[llvm] [LV] Add initial legality checks for ee loops with stores (PR #145663)

[David Sherwood via llvm-commits]

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@@ -1793,6 +1814,110 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
   return true;
 }
 
+bool LoopVectorizationLegality::canUncountedExitConditionLoadBeMoved(
+    BasicBlock *ExitingBlock) {
+  SmallVector<const SCEVPredicate *, 4> Predicates;
+  LoadInst *CriticalUncountedExitConditionLoad = nullptr;
+
+  // Try to find a load in the critical path for the uncounted 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.
+  if (BranchInst *Br = dyn_cast<BranchInst>(ExitingBlock->getTerminator())) {
+    // FIXME: Don't rely on operand ordering for the comparison.
+    ICmpInst *Cmp = dyn_cast<ICmpInst>(Br->getCondition());
+    if (Cmp && Cmp->hasOneUse() &&
+        TheLoop->isLoopInvariant(Cmp->getOperand(1))) {
+      LoadInst *Load = dyn_cast<LoadInst>(Cmp->getOperand(0));
+      if (Load && Load->hasOneUse() && !TheLoop->isLoopInvariant(Load)) {
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david-arm wrote:

If the algorithm for vectorising the loop absolutely depends upon the Load being variant, then `isLoopInvariant` isn't strong enough here. It will return true if the load is from an invariant address, but happens to be in the loop. If you really want to make sure it's loop invariant (to stop the loop vectoriser hoisting it to the preheader, etc.) then you probably need to call ScalarEvolution::isLoopInvariant

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