[PATCH] D103510: [RISCV] Use vmv.v.[v|i] if we know COPY is under the same vl and vtype.

[Craig Topper via Phabricator via llvm-commits]

craig.topper added inline comments.


================
Comment at: llvm/lib/Target/RISCV/RISCVInstrInfo.cpp:167
+      // If the vsetvli is tail undisturbed, keep the whole register move.
+      if (!RISCVVType::isTailAgnostic(VType))
+        return false;
----------------
craig.topper wrote:
> What if the defining instruction something that doesn't obey vsetvli for example an earlier COPY that became a whole register move. Or a whole register load which could be a spill reload or just a VLMAX load.
> 
> Or a reduction which only writes element 0 regardless of the tail policy. I think we currently force tail agnostic on the vsetvli for those, but I might update it to keep whatever tail policy the previous instruction used.
Looks like I was wrong about reductions. The behavior changed after 0.9 and our implementation is incorrectly forcing tail agnostic for those.


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https://reviews.llvm.org/D103510