[llvm] [LV] Use getFixedValue instead of getKnownMinValue when appropriate (PR #143526)

[David Sherwood via llvm-commits]

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@@ -5342,6 +5342,10 @@ LoopVectorizationCostModel::getUniformMemOpCost(Instruction *I,
   StoreInst *SI = cast<StoreInst>(I);
 
   bool IsLoopInvariantStoreValue = Legal->isInvariant(SI->getValueOperand());
+  // TODO: We have tests that request the cost of extracting element
+  // VF.getKnownMinValue() - 1 from a scalable vector. This is actually
+  // meaningless, given what we actually want is the last lane and is likely
+  // to be more expensive.
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david-arm wrote:

I don't think there is such a TTI hook. Maybe an argument for adding one, or at least modifying an existing interface? I'm happy to look into it.

https://github.com/llvm/llvm-project/pull/143526