bitreverse+bswap => bitreverse8

Description

define <2 x i64> @foo(<2 x i64> %0) unnamed_addr align 4 {
Entry:
  %1 = tail call <2 x i64> @llvm.bitreverse.v2i64(<2 x i64> %0)
  %2 = tail call <2 x i64> @llvm.bswap.v2i64(<2 x i64> %1)
  ret <2 x i64> %2
}
declare <2 x i64> @llvm.bitreverse.v2i64(<2 x i64>) #1
declare <2 x i64> @llvm.bswap.v2i64(<2 x i64>) #1

define <2 x i64> @bar(<2 x i64> %0) unnamed_addr align 4 {
Entry:
  %1 = bitcast <2 x i64> %0 to <16 x i8>
  %2 = tail call <16 x i8> @llvm.bitreverse.v16i8(<16 x i8> %1)
  %3 = bitcast <16 x i8> %2 to <2 x i64>
  ret <2 x i64> %3
}

declare <16 x i8> @llvm.bitreverse.v16i8(<16 x i8>) #1

foo:
-       rev64   v0.16b, v0.16b
-       rev64   v0.16b, v0.16b
        rbit    v0.16b, v0.16b
        ret

Contributor guide

Tech stack

cpp

Domain

backend

Issue type

feature

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

3

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

1-2 days

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

active

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

C++LLVMGit

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

60

Research direction

Implement a target specific DAG combine for AArch64 that replaces a bitreverse followed by bswap with a bitreverse on the element type (e.g., v16i8). Look at existing patterns in the AArch64 target directory for similar combines.