Re: [PATCH bpf-next] bpf: arm64: Redefine MOV consistent with arch insn

[Jianlin Lv]

On Tue, Mar 30, 2021 at 5:31 PM Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Tue, Mar 30, 2021 at 03:42:35PM +0800, Jianlin Lv wrote:
> > A64_MOV is currently mapped to Add Instruction. Architecturally MOV
> > (register) is an alias of ORR (shifted register) and MOV (to or from SP)
> > is an alias of ADD (immediate).
> > This patch redefines A64_MOV and uses existing functionality
> > aarch64_insn_gen_move_reg() in insn.c to encode MOV (register) instruction.
> > For moving between register and stack pointer, rename macro to A64_MOV_SP.
>
> What does this gain us? There's no requirement for a BPF "MOV" to match an
> arm64 architectural "MOV", so what's the up-side of aligning them like this?
>
> Cheers,
>
> Will

According to the description in the Arm Software Optimization Guide,
Arithmetic(basic) and Logical(basic) instructions have the same
Exec Latency and Execution Throughput.
This change did not bring about a performance improvement.
The original intention was to make the instruction map more 'natively'.

Jianlin