Re: [PATCH, AArch64] Disable reg offset in quad-word store for Falkor.

[Andrew Pinski <pinskia at gmail dot com>]

On Fri, Sep 22, 2017 at 8:59 AM, Jim Wilson <jim.wilson@linaro.org> wrote:
> On Fri, Sep 22, 2017 at 8:49 AM, Jim Wilson <jim.wilson@linaro.org> wrote:
>> On Falkor, because of an idiosyncracy of how the pipelines are designed, a
>> quad-word store using a reg+reg addressing mode is almost twice as slow as an
>> add followed by a quad-word store with a single reg addressing mode.  So we
>> get better performance if we disallow addressing modes using register offsets
>> with quad-word stores.
>
> This was tested with a bootstrap and make check of course.  Also,
> gcc.c-torture/compile/20021212-1.c compiled with -O3 -mcpu=falkor
> makes a nice testcase to demonstrate that the patch works.
>
> OK?

Two overall comments:
* What about splitting register_offset into two different elements,
one for non 128bit modes and one for 128bit (and more; OI, etc.) modes
so you get better address generation right away for the simd load
cases rather than having LRA/reload having to reload the address into
a register.
* Maybe adding a testcase to the testsuite to show this change.

One extra comment:
* should we change the generic tuning to avoid reg+reg for 128bit modes?

Thanks,
Andrew

>
> Jim