[PATCH] D112236: [RISCV] Expand scalable vector bswap. Fix crash for bitreverse.

Fri Oct 29 09:58:24 PDT 2021

craig.topper added a comment.

In D112236#3086805 <https://reviews.llvm.org/D112236#3086805>, @frasercrmck wrote:

> It's not very clear to me why we have different ways of expanding bswap. Why doesn't `VectorLegalizer::ExpandBSWAP` always call `TLI.expandBSWAP`, for instance?

ExpandBSWAP is using the shift+and+or expansion. VectorLegalizer::ExpandBSWAP tries to use a shuffle+bitcast which would only be legal for a vector. I suppose we could sink that into expandBSWAP with a .isFixedVector() check if that's what you're asking?

================
Comment at: llvm/lib/CodeGen/SelectionDAG/LegalizeVectorOps.cpp:1099
+  // Scalable vectors can't use shuffle expansion.
+  if (VT.isScalableVector())
+    return TLI.expandBSWAP(Node, DAG);
----------------
I've made a bold assumption here that SHL/SRL/AND/OR are all LegalOrCustom for a scalable vector. Otherwise the unroll fallback would crash.

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https://reviews.llvm.org/D112236