[llvm] [RISCV] Don't set AVL if only zeroness is demanded (PR #74049)

[Luke Lau via llvm-commits]

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@@ -1086,6 +1095,14 @@ void RISCVInsertVSETVLI::transferBefore(VSETVLIInfo &Info,
           IncomingInfo.getTailAgnostic(),
       (Demanded.MaskPolicy ? IncomingInfo : Info).getMaskAgnostic() ||
           IncomingInfo.getMaskAgnostic());
+
+  // If we only knew the sew/lmul ratio previously, replace the VTYPE but keep
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lukel97 wrote:

We have to handle the case where the AVL might be preserved so we can't bail before the AVL gets set on line 1084. We could maybe do an early exit after that though, before we set VTYPE.

As a side note, this is a bit awkward because calling `setVTYPE` isn't enough to clear the hasSEWLMULRatioOnly bit. Hence why we need to copy over incoming info.

https://github.com/llvm/llvm-project/pull/74049