[ValueTracking] Add a new predicate: isKnownNonEqual()

isKnownNonEqual(A, B) returns true if it can be determined that A != B.

At the moment it only knows two facts, that a non-wrapping add of nonzero to a value cannot be that value:

A + B != A [where B != 0, addition is nsw or nuw]

and that contradictory known bits imply two values are not equal.

This patch also hooks this up to InstSimplify; InstSimplify had a peephole for the first fact but not the second so this teaches InstSimplify a new trick too (alas no measured performance impact!)

llvm-svn: 251012

Author: James Molloy

llvm/include/llvm/Analysis/ValueTracking.h

@@ -90,6 +90,13 @@ namespace llvm {
                           const Instruction *CxtI = nullptr,
                           const DominatorTree *DT = nullptr);
 
+  /// isKnownNonEqual - Return true if the given values are known to be
+  /// non-equal when defined. Supports scalar integer types only.
+  bool isKnownNonEqual(Value *V1, Value *V2, const DataLayout &DL,
+                      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.

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -2642,6 +2642,14 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
     }
   }
 
+  // icmp eq|ne X, Y -> false|true if X != Y
+  if ((Pred == ICmpInst::ICMP_EQ || Pred == ICmpInst::ICMP_NE) &&
+      isKnownNonEqual(LHS, RHS, Q.DL, Q.AC, Q.CxtI, Q.DT)) {
+    LLVMContext &Ctx = LHS->getType()->getContext();
+    return Pred == ICmpInst::ICMP_NE ?
+      ConstantInt::getTrue(Ctx) : ConstantInt::getFalse(Ctx);
+  }
+  
   // Special logic for binary operators.
   BinaryOperator *LBO = dyn_cast<BinaryOperator>(LHS);
   BinaryOperator *RBO = dyn_cast<BinaryOperator>(RHS);

llvm/lib/Analysis/ValueTracking.cpp

@@ -193,6 +193,17 @@ bool llvm::isKnownNonNegative(Value *V, const DataLayout &DL, unsigned Depth,
   return NonNegative;
 }
 
+static bool isKnownNonEqual(Value *V1, Value *V2, const DataLayout &DL,
+                           const Query &Q);
+
+bool llvm::isKnownNonEqual(Value *V1, Value *V2, const DataLayout &DL,
+                          AssumptionCache *AC, const Instruction *CxtI,
+                          const DominatorTree *DT) {
+  return ::isKnownNonEqual(V1, V2, DL, Query(AC,
+                                             safeCxtI(V1, safeCxtI(V2, CxtI)),
+                                             DT));
+}
+
 static bool MaskedValueIsZero(Value *V, const APInt &Mask, const DataLayout &DL,
                               unsigned Depth, const Query &Q);
 
@@ -1950,6 +1961,51 @@ bool isKnownNonZero(Value *V, const DataLayout &DL, unsigned Depth,
   return KnownOne != 0;
 }
 
+/// Return true if V2 == V1 + X, where X is known non-zero.
+static bool isAddOfNonZero(Value *V1, Value *V2, const DataLayout &DL,
+                           const Query &Q) {
+  BinaryOperator *BO = dyn_cast<BinaryOperator>(V1);
+  if (!BO || BO->getOpcode() != Instruction::Add)
+    return false;
+  Value *Op = nullptr;
+  if (V2 == BO->getOperand(0))
+    Op = BO->getOperand(1);
+  else if (V2 == BO->getOperand(1))
+    Op = BO->getOperand(0);
+  else
+    return false;
+  return isKnownNonZero(Op, DL, 0, Q);
+}
+
+/// Return true if it is known that V1 != V2.
+static bool isKnownNonEqual(Value *V1, Value *V2, const DataLayout &DL,
+                            const Query &Q) {
+  if (V1->getType()->isVectorTy() || V1 == V2)
+    return false;
+  if (V1->getType() != V2->getType())
+    // We can't look through casts yet.
+    return false;
+  if (isAddOfNonZero(V1, V2, DL, Q) || isAddOfNonZero(V2, V1, DL, Q))
+    return true;
+
+  if (IntegerType *Ty = dyn_cast<IntegerType>(V1->getType())) {
+    // Are any known bits in V1 contradictory to known bits in V2? If V1
+    // has a known zero where V2 has a known one, they must not be equal.
+    auto BitWidth = Ty->getBitWidth();
+    APInt KnownZero1(BitWidth, 0);
+    APInt KnownOne1(BitWidth, 0);
+    computeKnownBits(V1, KnownZero1, KnownOne1, DL, 0, Q);
+    APInt KnownZero2(BitWidth, 0);
+    APInt KnownOne2(BitWidth, 0);
+    computeKnownBits(V2, KnownZero2, KnownOne2, DL, 0, Q);
+
+    auto OppositeBits = (KnownZero1 & KnownOne2) | (KnownZero2 & KnownOne1);
+    if (OppositeBits.getBoolValue())
+      return true;
+  }
+  return false;
+}
+
 /// 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.

llvm/test/Analysis/ValueTracking/known-non-equal.ll

@@ -0,0 +1,21 @@
+; RUN: opt -instsimplify < %s -S | FileCheck %s
+
+; CHECK: define i1 @test
+define i1 @test(i8* %pq, i8 %B) {
+  %q = load i8, i8* %pq, !range !0 ; %q is known nonzero; no known bits
+  %A = add nsw i8 %B, %q
+  %cmp = icmp eq i8 %A, %B
+  ; CHECK: ret i1 false
+  ret i1 %cmp
+}
+
+; CHECK: define i1 @test2
+define i1 @test2(i8 %a, i8 %b) {
+  %A = or i8 %a, 2    ; %A[1] = 1
+  %B = and i8 %b, -3  ; %B[1] = 0
+  %cmp = icmp eq i8 %A, %B ; %A[1] and %B[1] are contradictory.
+  ; CHECK: ret i1 false
+  ret i1 %cmp
+}
+
+!0 = !{ i8 1, i8 5 }