[llvm] [RISCV] Check for COPY_TO_REGCLASS in usesAllOnesMask (PR #67037)

[Philip Reames via llvm-commits]

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@@ -3188,6 +3188,12 @@ static bool usesAllOnesMask(SDValue MaskOp, SDValue GlueOp) {
   // Check the instruction defining V0; it needs to be a VMSET pseudo.
   SDValue MaskSetter = Glued->getOperand(2);
 
+  // Sometimes the VMSET is wrapped in a COPY_TO_REGCLASS node, e.g. if the mask
+  // was extracted from a larger register.
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preames wrote:

There's something off here.  The docs and lowering code for COPY_TO_REGCLASS say that it is "like a copy", but fixes the destination register class.  Your comment seems to say that it's more like a EXTRACT_SUBREG.  I can't reconcile these two.  Are you really sure the two register classes here are different sizes?  Can you give an example of the before and after register class?

https://github.com/llvm/llvm-project/pull/67037