[all-commits] [llvm/llvm-project] 2663d2: [RISCV] Adjust select shuffle cost to reflect mask...

[Philip Reames via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 2663d2cb9c9361f0b234c40a0f50c7ba0748eb26
      https://github.com/llvm/llvm-project/commit/2663d2cb9c9361f0b234c40a0f50c7ba0748eb26
  Author: Philip Reames <preames at rivosinc.com>
  Date:   2024-01-18 (Thu, 18 Jan 2024)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
    M llvm/test/Analysis/CostModel/RISCV/shuffle-insert_subvector.ll
    M llvm/test/Analysis/CostModel/RISCV/shuffle-select.ll

  Log Message:
  -----------
  [RISCV] Adjust select shuffle cost to reflect mask creation cost (#77963)

This is inspired by
https://github.com/llvm/llvm-project/pull/77342#pullrequestreview-1814673242,
and is split off of same with some differences in style.

A select is a vmerge.vv with the additional cost of materializing the
bitmask vector in a vreg. All masks fit within a single vector register
(e8 + m8 is the worst case), and thus our worst case cost should be
roughly 3 (2 scalar to produce the address, one vector load op). Given
most shuffles are small, and the mask will be instead produced by
LUI/ADDI + vmv.s.x or ADDI + vmv.s.x, using 2 as the default seems quite
reasonable. At worst, we're not going to be off by much.

The prior lowering scaled the cost of the bitmask with LMUL, which I
don't understand. At m1 it did use the same base cost of 2. (@lukel97
You wrote the original code here, anything I'm missing here?)