[U-Boot] [PATCH 1/2] armv8: apply -mgeneral-regs-only

[Peng Fan]

> -----Original Message-----
> From: Dr. Philipp Tomsich [mailto:philipp.tomsich at theobroma-systems.com]
> Sent: Wednesday, July 05, 2017 4:38 PM
> To: Peng Fan <peng.fan at nxp.com>
> Cc: Tom Rini <trini at konsulko.com>; u-boot at lists.denx.de
> Subject: Re: [U-Boot] [PATCH 1/2] armv8: apply -mgeneral-regs-only
> 
> 
> > On 05 Jul 2017, at 03:23, Peng Fan <peng.fan at nxp.com> wrote:
> >
> >
> >
> >> -----Original Message-----
> >> From: Tom Rini [mailto:trini at konsulko.com]
> >> Sent: Wednesday, July 05, 2017 3:38 AM
> >> To: Dr. Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> >> Cc: Peng Fan <peng.fan at nxp.com>; u-boot at lists.denx.de
> >> Subject: Re: [U-Boot] [PATCH 1/2] armv8: apply -mgeneral-regs-only
> >>
> >> On Tue, Jul 04, 2017 at 06:38:10PM +0200, Dr. Philipp Tomsich wrote:
> >>> Tom,
> >>>
> >>> Please note that some GCC versions had a code-generation bug when
> >>> -mgeneral-regs-only was used (we hit this for a customer in a vendor
> >>> GCC we maintain):
> >>> 	https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64304
> >>>
> >>> Until everyone upgrades to a recently recent GCC (or until UBoot
> >>> enforces this next January), some people may hit hard-to-reproduce
> >>> issues.
> >>>
> >>> A better way to suppress SIMD generation would be to use:
> >>> 	-march=armv8-a+nosimd
> >>
> >> Ah, good to know.  Peng, please re-spin.
> >
> > According to https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html,
> > There is no +nosimd for armv8-a. I also tried +nosimd,
> 
> This is odd. When you look at
> 	gcc/config/aarch64/aarch64-option-extensions.def
> the “simd” option was there since the beginning of time for AArch64 (e.g. on
> the gcc_4.8-branch — see https://github.com/gcc-mirror/gcc/blob/gcc-4_8-
> branch/gcc/config/aarch64/aarch64-option-extensions.def)
> and is still there today (see https://github.com/gcc-
> mirror/gcc/blob/master/gcc/config/aarch64/aarch64-option-extensions.def)

I am not GCC expert, what I can follow is the doc, although doc is not always accurate.

> 
> I just tested the “armv8-a+nosimd” specification on (configured as aarch64-*-
> linux-gnueabi) 4.9.1, 4.9.3, 5.4.0, 6.3.0 and (configured as aarch64-*-elf) 6.3.0
> and 7.1.0… …unfortunately this was all that I could find installed on our target
> systems.
> 

Then did you see simd registers are used?

> > There is still "str     q0, [x29,#112] ".  If I use armv8-a+nofp, there is no
> > such instructions. Then, do you agree using " armv8-a+nofp “?
> 
> I don’t mind if we go with +nofp, but I would like to understand how it comes
> that your GCC differs from the FSF-released GCC source tree… this worries me
> a bit, as users might encounter similar differences in the field.

I tried yocto 6.2.0 and android 4.9.x toolchain, both could generate instructions that use
simd registers, not simd instructions.

Regards,
Peng.
> 
> > Thanks,
> > Peng.
> >
> >>
> >>>
> >>> Regards,
> >>> Philipp.
> >>>
> >>>> On 04 Jul 2017, at 15:32, Tom Rini <trini at konsulko.com> wrote:
> >>>>
> >>>> On Mon, Jul 03, 2017 at 09:14:07PM +0800, Peng Fan wrote:
> >>>>
> >>>>> Pass -mgeneral-regs-only to GCC, if not compiler may generate
> >>>>> instructions that use SIMD registers.
> >>>>>
> >>>>> Signed-off-by: Peng Fan <peng.fan at nxp.com>
> >>>>> Cc: Albert Aribaud <albert.u.boot at aribaud.net>
> >>>>> Cc: Tom Rini <trini at konsulko.com>
> >>>>
> >>>> Reviewed-by: Tom Rini <trini at konsulko.com>
> >>>>
> >>>> --
> >>>> Tom
> >>>> _______________________________________________
> >>>> U-Boot mailing list
> >>>> U-Boot at lists.denx.de
> >>>> https://lists.denx.de/listinfo/u-boot
> >>>
> >>
> >> --
> >> Tom