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

[David Sherwood via llvm-commits]

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@@ -1771,13 +1821,65 @@ bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
 
   // TODO: Handle loops that may fault.
   Predicates.clear();
-  if (!isDereferenceableReadOnlyLoop(TheLoop, PSE.getSE(), DT, AC,
-                                     &Predicates)) {
-    reportVectorizationFailure(
-        "Loop may fault",
-        "Cannot vectorize potentially faulting early exit loop",
-        "PotentiallyFaultingEarlyExitLoop", ORE, TheLoop);
-    return false;
+  if (HasStore) {
+    // Record load for analysis by isDereferenceableAndAlignedInLoop
+    // and later by dependence analysis.
+    if (BranchInst *Br = dyn_cast<BranchInst>(
+            SingleUncountableEdge->first->getTerminator())) {
+      // FIXME: Handle exit conditions with multiple users, more complex exit
+      //        conditions than br(icmp(load, loop_inv)).
+      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)) {
+          if (isDereferenceableAndAlignedInLoop(Load, TheLoop, *PSE.getSE(),
+                                                *DT, AC, &Predicates)) {
+            ICFLoopSafetyInfo SafetyInfo;
+            SafetyInfo.computeLoopSafetyInfo(TheLoop);
+            // FIXME: We may have multiple levels of conditional loads, so will
+            //        need to improve on outright rejection at some point.
+            if (SafetyInfo.isGuaranteedToExecute(*Load, DT, TheLoop)) {
+              EarlyExitLoad = Load;
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david-arm wrote:

I think this variable should be named something else because it gives the impression that for any arbitrary early exit loop there is only one load permitted inside it. Plus, the loop could have a countable early exit too.

How about CriticalUncountableEarlyExitLoad or something like that? It's hard to come up with a decent name.

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