Re: [Qemu-devel] [RFC v1 3/3] target/arm: Correct exclusive store return value

[Richard Henderson]

On 08/11/2017 11:19 AM, Alistair Francis wrote:
> The exclusive store operation should return 0 if the operation updates
> memory and 1 if it doesn't. This means that storing tmp in the rd
> register is incorrect.

I'm confused as to what you believe is wrong.

>              tcg_gen_atomic_cmpxchg_i64(tmp, addr, val, tmp,
>                                         get_mem_index(s),
> -                                       size | MO_ALIGN | s->be_data);
> -            tcg_gen_setcond_i64(TCG_COND_NE, tmp, tmp, val);

Yes, we load the result of the cmpxchg into tmp, but then we immediately
overwrite tmp with 0/1 depending on whether the operation succeeded.

> 
> This patch updates the succesful opertion to store 0 into the rd
> register instead of tmp. It also adds a branch to fail if the memory
> isn't updated.

Since we use setcond, a branch is not required.

> +            tcg_gen_ext_i64(val, val, memop);

What is this addition intended to accomplish?  Because of the position within
the code, you know that memop contains MO_64, so that this is a no-op.


r~