[llvm] 5ae5d25 - [ValueTracking] match negative-stepping non-zero recurrence

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2021-04-14T08:57:53-04:00
New Revision: 5ae5d25e38efad1d59ed97d969a5e930b58a5e16

URL: https://github.com/llvm/llvm-project/commit/5ae5d25e38efad1d59ed97d969a5e930b58a5e16
DIFF: https://github.com/llvm/llvm-project/commit/5ae5d25e38efad1d59ed97d969a5e930b58a5e16.diff

LOG: [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.

Added: 
    

Modified: 
    llvm/lib/Analysis/ValueTracking.cpp
    llvm/unittests/Analysis/ValueTrackingTest.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 2d933489447d..d78acdbb1cd9 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/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()) ||

diff  --git a/llvm/unittests/Analysis/ValueTrackingTest.cpp b/llvm/unittests/Analysis/ValueTrackingTest.cpp
index 85678bb66530..99a0f6daa84d 100644
--- a/llvm/unittests/Analysis/ValueTrackingTest.cpp
+++ b/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) {