loop vectorizer"),
     cl::init(false));
 
+static cl::opt<unsigned> DistributeSCEVCheckThreshold(
+    "loop-distribute-scev-check-threshold", cl::init(8), cl::Hidden,
+    cl::desc("The maximum number of SCEV checks allowed for Loop "
+             "Distribution"));
+
 STATISTIC(NumLoopsDistributed, "Number of loops distributed");
 
 namespace {
@@ -577,6 +582,7 @@ public:
     LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
     LAA = &getAnalysis<LoopAccessAnalysis>();
     DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+    SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
 
     // Build up a worklist of inner-loops to vectorize. This is necessary as the
     // act of distributing a loop creates new loops and can invalidate iterators
@@ -599,6 +605,7 @@ public:
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<ScalarEvolutionWrapperPass>();
     AU.addRequired<LoopInfoWrapperPass>();
     AU.addPreserved<LoopInfoWrapperPass>();
     AU.addRequired<LoopAccessAnalysis>();
@@ -753,6 +760,13 @@ private:
         return false;
     }
 
+    // Don't distribute the loop if we need too many SCEV run-time checks.
+    const SCEVUnionPredicate &Pred = LAI.Preds;
+    if (Pred.getComplexity() > DistributeSCEVCheckThreshold) {
+      DEBUG(dbgs() << "Too many SCEV run-time checks needed.\n");
+      return false;
+    }
+
     DEBUG(dbgs() << "\nDistributing loop: " << *L << "\n");
     // We're done forming the partitions set up the reverse mapping from
     // instructions to partitions.
@@ -764,17 +778,19 @@ private:
     if (!PH->getSinglePredecessor() || &*PH->begin() != PH->getTerminator())
       SplitBlock(PH, PH->getTerminator(), DT, LI);
 
-    // If we need run-time checks to disambiguate pointers are run-time, version
-    // the loop now.
+    // If we need run-time checks, version the loop now.
     auto PtrToPartition = Partitions.computePartitionSetForPointers(LAI);
     const auto *RtPtrChecking = LAI.getRuntimePointerChecking();
     const auto &AllChecks = RtPtrChecking->getChecks();
     auto Checks = includeOnlyCrossPartitionChecks(AllChecks, PtrToPartition,
                                                   RtPtrChecking);
-    if (!Checks.empty()) {
+
+    if (!Pred.isAlwaysTrue() || !Checks.empty()) {
       DEBUG(dbgs() << "\nPointers:\n");
       DEBUG(LAI.getRuntimePointerChecking()->printChecks(dbgs(), Checks));
-      LoopVersioning LVer(std::move(Checks), LAI, L, LI, DT);
+      LoopVersioning LVer(LAI, L, LI, DT, SE, false);
+      LVer.setAliasChecks(std::move(Checks));
+      LVer.setSCEVChecks(LAI.Preds);
       LVer.versionLoop(DefsUsedOutside);
     }
 
@@ -801,6 +817,7 @@ private:
   LoopInfo *LI;
   LoopAccessAnalysis *LAA;
   DominatorTree *DT;
+  ScalarEvolution *SE;
 };
 } // anonymous namespace
 
@@ -811,6 +828,7 @@ INITIALIZE_PASS_BEGIN(LoopDistribute, LDIST_NAME, ldist_name, false, false)
 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(LoopAccessAnalysis)
 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
 INITIALIZE_PASS_END(LoopDistribute, LDIST_NAME, ldist_name, false, false)
 
 namespace llvm {