[SCEV][LAA] Add no wrap SCEV predicates and use use them to improve strided pointer detection

Summary:
This change adds no wrap SCEV predicates with:
  - support for runtime checking
  - support for expression rewriting:
      (sext ({x,+,y}) -> {sext(x),+,sext(y)}
      (zext ({x,+,y}) -> {zext(x),+,sext(y)}

Note that we are sign extending the increment of the SCEV, even for
the zext case. This is needed to cover the fairly common case where y would
be a (small) negative integer. In order to do this, this change adds two new
flags: nusw and nssw that are applicable to AddRecExprs and permit the
transformations above.

We also change isStridedPtr in LAA to be able to make use of
these predicates. With this feature we should now always be able to
work around overflow issues in the dependence analysis.

Reviewers: mzolotukhin, sanjoy, anemet

Subscribers: mzolotukhin, sanjoy, llvm-commits, rengolin, jmolloy, hfinkel

Differential Revision: http://reviews.llvm.org/D15412

llvm-svn: 260085

1 files changed, 68 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolutionExpander.cpp b/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
index 86cfe2d..49cdcf7 100644
--- a/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Analysis/ScalarEvolutionExpander.cpp
@@ -1971,6 +1971,10 @@ Value *SCEVExpander::expandCodeForPredicate(const SCEVPredicate *Pred,
     return expandUnionPredicate(cast<SCEVUnionPredicate>(Pred), IP);
   case SCEVPredicate::P_Equal:
     return expandEqualPredicate(cast<SCEVEqualPredicate>(Pred), IP);
+  case SCEVPredicate::P_Wrap: {
+    auto *AddRecPred = cast<SCEVWrapPredicate>(Pred);
+    return expandWrapPredicate(AddRecPred, IP);
+  }
   }
   llvm_unreachable("Unknown SCEV predicate type");
 }
@@ -1985,6 +1989,70 @@ Value *SCEVExpander::expandEqualPredicate(const SCEVEqualPredicate *Pred,
   return I;
 }
 
+Value *SCEVExpander::generateOverflowCheck(const SCEVAddRecExpr *AR,
+                                           Instruction *Loc, bool Signed) {
+  assert(AR->isAffine() && "Cannot generate RT check for "
+                           "non-affine expression");
+
+  const SCEV *ExitCount = SE.getBackedgeTakenCount(AR->getLoop());
+  const SCEV *Step = AR->getStepRecurrence(SE);
+  const SCEV *Start = AR->getStart();
+
+  unsigned DstBits = SE.getTypeSizeInBits(AR->getType());
+  unsigned SrcBits = SE.getTypeSizeInBits(ExitCount->getType());
+  unsigned MaxBits = 2 * std::max(DstBits, SrcBits);
+
+  auto *TripCount = SE.getTruncateOrZeroExtend(ExitCount, AR->getType());
+  IntegerType *MaxTy = IntegerType::get(Loc->getContext(), MaxBits);
+
+  assert(ExitCount != SE.getCouldNotCompute() && "Invalid loop count");
+
+  const auto *ExtendedTripCount = SE.getZeroExtendExpr(ExitCount, MaxTy);
+  const auto *ExtendedStep = SE.getSignExtendExpr(Step, MaxTy);
+  const auto *ExtendedStart = Signed ? SE.getSignExtendExpr(Start, MaxTy)
+                                     : SE.getZeroExtendExpr(Start, MaxTy);
+
+  const SCEV *End = SE.getAddExpr(Start, SE.getMulExpr(TripCount, Step));
+  const SCEV *RHS = Signed ? SE.getSignExtendExpr(End, MaxTy)
+                           : SE.getZeroExtendExpr(End, MaxTy);
+
+  const SCEV *LHS = SE.getAddExpr(
+      ExtendedStart, SE.getMulExpr(ExtendedTripCount, ExtendedStep));
+
+  // Do all SCEV expansions now.
+  Value *LHSVal = expandCodeFor(LHS, MaxTy, Loc);
+  Value *RHSVal = expandCodeFor(RHS, MaxTy, Loc);
+
+  Builder.SetInsertPoint(Loc);
+
+  return Builder.CreateICmp(ICmpInst::ICMP_NE, RHSVal, LHSVal);
+}
+
+Value *SCEVExpander::expandWrapPredicate(const SCEVWrapPredicate *Pred,
+                                         Instruction *IP) {
+  const auto *A = cast<SCEVAddRecExpr>(Pred->getExpr());
+  Value *NSSWCheck = nullptr, *NUSWCheck = nullptr;
+
+  // Add a check for NUSW
+  if (Pred->getFlags() & SCEVWrapPredicate::IncrementNUSW)
+    NUSWCheck = generateOverflowCheck(A, IP, false);
+
+  // Add a check for NSSW
+  if (Pred->getFlags() & SCEVWrapPredicate::IncrementNSSW)
+    NSSWCheck = generateOverflowCheck(A, IP, true);
+
+  if (NUSWCheck && NSSWCheck)
+    return Builder.CreateOr(NUSWCheck, NSSWCheck);
+
+  if (NUSWCheck)
+    return NUSWCheck;
+
+  if (NSSWCheck)
+    return NSSWCheck;
+
+  return ConstantInt::getFalse(IP->getContext());
+}
+
 Value *SCEVExpander::expandUnionPredicate(const SCEVUnionPredicate *Union,
                                           Instruction *IP) {
   auto *BoolType = IntegerType::get(IP->getContext(), 1);