Re: [PATCH] LoongArch: Make -mstrict-align be configurable

[Jianmin Lv]

On 2023/2/6 下午9:30, Xi Ruoyao wrote:
> On Mon, 2023-02-06 at 21:13 +0800, Jianmin Lv wrote:
> > > (1) Is the difference contributed by a bad code generation of GCC?  If
> > > true, it's better to improve GCC before someone starts to build a distro
> > > for LA264 as it would benefit the user space as well.
> > AFAIK, GCC builds to produce unaligned-access-enabled target binary by
> > default (without -mstrict-align) for improving user space performance
> > (small size and runtime high performance), which is also based the fact
> > that the vast majority of LoongArch CPUs support unaligned-access.
>
> I mean: if someone starts to build a distro for a less-capable LoongArch
> processor, (s)he will need an entire user space compiled with -mstrict-
> align.  So it would be better to start preparation now.
>
> And it's likely (s)he will either submit a GCC patch to make GCC
> enable/disable -mstrict-align based on the -march= (--with-arch at
> configure time) value, or hack GCC to enable -mstrict-align by default
> for the distro.  So I think we'll also need:
>
> > +ifdef CONFIG_ARCH_STRICT_ALIGN may enable strict align by default.
> >   # Don't emit unaligned accesses.
> >   # Not all LoongArch cores support unaligned access, and as kernel we can't
> >   # rely on others to provide emulation for these accesses.
> >   KBUILD_CFLAGS += $(call cc-option,-mstrict-align)
>    +else
>    +# Distros designed for running on both kind of processors may disable
>    +# strict align by default, but the user may want a no-strict-align
>    +# kernel for his/her specific hardware.
>     KBUILD_CFLAGS += $(call cc-option,-mno-strict-align)
> > +endif

Thanks, Ruoyao, I think it's good suggestion. After talking about it 
with GCC colleague, it's very likely make GCC enable/disable 
-mstrict-align based on the -march= in future, just as you said.