[PATCH] D99912: Look through invertible recurrences in isKnownNonEqual

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added a reviewer: nikic.
Herald added subscribers: dantrushin, hiraditya, mcrosier.
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This extends the phi handling in isKnownNonEqual with a special case based on invertible recurrences.  If we can prove the recurrence is invertible (which many common ones are), we can recurse through the start operands of the recurrence skipping the phi cycle.

(Side note: Instcombine currently does not push back through these cases.  I will implement that in a follow up change w/separate review.)


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D99912

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Analysis/ValueTracking/known-non-equal.ll


Index: llvm/test/Analysis/ValueTracking/known-non-equal.ll
===================================================================
--- llvm/test/Analysis/ValueTracking/known-non-equal.ll
+++ llvm/test/Analysis/ValueTracking/known-non-equal.ll
@@ -500,8 +500,7 @@
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i64 [[IV_NEXT]], 10
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[RES:%.*]] = icmp eq i8 [[A_IV]], [[B_IV]]
-; CHECK-NEXT:    ret i1 [[RES]]
+; CHECK-NEXT:    ret i1 false
 ;
 entry:
   %B = add i8 %A, 1
@@ -603,8 +602,7 @@
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i64 [[IV_NEXT]], 10
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[RES:%.*]] = icmp eq i8 [[A_IV]], [[B_IV]]
-; CHECK-NEXT:    ret i1 [[RES]]
+; CHECK-NEXT:    ret i1 false
 ;
 entry:
   %B = add i8 %A, 1
@@ -705,8 +703,7 @@
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i64 [[IV_NEXT]], 10
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[RES:%.*]] = icmp eq i8 [[A_IV]], [[B_IV]]
-; CHECK-NEXT:    ret i1 [[RES]]
+; CHECK-NEXT:    ret i1 false
 ;
 entry:
   %B = add i8 %A, 1
@@ -840,8 +837,7 @@
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i64 [[IV_NEXT]], 10
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[RES:%.*]] = icmp eq i8 [[A_IV]], [[B_IV]]
-; CHECK-NEXT:    ret i1 [[RES]]
+; CHECK-NEXT:    ret i1 false
 ;
 entry:
   %B = add i8 %A, 1
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -2593,6 +2593,35 @@
     if (Op1->getOperand(0)->getType() == Op2->getOperand(0)->getType())
       return 0;
     break;
+  case Instruction::PHI: {
+    const PHINode *PN1 = cast<PHINode>(Op1);
+    const PHINode *PN2 = cast<PHINode>(Op2);
+
+    // If PN1 and PN2 are both recurrences, can we prove the entire recurrences
+    // are a single invertible function of the start values? Note that repeated
+    // application of an invertible function is also invertible
+    BinaryOperator *BO1 = nullptr;
+    Value *Start1 = nullptr, *Step1 = nullptr;
+    BinaryOperator *BO2 = nullptr;
+    Value *Start2 = nullptr, *Step2 = nullptr;
+    if (PN1->getParent() != PN2->getParent() ||
+        !matchSimpleRecurrence(PN1, BO1, Start1, Step1) ||
+        !matchSimpleRecurrence(PN2, BO2, Start2, Step2))
+      break;
+
+    Optional<unsigned> Idx = getInvertibleOperand(cast<Operator>(BO1),
+                                                  cast<Operator>(BO2));
+    if (!Idx || *Idx != 0)
+      break;
+    assert(BO1->getOperand(*Idx) == PN1 && BO2->getOperand(*Idx) == PN2);
+
+    if (PN1->getOperand(0) == BO1) {
+      assert(PN2->getOperand(0) == BO2);
+      return 1;
+    }
+    assert(PN1->getOperand(1) == BO1 && PN2->getOperand(1) == BO2);
+    return 0;
+  }
   }
   return None;
 }


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