[SCEV] Ensure ScalarEvolution::createAddRecFromPHIWithCastsImpl properly handles out of range truncations of the start and accum values

Summary:
 When constructing the predicate P1 in ScalarEvolution::createAddRecFromPHIWithCastsImpl() it is possible
for the PHISCEV from which the predicate is constructed to be a SCEVConstant instead of a SCEVAddRec. If
this happens, then the cast<SCEVAddRec>(PHISCEV) in the code will assert.

 Such a PHISCEV is possible if either the start value or the accumulator value is a constant value
that not equal to its truncated value, and if the truncated value is zero.

 This patch adds tests that demonstrate the cast<> assertion, and fixes this problem by checking
whether the PHISCEV is a constant before constructing the P1 predicate; if it is, then P1 is
equivalent to one of P2 or P3. Additionally, if we know that the start value or accumulator
value are constants then we check whether the P2 and/or P3 predicates are known false at compile
time; if either is, then we bail out of constructing the AddRec.

Reviewers: sanjoy, mkazantsev, silviu.baranga

Reviewed By: mkazantsev

Subscribers: mkazantsev, llvm-commits

Differential Revision: https://reviews.llvm.org/D37265

llvm-svn: 312568

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -4443,7 +4443,7 @@ ScalarEvolution::createAddRecFromPHIWithCastsImpl(const SCEVUnknown *SymbolicPHI
   // varying inside the loop.
   if (!isLoopInvariant(Accum, L))
     return None;
-  
+
   // *** Part2: Create the predicates 
 
   // Analysis was successful: we have a phi-with-cast pattern for which we
@@ -4493,27 +4493,71 @@ ScalarEvolution::createAddRecFromPHIWithCastsImpl(const SCEVUnknown *SymbolicPHI
   //
   // By induction, the same applies to all iterations 1<=i<n:
   //
-  
+
   // Create a truncated addrec for which we will add a no overflow check (P1).
   const SCEV *StartVal = getSCEV(StartValueV);
-  const SCEV *PHISCEV = 
+  const SCEV *PHISCEV =
       getAddRecExpr(getTruncateExpr(StartVal, TruncTy),
-                    getTruncateExpr(Accum, TruncTy), L, SCEV::FlagAnyWrap); 
-  const auto *AR = cast<SCEVAddRecExpr>(PHISCEV);
+                    getTruncateExpr(Accum, TruncTy), L, SCEV::FlagAnyWrap);
 
-  SCEVWrapPredicate::IncrementWrapFlags AddedFlags =
-      Signed ? SCEVWrapPredicate::IncrementNSSW
-             : SCEVWrapPredicate::IncrementNUSW;
-  const SCEVPredicate *AddRecPred = getWrapPredicate(AR, AddedFlags);
-  Predicates.push_back(AddRecPred);
+  // PHISCEV can be either a SCEVConstant or a SCEVAddRecExpr.
+  // ex: If truncated Accum is 0 and StartVal is a constant, then PHISCEV
+  // will be constant.
+  //
+  //  If PHISCEV is a constant, then P1 degenerates into P2 or P3, so we don't
+  // add P1.
+  if (const auto *AR = dyn_cast<SCEVAddRecExpr>(PHISCEV)) {
+    SCEVWrapPredicate::IncrementWrapFlags AddedFlags =
+        Signed ? SCEVWrapPredicate::IncrementNSSW
+               : SCEVWrapPredicate::IncrementNUSW;
+    const SCEVPredicate *AddRecPred = getWrapPredicate(AR, AddedFlags);
+    Predicates.push_back(AddRecPred);
+  } else
+    assert(isa<SCEVConstant>(PHISCEV) && "Expected constant SCEV");
 
   // Create the Equal Predicates P2,P3:
-  auto AppendPredicate = [&](const SCEV *Expr) -> void {
+
+  // It is possible that the predicates P2 and/or P3 are computable at
+  // compile time due to StartVal and/or Accum being constants.
+  // If either one is, then we can check that now and escape if either P2
+  // or P3 is false.
+
+  // Construct the extended SCEV: (Ext ix (Trunc iy (Expr) to ix) to iy)
+  // for each of StartVal and Accum
+  auto GetExtendedExpr = [&](const SCEV *Expr) -> const SCEV * {
     assert(isLoopInvariant(Expr, L) && "Expr is expected to be invariant");
     const SCEV *TruncatedExpr = getTruncateExpr(Expr, TruncTy);
     const SCEV *ExtendedExpr =
         Signed ? getSignExtendExpr(TruncatedExpr, Expr->getType())
                : getZeroExtendExpr(TruncatedExpr, Expr->getType());
+    return ExtendedExpr;
+  };
+
+  // Given:
+  //  ExtendedExpr = (Ext ix (Trunc iy (Expr) to ix) to iy
+  //               = GetExtendedExpr(Expr)
+  // Determine whether the predicate P: Expr == ExtendedExpr
+  // is known to be false at compile time
+  auto PredIsKnownFalse = [&](const SCEV *Expr,
+                              const SCEV *ExtendedExpr) -> bool {
+    return Expr != ExtendedExpr &&
+           isKnownPredicate(ICmpInst::ICMP_NE, Expr, ExtendedExpr);
+  };
+
+  const SCEV *StartExtended = GetExtendedExpr(StartVal);
+  if (PredIsKnownFalse(StartVal, StartExtended)) {
+    DEBUG(dbgs() << "P2 is compile-time false\n";);
+    return None;
+  }
+
+  const SCEV *AccumExtended = GetExtendedExpr(Accum);
+  if (PredIsKnownFalse(Accum, AccumExtended)) {
+    DEBUG(dbgs() << "P3 is compile-time false\n";);
+    return None;
+  }
+
+  auto AppendPredicate = [&](const SCEV *Expr,
+                             const SCEV *ExtendedExpr) -> void {
     if (Expr != ExtendedExpr &&
         !isKnownPredicate(ICmpInst::ICMP_EQ, Expr, ExtendedExpr)) {
       const SCEVPredicate *Pred = getEqualPredicate(Expr, ExtendedExpr);
@@ -4521,10 +4565,10 @@ ScalarEvolution::createAddRecFromPHIWithCastsImpl(const SCEVUnknown *SymbolicPHI
       Predicates.push_back(Pred);
     }
   };
-  
-  AppendPredicate(StartVal);
-  AppendPredicate(Accum);
-  
+
+  AppendPredicate(StartVal, StartExtended);
+  AppendPredicate(Accum, AccumExtended);
+
   // *** Part3: Predicates are ready. Now go ahead and create the new addrec in
   // which the casts had been folded away. The caller can rewrite SymbolicPHI
   // into NewAR if it will also add the runtime overflow checks specified in

llvm/unittests/Analysis/ScalarEvolutionTest.cpp

@@ -1095,5 +1095,60 @@ TEST_F(ScalarEvolutionsTest, SCEVExitLimitForgetValue) {
   EXPECT_EQ(cast<SCEVConstant>(NewEC)->getAPInt().getLimitedValue(), 1999u);
 }
 
+TEST_F(ScalarEvolutionsTest, SCEVAddRecFromPHIwithLargeConstants) {
+  // Reference: https://reviews.llvm.org/D37265
+  // Make sure that SCEV does not blow up when constructing an AddRec
+  // with predicates for a phi with the update pattern:
+  //  (SExt/ZExt ix (Trunc iy (%SymbolicPHI) to ix) to iy) + InvariantAccum
+  // when either the initial value of the Phi or the InvariantAccum are
+  // constants that are too large to fit in an ix but are zero when truncated to
+  // ix.
+  FunctionType *FTy =
+      FunctionType::get(Type::getVoidTy(Context), std::vector<Type *>(), false);
+  Function *F = cast<Function>(M.getOrInsertFunction("addrecphitest", FTy));
+
+  /*
+    Create IR:
+    entry:
+     br label %loop
+    loop:
+     %0 = phi i64 [-9223372036854775808, %entry], [%3, %loop]
+     %1 = shl i64 %0, 32
+     %2 = ashr exact i64 %1, 32
+     %3 = add i64 %2, -9223372036854775808
+     br i1 undef, label %exit, label %loop
+    exit:
+     ret void
+   */
+  BasicBlock *EntryBB = BasicBlock::Create(Context, "entry", F);
+  BasicBlock *LoopBB = BasicBlock::Create(Context, "loop", F);
+  BasicBlock *ExitBB = BasicBlock::Create(Context, "exit", F);
+
+  // entry:
+  BranchInst::Create(LoopBB, EntryBB);
+  // loop:
+  auto *MinInt64 =
+      ConstantInt::get(Context, APInt(64, 0x8000000000000000U, true));
+  auto *Int64_32 = ConstantInt::get(Context, APInt(64, 32));
+  auto *Br = BranchInst::Create(
+      LoopBB, ExitBB, UndefValue::get(Type::getInt1Ty(Context)), LoopBB);
+  auto *Phi = PHINode::Create(Type::getInt64Ty(Context), 2, "", Br);
+  auto *Shl = BinaryOperator::CreateShl(Phi, Int64_32, "", Br);
+  auto *AShr = BinaryOperator::CreateExactAShr(Shl, Int64_32, "", Br);
+  auto *Add = BinaryOperator::CreateAdd(AShr, MinInt64, "", Br);
+  Phi->addIncoming(MinInt64, EntryBB);
+  Phi->addIncoming(Add, LoopBB);
+  // exit:
+  ReturnInst::Create(Context, nullptr, ExitBB);
+
+  // Make sure that SCEV doesn't blow up
+  ScalarEvolution SE = buildSE(*F);
+  SCEVUnionPredicate Preds;
+  const SCEV *Expr = SE.getSCEV(Phi);
+  EXPECT_NE(nullptr, Expr);
+  EXPECT_TRUE(isa<SCEVUnknown>(Expr));
+  auto Result = SE.createAddRecFromPHIWithCasts(cast<SCEVUnknown>(Expr));
+}
+
 }  // end anonymous namespace
 }  // end namespace llvm