Re: [PATCH RFC bpf-next v1 2/4] bpf: Introduce load-acquire and store-release instructions

[Peilin Ye <yepeilin@google.com>]

On Thu, Dec 26, 2024 at 11:07:10PM +0000, Peilin Ye wrote:
> > > +	if (BPF_ATOMIC_TYPE(insn->imm) == BPF_ATOMIC_LOAD)
> > > +		ptr = src;
> > > +	else
> > > +		ptr = dst;
> > > +
> > > +	if (off) {
> > > +		emit_a64_mov_i(true, tmp, off, ctx);
> > > +		emit(A64_ADD(true, tmp, tmp, ptr), ctx);
> > 
> > The mov and add instructions can be optimized to a single A64_ADD_I
> > if is_addsub_imm(off) is true.
> 
> Thanks!  I'll try this.

The following diff seems to work:

--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -658,9 +658,15 @@ static int emit_atomic_load_store(const struct bpf_insn *insn, struct jit_ctx *c
                ptr = dst;

        if (off) {
-               emit_a64_mov_i(true, tmp, off, ctx);
-               emit(A64_ADD(true, tmp, tmp, ptr), ctx);
-               ptr = tmp;
+               if (is_addsub_imm(off)) {
+                       emit(A64_ADD_I(true, ptr, ptr, off), ctx);
+               } else if (is_addsub_imm(-off)) {
+                       emit(A64_SUB_I(true, ptr, ptr, -off), ctx);
+               } else {
+                       emit_a64_mov_i(true, tmp, off, ctx);
+                       emit(A64_ADD(true, tmp, tmp, ptr), ctx);
+                       ptr = tmp;
+               }
        }
        if (arena) {
                emit(A64_ADD(true, tmp, ptr, arena_vm_base), ctx);

I'll include it in the next version.  I think the same thing can be done
for emit_lse_atomic() and emit_ll_sc_atomic(); let me do that in a
separate patch.

Thanks,
Peilin Ye