Re: [PATCH bpf-next] libbpf: generate more efficient BPF_CORE_READ code

[Daniel Borkmann <daniel@iogearbox.net>]

On Thu, Oct 10, 2019 at 07:38:47PM -0700, Andrii Nakryiko wrote:
> Existing BPF_CORE_READ() macro generates slightly suboptimal code. If
> there are intermediate pointers to be read, initial source pointer is
> going to be assigned into a temporary variable and then temporary
> variable is going to be uniformly used as a "source" pointer for all
> intermediate pointer reads. Schematically (ignoring all the type casts),
> BPF_CORE_READ(s, a, b, c) is expanded into:
> ({
> 	const void *__t = src;
> 	bpf_probe_read(&__t, sizeof(*__t), &__t->a);
> 	bpf_probe_read(&__t, sizeof(*__t), &__t->b);
> 
> 	typeof(s->a->b->c) __r;
> 	bpf_probe_read(&__r, sizeof(*__r), &__t->c);
> })
> 
> This initial `__t = src` makes calls more uniform, but causes slightly
> less optimal register usage sometimes when compiled with Clang. This can
> cascase into, e.g., more register spills.
> 
> This patch fixes this issue by generating more optimal sequence:
> ({
> 	const void *__t;
> 	bpf_probe_read(&__t, sizeof(*__t), &src->a); /* <-- src here */
> 	bpf_probe_read(&__t, sizeof(*__t), &__t->b);
> 
> 	typeof(s->a->b->c) __r;
> 	bpf_probe_read(&__r, sizeof(*__r), &__t->c);
> })
> 
> Fixes: 7db3822ab991 ("libbpf: Add BPF_CORE_READ/BPF_CORE_READ_INTO helpers")
> Signed-off-by: Andrii Nakryiko <andriin@fb.com>

Applied, thanks!