Re: Surprising code generated for vdso_read_begin()

[Christophe Leroy]

Le 16/02/2020 Ã 19:10, Arnd Bergmann a ÃcritÂ:
> On Sat, Jan 11, 2020 at 12:33 PM Segher Boessenkool
> <segher@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Fri, Jan 10, 2020 at 07:45:44AM +0100, Christophe Leroy wrote:
> > > Le 09/01/2020 Ã 21:07, Segher Boessenkool a Ãcrit :
> > > > It looks like the compiler did loop peeling.  What GCC version is this?
> > > > Please try current trunk (to become GCC 10), or at least GCC 9?
> > >
> > > It is with GCC 5.5
> > >
> > > https://mirrors.edge.kernel.org/pub/tools/crosstool/ doesn't have more
> > > recent than 8.1
> >
> > Arnd, can you update the tools?  We are at 8.3 and 9.2 now :-)  Or is
> > this hard and/or painful to do?
>
> To follow up on this older thread, I have now uploaded 6.5, 7.5, 8.3 and 9.2
> binaries, as well as a recent 10.0 snapshot.

Thanks Arnd,

I have built the VDSO with 9.2, I get less performant result than with 
8.2 (same performance as with 5.5).

After a quick look, I see:
- Irrelevant NOPs to align loops and stuff, allthough -mpcu=860 should 
avoid that.
- A stack frame is set for saving r31 in __c_kernel_clock_gettime. GCC 
8.1 don't need that, all VDSO functions are frameless with 8.1

Christophe