[llvm] Extending LoopVersioningLICM to handle cases where loopbound is invariant (PR #192902)

[Ramkumar Ramachandra via llvm-commits]

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@@ -213,12 +232,124 @@ bool LoopVersioningLICM::legalLoopStructure() {
   // to generate the bound checks.
   const SCEV *ExitCount = SE->getBackedgeTakenCount(CurLoop);
   if (isa<SCEVCouldNotCompute>(ExitCount)) {
+    IsDynamicBound = true;
     LLVM_DEBUG(dbgs() << "    loop does not have trip count\n");
+    LLVM_DEBUG(dbgs() << "    checking the possibility of dynamic trip count\n");
+
+    HoistedDependencies.clear();
+
+    // Check if the loop trip count is dynamic
+    BasicBlock *ExitBlock = CurLoop->getExitingBlock();
+    assert(ExitBlock && "Exiting block guaranteed by earlier getExitingBlock() check");
+    CondBrInst *ExitBranch = dyn_cast<CondBrInst>(ExitBlock->getTerminator());
+    if (!ExitBranch) {
+      LLVM_DEBUG(dbgs() << "    loop exit condition is not a conditional branch\n");
+      return false;
+    }
+    ICmpInst *ExitCmp = dyn_cast<ICmpInst>(ExitBranch->getCondition());
+    if (!ExitCmp) {
+      LLVM_DEBUG(dbgs() << "    loop exit condition is not an icmp instruction\n");
+      return false;
+    }
+    
+    PHINode *IndVar = CurLoop->getInductionVariable(*SE);
+    if (!IndVar) {
+      LLVM_DEBUG(dbgs() << "    unable to find loop induction variable\n");
+      return false;
+    }
+    LLVM_DEBUG(dbgs() << "    Induction variable: " << *IndVar << "\n");
+
+    Value *StepInst = IndVar->getIncomingValueForBlock(CurLoop->getLoopLatch());
+
+    auto IsIVOrStep = [&](Value *V) {
+      return V == IndVar || V == StepInst;
+    };
+
+    Value *DynamicUpperBound = nullptr;
+    if (IsIVOrStep(ExitCmp->getOperand(0))) {
+      DynamicUpperBound = ExitCmp->getOperand(1);
+    }
+    else if (IsIVOrStep(ExitCmp->getOperand(1))) {
+      DynamicUpperBound = ExitCmp->getOperand(0);
+    }
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artagnon wrote:

Ah, I read it as the terminator; never mind.

https://github.com/llvm/llvm-project/pull/192902