[ValueTracking] match negative-stepping non-zero recurrence

This is pulled out of D100408.

This avoids a regression that would be exposed by making the
calling code from InstSimplify more efficient.

Author: rotateright

llvm/lib/Analysis/ValueTracking.cpp

@@ -2213,8 +2213,8 @@ static bool rangeMetadataExcludesValue(const MDNode* Ranges, const APInt& Value)
   return true;
 }
 
-/// Try to detect a recurrence that monotonically increases from a non-zero
-/// starting value. These are common as induction variables.
+/// Try to detect a recurrence that monotonically increases/decreases from a
+/// non-zero starting value. These are common as induction variables.
 static bool isNonZeroRecurrence(const PHINode *PN) {
   BinaryOperator *BO = nullptr;
   Value *Start = nullptr, *Step = nullptr;
@@ -2225,10 +2225,13 @@ static bool isNonZeroRecurrence(const PHINode *PN) {
 
   switch (BO->getOpcode()) {
   case Instruction::Add:
+    // Starting from non-zero and stepping away from zero can never wrap back
+    // to zero.
+    // TODO: The constant step requirement is not needed with NUW.
     return match(Step, m_APInt(StepC)) &&
            ((BO->hasNoUnsignedWrap() && !StepC->isNullValue()) ||
-            (BO->hasNoSignedWrap() && StartC->isStrictlyPositive() &&
-             StepC->isNonNegative()));
+            (BO->hasNoSignedWrap() &&
+             StartC->isNegative() == StepC->isNegative()));
   case Instruction::Mul:
     return match(Step, m_APInt(StepC)) &&
            ((BO->hasNoUnsignedWrap() && !StepC->isNullValue()) ||

llvm/unittests/Analysis/ValueTrackingTest.cpp

@@ -1191,7 +1191,7 @@ TEST_F(ValueTrackingTest, isNonZeroRecurrence) {
   )");
   DataLayout DL = M->getDataLayout();
   AssumptionCache AC(*F);
-  EXPECT_FALSE(isKnownNonZero(A, DL, 0, &AC, CxtI));
+  EXPECT_TRUE(isKnownNonZero(A, DL, 0, &AC, CxtI));
 }
 
 TEST_F(ValueTrackingTest, KnownNonZeroFromDomCond) {