[ValueTracking] computeOverflowForSignedAdd and isKnownNonNegative

Summary:
Refactor, NFC

Extracts computeOverflowForSignedAdd and isKnownNonNegative from NaryReassociate to ValueTracking in case
others need it.

Reviewers: reames

Subscribers: majnemer, llvm-commits

Differential Revision: http://reviews.llvm.org/D11313

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245591 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/ValueTracking.h

@@ -20,17 +20,18 @@
 #include "llvm/Support/DataTypes.h"
 
 namespace llvm {
-  class Value;
-  class Instruction;
   class APInt;
-  class DataLayout;
-  class StringRef;
-  class MDNode;
+  class AddOperator;
   class AssumptionCache;
+  class DataLayout;
   class DominatorTree;
-  class TargetLibraryInfo;
-  class LoopInfo;
+  class Instruction;
   class Loop;
+  class LoopInfo;
+  class MDNode;
+  class StringRef;
+  class TargetLibraryInfo;
+  class Value;
 
   /// Determine which bits of V are known to be either zero or one and return
   /// them in the KnownZero/KnownOne bit sets.
@@ -83,6 +84,12 @@ namespace llvm {
                       const Instruction *CxtI = nullptr,
                       const DominatorTree *DT = nullptr);
 
+  /// Returns true if the give value is known to be non-negative.
+  bool isKnownNonNegative(Value *V, const DataLayout &DL, unsigned Depth = 0,
+                          AssumptionCache *AC = nullptr,
+                          const Instruction *CxtI = nullptr,
+                          const DominatorTree *DT = nullptr);
+
   /// MaskedValueIsZero - Return true if 'V & Mask' is known to be zero.  We use
   /// this predicate to simplify operations downstream.  Mask is known to be
   /// zero for bits that V cannot have.
@@ -309,6 +316,17 @@ namespace llvm {
                                                AssumptionCache *AC,
                                                const Instruction *CxtI,
                                                const DominatorTree *DT);
+  OverflowResult computeOverflowForSignedAdd(Value *LHS, Value *RHS,
+                                             const DataLayout &DL,
+                                             AssumptionCache *AC = nullptr,
+                                             const Instruction *CxtI = nullptr,
+                                             const DominatorTree *DT = nullptr);
+  /// This version also leverages the sign bit of Add if known.
+  OverflowResult computeOverflowForSignedAdd(AddOperator *Add,
+                                             const DataLayout &DL,
+                                             AssumptionCache *AC = nullptr,
+                                             const Instruction *CxtI = nullptr,
+                                             const DominatorTree *DT = nullptr);
 
   /// Return true if this function can prove that the instruction I will
   /// always transfer execution to one of its successors (including the next

lib/Analysis/ValueTracking.cpp

@@ -185,6 +185,14 @@ bool llvm::isKnownNonZero(Value *V, const DataLayout &DL, unsigned Depth,
   return ::isKnownNonZero(V, DL, Depth, Query(AC, safeCxtI(V, CxtI), DT));
 }
 
+bool llvm::isKnownNonNegative(Value *V, const DataLayout &DL, unsigned Depth,
+                              AssumptionCache *AC, const Instruction *CxtI,
+                              const DominatorTree *DT) {
+  bool NonNegative, Negative;
+  ComputeSignBit(V, NonNegative, Negative, DL, Depth, AC, CxtI, DT);
+  return NonNegative;
+}
+
 static bool MaskedValueIsZero(Value *V, const APInt &Mask, const DataLayout &DL,
                               unsigned Depth, const Query &Q);
 
@@ -3370,6 +3378,67 @@ OverflowResult llvm::computeOverflowForUnsignedAdd(Value *LHS, Value *RHS,
   return OverflowResult::MayOverflow;
 }
 
+static OverflowResult computeOverflowForSignedAdd(
+    Value *LHS, Value *RHS, AddOperator *Add, const DataLayout &DL,
+    AssumptionCache *AC, const Instruction *CxtI, const DominatorTree *DT) {
+  if (Add && Add->hasNoSignedWrap()) {
+    return OverflowResult::NeverOverflows;
+  }
+
+  bool LHSKnownNonNegative, LHSKnownNegative;
+  bool RHSKnownNonNegative, RHSKnownNegative;
+  ComputeSignBit(LHS, LHSKnownNonNegative, LHSKnownNegative, DL, /*Depth=*/0,
+                 AC, CxtI, DT);
+  ComputeSignBit(RHS, RHSKnownNonNegative, RHSKnownNegative, DL, /*Depth=*/0,
+                 AC, CxtI, DT);
+
+  if ((LHSKnownNonNegative && RHSKnownNegative) ||
+      (LHSKnownNegative && RHSKnownNonNegative)) {
+    // The sign bits are opposite: this CANNOT overflow.
+    return OverflowResult::NeverOverflows;
+  }
+
+  // The remaining code needs Add to be available. Early returns if not so.
+  if (!Add)
+    return OverflowResult::MayOverflow;
+
+  // If the sign of Add is the same as at least one of the operands, this add
+  // CANNOT overflow. This is particularly useful when the sum is
+  // @llvm.assume'ed non-negative rather than proved so from analyzing its
+  // operands.
+  bool LHSOrRHSKnownNonNegative =
+      (LHSKnownNonNegative || RHSKnownNonNegative);
+  bool LHSOrRHSKnownNegative = (LHSKnownNegative || RHSKnownNegative);
+  if (LHSOrRHSKnownNonNegative || LHSOrRHSKnownNegative) {
+    bool AddKnownNonNegative, AddKnownNegative;
+    ComputeSignBit(Add, AddKnownNonNegative, AddKnownNegative, DL,
+                   /*Depth=*/0, AC, CxtI, DT);
+    if ((AddKnownNonNegative && LHSOrRHSKnownNonNegative) ||
+        (AddKnownNegative && LHSOrRHSKnownNegative)) {
+      return OverflowResult::NeverOverflows;
+    }
+  }
+
+  return OverflowResult::MayOverflow;
+}
+
+OverflowResult llvm::computeOverflowForSignedAdd(AddOperator *Add,
+                                                 const DataLayout &DL,
+                                                 AssumptionCache *AC,
+                                                 const Instruction *CxtI,
+                                                 const DominatorTree *DT) {
+  return ::computeOverflowForSignedAdd(Add->getOperand(0), Add->getOperand(1),
+                                       Add, DL, AC, CxtI, DT);
+}
+
+OverflowResult llvm::computeOverflowForSignedAdd(Value *LHS, Value *RHS,
+                                                 const DataLayout &DL,
+                                                 AssumptionCache *AC,
+                                                 const Instruction *CxtI,
+                                                 const DominatorTree *DT) {
+  return ::computeOverflowForSignedAdd(LHS, RHS, nullptr, DL, AC, CxtI, DT);
+}
+
 bool llvm::isGuaranteedToTransferExecutionToSuccessor(const Instruction *I) {
   // FIXME: This conservative implementation can be relaxed. E.g. most
   // atomic operations are guaranteed to terminate on most platforms

lib/Transforms/Scalar/NaryReassociate.cpp

@@ -161,11 +161,6 @@ private:
   // GEP's pointer size, i.e., whether Index needs to be sign-extended in order
   // to be an index of GEP.
   bool requiresSignExtension(Value *Index, GetElementPtrInst *GEP);
-  // Returns whether V is known to be non-negative at context \c Ctxt.
-  bool isKnownNonNegative(Value *V, Instruction *Ctxt);
-  // Returns whether AO may sign overflow at context \c Ctxt. It computes a
-  // conservative result -- it answers true when not sure.
-  bool maySignOverflow(AddOperator *AO, Instruction *Ctxt);
 
   AssumptionCache *AC;
   const DataLayout *DL;
@@ -352,27 +347,6 @@ bool NaryReassociate::requiresSignExtension(Value *Index,
   return cast<IntegerType>(Index->getType())->getBitWidth() < PointerSizeInBits;
 }
 
-bool NaryReassociate::isKnownNonNegative(Value *V, Instruction *Ctxt) {
-  bool NonNegative, Negative;
-  // TODO: ComputeSignBits is expensive. Consider caching the results.
-  ComputeSignBit(V, NonNegative, Negative, *DL, 0, AC, Ctxt, DT);
-  return NonNegative;
-}
-
-bool NaryReassociate::maySignOverflow(AddOperator *AO, Instruction *Ctxt) {
-  if (AO->hasNoSignedWrap())
-    return false;
-
-  Value *LHS = AO->getOperand(0), *RHS = AO->getOperand(1);
-  // If LHS or RHS has the same sign as the sum, AO doesn't sign overflow.
-  // TODO: handle the negative case as well.
-  if (isKnownNonNegative(AO, Ctxt) &&
-      (isKnownNonNegative(LHS, Ctxt) || isKnownNonNegative(RHS, Ctxt)))
-    return false;
-
-  return true;
-}
-
 GetElementPtrInst *
 NaryReassociate::tryReassociateGEPAtIndex(GetElementPtrInst *GEP, unsigned I,
                                           Type *IndexedType) {
@@ -381,7 +355,7 @@ NaryReassociate::tryReassociateGEPAtIndex(GetElementPtrInst *GEP, unsigned I,
     IndexToSplit = SExt->getOperand(0);
   } else if (ZExtInst *ZExt = dyn_cast<ZExtInst>(IndexToSplit)) {
     // zext can be treated as sext if the source is non-negative.
-    if (isKnownNonNegative(ZExt->getOperand(0), GEP))
+    if (isKnownNonNegative(ZExt->getOperand(0), *DL, 0, AC, GEP, DT))
       IndexToSplit = ZExt->getOperand(0);
   }
 
@@ -389,8 +363,11 @@ NaryReassociate::tryReassociateGEPAtIndex(GetElementPtrInst *GEP, unsigned I,
     // If the I-th index needs sext and the underlying add is not equipped with
     // nsw, we cannot split the add because
     //   sext(LHS + RHS) != sext(LHS) + sext(RHS).
-    if (requiresSignExtension(IndexToSplit, GEP) && maySignOverflow(AO, GEP))
+    if (requiresSignExtension(IndexToSplit, GEP) &&
+        computeOverflowForSignedAdd(AO, *DL, AC, GEP, DT) !=
+            OverflowResult::NeverOverflows)
       return nullptr;
+
     Value *LHS = AO->getOperand(0), *RHS = AO->getOperand(1);
     // IndexToSplit = LHS + RHS.
     if (auto *NewGEP = tryReassociateGEPAtIndex(GEP, I, LHS, RHS, IndexedType))
@@ -415,7 +392,7 @@ GetElementPtrInst *NaryReassociate::tryReassociateGEPAtIndex(
     IndexExprs.push_back(SE->getSCEV(*Index));
   // Replace the I-th index with LHS.
   IndexExprs[I] = SE->getSCEV(LHS);
-  if (isKnownNonNegative(LHS, GEP) &&
+  if (isKnownNonNegative(LHS, *DL, 0, AC, GEP, DT) &&
       DL->getTypeSizeInBits(LHS->getType()) <
           DL->getTypeSizeInBits(GEP->getOperand(I)->getType())) {
     // Zero-extend LHS if it is non-negative. InstCombine canonicalizes sext to