[all-commits] [llvm/llvm-project] 8adde6: [RISCV] Use vmv.v.i for insertion into lane 0 of u...

[Philip Reames via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 8adde6941a4b31bbabab41f03dda76c4a05c94b1
      https://github.com/llvm/llvm-project/commit/8adde6941a4b31bbabab41f03dda76c4a05c94b1
  Author: Philip Reames <preames at rivosinc.com>
  Date:   2022-12-13 (Tue, 13 Dec 2022)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVISelLowering.cpp
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-extract.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-buildvec.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-shuffles.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-stepvector.ll

  Log Message:
  -----------
  [RISCV] Use vmv.v.i for insertion into lane 0 of undef vector when profitable

If we're initializing lane 0 of an undef vector, we can optionally write to other lanes of the vector. Doing so may require additional work, so we don't want to e.g. always use a splat. However, since we don't have an immediate form of vmv.s.x it's useful to use a vmv.v.i if the work required is expected to be equal in practice.  We restrict this to when LMUL <= 1 to a) prevent doing additional work at higher LMULs, and b) avoid overconstraining the register allocator.

At the moment, the new utility is only used by one case in INSERT_VECTOR_ELT lowering. My expectation is that we will reuse this in a couple other places, but each of those deserve individual review.

This change is inspired by D137530, but is not directly related to it. I vaguely remember we discussed the tradeoffs of using vmv.v.i in another recent review, but couldn't find it.

Differential Revision: https://reviews.llvm.org/D139648