[Mesa-dev] [PATCH] util: use GCC atomic intrinsics with explicit memory model

[Emil Velikov]

On 4 October 2016 at 15:14, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> From: Nicolai Hähnle <nicolai.haehnle at amd.com>
>
> This is motivated by the fact that p_atomic_read and p_atomic_set may
> somewhat surprisingly not do the right thing in the old version: while
> stores and loads are de facto atomic at least on x86, the compiler may
> apply re-ordering and speculation quite liberally. Basically, the old
> version uses the "relaxed" memory ordering.
>
> The new ordering always uses acquire/release ordering. This is the
> strongest possible memory ordering that doesn't require additional
> fence instructions on x86. (And the only stronger ordering is
> "sequentially consistent", which is usually more than you need anyway.)
>
> I would feel more comfortable if p_atomic_set/read in the old
> implementation were at least using volatile loads and stores, but I
> don't see a way to get there without typeof (which we cannot use here
> since the code is compiled with -std=c99).
>
> Eventually, we should really just move to something that is based on
> the atomics in C11 / C++11.
> ---
>  configure.ac        | 11 +++++++++++
>  src/util/u_atomic.h | 21 +++++++++++++++++++++
>  2 files changed, 32 insertions(+)
>
On the build side the patch looks great, I haven't looked at the
atomic specifics.
Reviewed-by: Emil Velikov <emil.velikov at collabora.com>

-Emil