[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:

So I'm looking at changing the `getVectorInstrCost` interface to add support for other "unknown" indices that are more meaningful, such as the last element of a vector. In the meantime, I'm happy to update the comment. There are already quite a few tests exercising this code path (since we make different vectorisation decisions with my downstream patch). Once I've got something working I'll put up a WIP patch for people to look at.

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