[ValueTracking] Handle two PHIs in isKnownNonEqual()

loop:
  %cmp.0 = phi i32 [ 3, %entry ], [ %inc, %loop ]
  %pos.0 = phi i32 [ 1, %entry ], [ %cmp.0, %loop ]
  ...
  %inc = add i32 %cmp.0, 1
  br label %loop

On above example, %pos.0 uses previous iteration's %cmp.0 with backedge
according to PHI's instruction's defintion. If the %inc is not same among
iterations, we can say the two PHIs are not same.

Differential Revision: https://reviews.llvm.org/D98422

Author: jaykang10

llvm/lib/Analysis/ValueTracking.cpp

@@ -2548,6 +2548,36 @@ static bool isNonEqualMul(const Value *V1, const Value *V2, unsigned Depth,
   return false;
 }
 
+static bool isNonEqualPHIs(const PHINode *PN1, const PHINode *PN2,
+                           unsigned Depth, const Query &Q) {
+  // Check two PHIs are in same block.
+  if (PN1->getParent() != PN2->getParent())
+    return false;
+
+  SmallPtrSet<const BasicBlock *, 8> VisitedBBs;
+  bool UsedFullRecursion = false;
+  for (const BasicBlock *IncomBB : PN1->blocks()) {
+    if (!VisitedBBs.insert(IncomBB).second)
+      continue; // Don't reprocess blocks that we have dealt with already.
+    const Value *IV1 = PN1->getIncomingValueForBlock(IncomBB);
+    const Value *IV2 = PN2->getIncomingValueForBlock(IncomBB);
+    const APInt *C1, *C2;
+    if (match(IV1, m_APInt(C1)) && match(IV2, m_APInt(C2)) && *C1 != *C2)
+      continue;
+
+    // Only one pair of phi operands is allowed for full recursion.
+    if (UsedFullRecursion)
+      return false;
+
+    Query RecQ = Q;
+    RecQ.CxtI = IncomBB->getTerminator();
+    if (!isKnownNonEqual(IV1, IV2, Depth + 1, RecQ))
+      return false;
+    UsedFullRecursion = true;
+  }
+  return true;
+}
+
 /// Return true if it is known that V1 != V2.
 static bool isKnownNonEqual(const Value *V1, const Value *V2, unsigned Depth,
                             const Query &Q) {
@@ -2599,12 +2629,20 @@ static bool isKnownNonEqual(const Value *V1, const Value *V2, unsigned Depth,
     case Instruction::SExt:
     case Instruction::ZExt:
       if (O1->getOperand(0)->getType() == O2->getOperand(0)->getType())
-        return isKnownNonEqual(O1->getOperand(0), O2->getOperand(0),
-                               Depth + 1, Q);
+        return isKnownNonEqual(O1->getOperand(0), O2->getOperand(0), Depth + 1,
+                               Q);
+      break;
+    case Instruction::PHI:
+      const PHINode *PN1 = cast<PHINode>(V1);
+      const PHINode *PN2 = cast<PHINode>(V2);
+      // FIXME: This is missing a generalization to handle the case where one is
+      // a PHI and another one isn't.
+      if (isNonEqualPHIs(PN1, PN2, Depth, Q))
+        return true;
       break;
     };
   }
-  
+
   if (isAddOfNonZero(V1, V2, Depth, Q) || isAddOfNonZero(V2, V1, Depth, Q))
     return true;
 

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

@@ -291,4 +291,56 @@ define i1 @mul_other_may_be_zero_or_one(i16 %x, i16 %y) {
   ret i1 %cmp
 }
 
+define i1 @known_non_equal_phis(i8 %p, i8* %pq, i8 %n, i8 %r) {
+; CHECK-LABEL: @known_non_equal_phis(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[A:%.*]] = phi i8 [ 2, [[ENTRY:%.*]] ], [ [[NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[NEXT]] = mul nsw i8 [[A]], 2
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq i8 [[A]], [[N:%.*]]
+; CHECK-NEXT:    br i1 [[CMP1]], label [[EXIT:%.*]], label [[LOOP]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret i1 true
+;
+entry:
+  br label %loop
+loop:
+  %A = phi i8 [ 2, %entry ], [ %next, %loop ]
+  %B = phi i8 [ 3, %entry ], [ %A, %loop ]
+  %next = mul nsw i8 %A, 2
+  %cmp1 = icmp eq i8 %A, %n
+  br i1 %cmp1, label %exit, label %loop
+exit:
+  %cmp = icmp ne i8 %A, %B
+  ret i1 %cmp
+}
+
+define i1 @known_non_equal_phis_fail(i8 %p, i8* %pq, i8 %n, i8 %r) {
+; CHECK-LABEL: @known_non_equal_phis_fail(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[A:%.*]] = phi i8 [ 2, [[ENTRY:%.*]] ], [ [[NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[B:%.*]] = phi i8 [ 2, [[ENTRY]] ], [ [[A]], [[LOOP]] ]
+; CHECK-NEXT:    [[NEXT]] = mul nsw i8 [[A]], 2
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq i8 [[A]], [[N:%.*]]
+; CHECK-NEXT:    br i1 [[CMP1]], label [[EXIT:%.*]], label [[LOOP]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i8 [[A]], [[B]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  br label %loop
+loop:
+  %A = phi i8 [ 2, %entry ], [ %next, %loop ]
+  %B = phi i8 [ 2, %entry ], [ %A, %loop ]
+  %next = mul nsw i8 %A, 2
+  %cmp1 = icmp eq i8 %A, %n
+  br i1 %cmp1, label %exit, label %loop
+exit:
+  %cmp = icmp ne i8 %A, %B
+  ret i1 %cmp
+}
+
 !0 = !{ i8 1, i8 5 }