[llvm] [LV] Add initial support for partial alias masking (PR #177599)

[Florian Hahn via llvm-commits]

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@@ -3079,8 +3120,15 @@ bool LoopVectorizationCostModel::memoryInstructionCanBeWidened(
   auto *Ptr = getLoadStorePointerOperand(I);
   auto *ScalarTy = getLoadStoreType(I);
 
+  int Stride = Legal->isConsecutivePtr(ScalarTy, Ptr);
   // In order to be widened, the pointer should be consecutive, first of all.
-  if (!Legal->isConsecutivePtr(ScalarTy, Ptr))
+  if (!Stride)
+    return false;
+
+  // Currently, we can't handle alias masking in reverse. Reversing the alias
+  // mask is not correct (or necessary). When combined with tail-folding the ALM
+  // should only be reversed where the alias-mask is true.
+  if (Stride < 0 && maskPartialAliasing())
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fhahn wrote:

this looks like it should be the other way around, i.e. not using `maskPartialAliasing` if there are negative strides?

https://github.com/llvm/llvm-project/pull/177599