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

[Graham Hunter via llvm-commits]

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@@ -1207,8 +1209,42 @@ bool LoopVectorizationLegality::canVectorizeMemory() {
     });
   }
 
-  if (!LAI->canVectorizeMemory())
-    return canVectorizeIndirectUnsafeDependences();
+  if (LAI->canVectorizeMemory()) {
+    // FIXME: Remove or reduce this restriction. We're in a bit of an odd spot
+    //        since we're (potentially) doing the load out of its normal order
+    //        in the loop and that may throw off dependency checking.
+    //        A forward dependency should be fine, but a backwards dep may not
+    //        be even if LAA thinks it is due to performing the load for the
+    //        vector iteration i+1 in vector iteration i.
+    if (CriticalEELoad) {
+      const MemoryDepChecker &DepChecker = LAI->getDepChecker();
+      const auto *Deps = DepChecker.getDependences();
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huntergr-arm wrote:

Good point. Have we thought about making getDependences return a std::optional to make it more obvious that needs to be considered?

I'll add the test.

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