Fork channel

Create a new channel as a copy of main.

Rename channel

Rename main to:

Delete channel

Delete main? This cannot be undone.

test_2r_msa64.s
# RUN: llvm-mc %s -arch=mips64 -mcpu=mips64r2 -mattr=+msa -show-encoding | FileCheck %s
#
# CHECK:        fill.d  $w27, $9                # encoding: [0x7b,0x03,0x4e,0xde]

                fill.d  $w27, $9