On Wed, Feb 19, 2020 at 9:45 AM Christophe Leroy
[off-list ref] wrote:

Le 16/02/2020 à 19:10, Arnd Bergmann a écrit :

quoted

On Sat, Jan 11, 2020 at 12:33 PM Segher Boessenkool
[off-list ref] wrote:

quoted

On Fri, Jan 10, 2020 at 07:45:44AM +0100, Christophe Leroy wrote:

quoted

Le 09/01/2020 à 21:07, Segher Boessenkool a écrit :

quoted

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

If you think it should be fixed in gcc, maybe try to reproduce it in
https://godbolt.org/ and open a gcc bug against that.

Also, please try the gcc-10 snapshot, which has the highest chance
of getting fixes if it shows the same issue (or worse).

     Arnd

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