From b11b07e0b8a47409e73c7d2ed735d20c3cf67019 Mon Sep 17 00:00:00 2001
From: Rosie Sumpter <rosie.sumpter@arm.com>
Date: Thu, 15 Jul 2021 14:08:30 +0100
Subject: [PATCH] [LoopFlatten][LoopInfo] Use Loop to identify latch compare
 instruction

Make getLatchCmpInst non-static and use it in LoopFlatten as a more
robust way of identifying the compare.

Differential Revision: https://reviews.llvm.org/D106256
---
 include/llvm/Analysis/LoopInfo.h      |  3 +++
 lib/Analysis/LoopInfo.cpp             |  8 ++++----
 lib/Transforms/Scalar/LoopFlatten.cpp | 18 +++++++-----------
 3 files changed, 14 insertions(+), 15 deletions(-)

diff --git a/include/llvm/Analysis/LoopInfo.h b/include/llvm/Analysis/LoopInfo.h
index 6e149b5d14f..164ec50e47b 100644
--- a/include/llvm/Analysis/LoopInfo.h
+++ b/include/llvm/Analysis/LoopInfo.h
@@ -589,6 +589,9 @@ public:
   ///
   PHINode *getCanonicalInductionVariable() const;
 
+  /// Get the latch condition instruction.
+  ICmpInst *getLatchCmpInst() const;
+
   /// Obtain the unique incoming and back edge. Return false if they are
   /// non-unique or the loop is dead; otherwise, return true.
   bool getIncomingAndBackEdge(BasicBlock *&Incoming,
diff --git a/lib/Analysis/LoopInfo.cpp b/lib/Analysis/LoopInfo.cpp
index f45cc12d463..66aab4c195c 100644
--- a/lib/Analysis/LoopInfo.cpp
+++ b/lib/Analysis/LoopInfo.cpp
@@ -171,8 +171,8 @@ PHINode *Loop::getCanonicalInductionVariable() const {
 }
 
 /// Get the latch condition instruction.
-static ICmpInst *getLatchCmpInst(const Loop &L) {
-  if (BasicBlock *Latch = L.getLoopLatch())
+ICmpInst *Loop::getLatchCmpInst() const {
+  if (BasicBlock *Latch = getLoopLatch())
     if (BranchInst *BI = dyn_cast_or_null<BranchInst>(Latch->getTerminator()))
       if (BI->isConditional())
         return dyn_cast<ICmpInst>(BI->getCondition());
@@ -183,7 +183,7 @@ static ICmpInst *getLatchCmpInst(const Loop &L) {
 /// Return the final value of the loop induction variable if found.
 static Value *findFinalIVValue(const Loop &L, const PHINode &IndVar,
                                const Instruction &StepInst) {
-  ICmpInst *LatchCmpInst = getLatchCmpInst(L);
+  ICmpInst *LatchCmpInst = L.getLatchCmpInst();
   if (!LatchCmpInst)
     return nullptr;
 
@@ -297,7 +297,7 @@ PHINode *Loop::getInductionVariable(ScalarEvolution &SE) const {
 
   BasicBlock *Header = getHeader();
   assert(Header && "Expected a valid loop header");
-  ICmpInst *CmpInst = getLatchCmpInst(*this);
+  ICmpInst *CmpInst = getLatchCmpInst();
   if (!CmpInst)
     return nullptr;
 
diff --git a/lib/Transforms/Scalar/LoopFlatten.cpp b/lib/Transforms/Scalar/LoopFlatten.cpp
index 1cad134876a..38bf0bca6cd 100644
--- a/lib/Transforms/Scalar/LoopFlatten.cpp
+++ b/lib/Transforms/Scalar/LoopFlatten.cpp
@@ -111,15 +111,6 @@ static bool findLoopComponents(
     LLVM_DEBUG(dbgs() << "Exiting and latch block are different\n");
     return false;
   }
-  // Latch block must end in a conditional branch.
-  BackBranch = dyn_cast<BranchInst>(Latch->getTerminator());
-  if (!BackBranch || !BackBranch->isConditional()) {
-    LLVM_DEBUG(dbgs() << "Could not find back-branch\n");
-    return false;
-  }
-  IterationInstructions.insert(BackBranch);
-  LLVM_DEBUG(dbgs() << "Found back branch: "; BackBranch->dump());
-  bool ContinueOnTrue = L->contains(BackBranch->getSuccessor(0));
 
   // Find the induction PHI. If there is no induction PHI, we can't do the
   // transformation. TODO: could other variables trigger this? Do we have to
@@ -131,6 +122,7 @@ static bool findLoopComponents(
   }
   LLVM_DEBUG(dbgs() << "Found induction PHI: "; InductionPHI->dump());
 
+  bool ContinueOnTrue = L->contains(Latch->getTerminator()->getSuccessor(0));
   auto IsValidPredicate = [&](ICmpInst::Predicate Pred) {
     if (ContinueOnTrue)
       return Pred == CmpInst::ICMP_NE || Pred == CmpInst::ICMP_ULT;
@@ -138,13 +130,17 @@ static bool findLoopComponents(
       return Pred == CmpInst::ICMP_EQ;
   };
 
-  // Find Compare and make sure it is valid
-  ICmpInst *Compare = dyn_cast<ICmpInst>(BackBranch->getCondition());
+  // Find Compare and make sure it is valid. getLatchCmpInst checks that the
+  // back branch of the latch is conditional.
+  ICmpInst *Compare = L->getLatchCmpInst();
   if (!Compare || !IsValidPredicate(Compare->getUnsignedPredicate()) ||
       Compare->hasNUsesOrMore(2)) {
     LLVM_DEBUG(dbgs() << "Could not find valid comparison\n");
     return false;
   }
+  BackBranch = cast<BranchInst>(Latch->getTerminator());
+  IterationInstructions.insert(BackBranch);
+  LLVM_DEBUG(dbgs() << "Found back branch: "; BackBranch->dump());
   IterationInstructions.insert(Compare);
   LLVM_DEBUG(dbgs() << "Found comparison: "; Compare->dump());