Re: [RFC bpf-next 01/11] bpf: use branch predictions in opt_hard_wire_dead_code_branches()

[Eduard Zingerman <eddyz87@xxxxxxxxx>]

On Thu, 2024-11-14 at 16:19 -0800, Andrii Nakryiko wrote:

[...]

> I was also always hoping that we'll eventually optimize the following pattern:
> 
> r1 = *(global var)
> if r1 == 1 /* always 1 or 0 */
>    goto +...
> ...
> 
> 
> This is extremely common with .rodata global variables, and while the
> branches are dead code eliminated, memory reads are not. Not sure how
> involved it would be to do this.

Could you please elaborate a bit.
For a simple test like below compiler replaces 'flag' with 1,
so no action is needed from verifier:

    const int flag = 1;

    SEC("socket")
    __success
    __xlated("foobar")
    int rodata_test(void *ctx)
    {
    	if (flag)
    		return 1;
    	return 0;
    }