[llvm] [RISCV] Keep same SEW/LMUL ratio if possible in forward transfer (PR #69788)

[Philip Reames via llvm-commits]

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@@ -1042,6 +1042,26 @@ void RISCVInsertVSETVLI::transferBefore(VSETVLIInfo &Info,
   if (!RISCVII::hasVLOp(TSFlags))
     return;
 
+  // If we don't use LMUL or the SEW/LMUL ratio, then adjust LMUL so that we
+  // maintain the SEW/LMUL ratio. This allows us to eliminate VL toggles in more
+  // places.
+  DemandedFields Demanded = getDemanded(MI, MRI);
+  if (!Demanded.LMUL && !Demanded.SEWLMULRatio && Info.isValid() &&
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preames wrote:

I mentioned this offline, but basing this off the existing demanded feilds is much better than re-infering which is what I'd tried   This makes cornercase bugs around instructions (e.g. vrgather, or vslide*) much less likely because the existing getDemandedFeidls logic would have to be wrong as well.  I feel a bit silly in retrospect, but glad you got it structured properly.  :)

https://github.com/llvm/llvm-project/pull/69788