[all-commits] [llvm/llvm-project] 26bd22: [RISCV] Ensure shuffle splat operands are type-legal

[Fraser Cormack via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 26bd2250c1870f1983fd3439becfbb39e369b8a3
      https://github.com/llvm/llvm-project/commit/26bd2250c1870f1983fd3439becfbb39e369b8a3
  Author: Fraser Cormack <fraser at codeplay.com>
  Date:   2021-05-20 (Thu, 20 May 2021)

  Changed paths:
    M llvm/include/llvm/CodeGen/SelectionDAG.h
    M llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
    M llvm/lib/Target/RISCV/RISCVISelLowering.cpp
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-shuffles.ll

  Log Message:
  -----------
  [RISCV] Ensure shuffle splat operands are type-legal

The use of `SelectionDAG::getSplatValue` isn't guaranteed to return a
type-legal splat value as it may implicitly extract a vector element
from another shuffle. It is not permitted to introduce an illegal type
when lowering shuffles.

This patch addresses the crash by adding a boolean flag to
`getSplatValue`, defaulting to false, which when set will ensure a
type-legal return value. If it is unable to do that it will fail to
return a splat value.

I've been through the existing uses of `getSplatValue` in other targets
and was unable to find a need or test cases showing a need to update
their uses. In some cases, the call is made during `LegalizeVectorOps`
which may still produce illegal scalar types. In other situations, the
illegally-typed splat value may be quickly patched up to a legal type
(such as any-extending the returned `extract_vector_elt` up to a legal
type) before `LegalizeDAG` notices.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D102687