cmd/internal/obj/riscv: fix vector integer multiply add
The RISC-V integer vector multiply add instructions are not encoded
correctly; the first and second arguments are swapped. For example,
the instruction
VMACCVV V1, V2, V3
encodes to
b620a1d7 or vmacc.vv v3,v1,v2
and not
b61121d7 or vmacc.vv v3,v2,v1
as expected.
This is inconsistent with the argument ordering we use for 3
argument vector instructions, in which the argument order, as given
in the RISC-V specifications, is reversed, and also with the vector
FMA instructions which have the same argument ordering as the vector
integer multiply add instructions in the "The RISC-V Instruction Set
Manual Volume I". For example, in the ISA manual we have the
following instruction definitions
; Integer multiply-add, overwrite addend
vmacc.vv vd, vs1, vs2, vm # vd[i] = +(vs1[i] * vs2[i]) + vd[i]
; FP multiply-accumulate, overwrites addend
vfmacc.vv vd, vs1, vs2, vm # vd[i] = +(vs1[i] * vs2[i]) + vd[i]
It's reasonable to expect that the Go assembler would use the same
argument ordering for both of these instructions. It currently does
not.
We fix the issue by switching the argument ordering for the vector
integer multiply add instructions to match those of the vector FMA
instructions.
Change-Id: Ib98e9999617f991969e5c831734b3bb3324439f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/670335
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>