Re: [tip: locking/urgent] locking/ww_mutex: Simplify use_ww_ctx & ww_ctx handling

Peter Zijlstra
2021-03-17T14:18:00Z

On Wed, Mar 17, 2021 at 10:10:16AM -0400, Waiman Long wrote:
> On 3/17/21 9:55 AM, Peter Zijlstra wrote:
> > On Wed, Mar 17, 2021 at 09:43:20AM -0400, Waiman Long wrote:
> >
> > > Using gcc 8.4.1, the generated __mutex_lock function has the same size (with
> > > last instruction at offset +5179) with or without this patch. Well, you can
> > > say that this patch is an no-op wrt generated code.
> > OK, then GCC has gotten better. Because back then I tried really hard
> > but it wouldn't remove the if (ww_ctx) branches unless I had that extra
> > const bool argument.
> >
> I think ww_mutex was merged in 2013. That is almost 8 years ago. It could
> still be the case that older gcc compilers may not generate the right code.
> I will try the RHEL7 gcc compiler (4.8.5) to see how it fares.

I really don't care about code generation qualitee of anything before
8-ish at this point. That's already an old compiler.

If you run on ancient compilers, you simply don't care about code
quality.