[llvm] [LV] Add initial legality checks for early exit loops with side effects (PR #145663)

[Florian Hahn via llvm-commits]

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@@ -1813,6 +1840,99 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
                        "backedge taken count: "
                     << *SymbolicMaxBTC << '\n');
   UncountableExitingBB = SingleUncountableExitingBlock;
+  UncountableExitWithSideEffects = HasSideEffects;
+  return true;
+}
+
+bool LoopVectorizationLegality::canUncountableExitConditionLoadBeMoved(
+    BasicBlock *ExitingBlock) {
+  // Try to find a load in the critical path for the uncountable exit condition.
+  // This is currently matching about the simplest form we can, expecting
+  // only one in-loop load, the result of which is directly compared against
+  // a loop-invariant value.
+  // FIXME: We're insisting on a single use for now, because otherwise we will
+  // need to make PHI nodes for other users. That can be done once the initial
+  // transform code lands.
+  auto *Br = cast<BranchInst>(ExitingBlock->getTerminator());
+
+  using namespace llvm::PatternMatch;
+  Instruction *L = nullptr;
+  Value *Ptr = nullptr;
+  Value *R = nullptr;
+  if (!match(Br->getCondition(),
+             m_OneUse(m_ICmp(m_OneUse(m_Instruction(L, m_Load(m_Value(Ptr)))),
+                             m_Value(R))))) {
+    reportVectorizationFailure(
+        "Early exit loop with store but no supported condition load",
+        "NoConditionLoadForEarlyExitLoop", ORE, TheLoop);
+    return false;
+  }
+
+  // FIXME: Don't rely on operand ordering for the comparison.
+  if (!TheLoop->isLoopInvariant(R)) {
+    reportVectorizationFailure(
+        "Early exit loop with store but no supported condition load",
+        "NoConditionLoadForEarlyExitLoop", ORE, TheLoop);
+    return false;
+  }
+
+  // Make sure that the load address is not loop invariant; we want an
+  // address calculation that we can rotate to the next vector iteration.
+  const SCEV *PtrScev = PSE.getSE()->getSCEV(Ptr);
+  if (!isa<SCEVAddRecExpr>(PtrScev)) {
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fhahn wrote:

@huntergr-arm I see you pushed a fix, would be helpful for visibility to mention it somewhere here

https://github.com/llvm/llvm-project/pull/145663