Re: [PATCH v3] LoongArch/percpu: Simplify _percpu_read() and _percpu_write()

From: Uros Bizjak
Date: Thu Sep 05 2024 - 08:50:08 EST


On Thu, Sep 5, 2024 at 2:16 PM Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
>
> On Thu, Sep 5, 2024 at 2:09 PM Xi Ruoyao <xry111@xxxxxxxxxxx> wrote:
> >
> > On Thu, 2024-09-05 at 14:02 +0200, Uros Bizjak wrote:
> > > > If the input value is less than 0xff, then "& 0xff" is meaningless, if
> > > > the input value is more than 0xff, this conversion still cannot give a
> > > > correct result for the caller. So I think for all sizes it is enough
> > > > to just use "((unsigned long) val)".
> > >
> > > This part is used to force unsigned extension, otherwise the compiler
> > > will use sign-extension of the possibly signed variable.
> >
> > It's not relevant. For example when size is 2 __pcpu_op_##size("stx")
> > is expanded to stx.h, and stx.h only stores the lower 16 bits of a
> > register into MEM[r21 + ptr], the high bits are ignored anyway.
> >
> > Thus we can just have
> >
> > +#define _percpu_write(size, _pcp, _val) \
> > +do { \
> > + if (0) { \
> > + typeof(_pcp) pto_tmp__; \
> > + pto_tmp__ = (_val); \
> > + (void)pto_tmp__; \
> > + } \
> > + __asm__ __volatile__( \
> > + __pcpu_op_##size("stx") "%[val], $r21, %[ptr] \n" \
> > + : \
> > + : [val] "r"(_val), [ptr] "r"(&(_pcp)) \
> > + : "memory"); \
> > +} while (0)
>
> Nice, the less code, the better. If it works for loongson target, then
> we don't need this paranoia.
>
> I just played safe and took the approach that x86 took.

Please note that the original code extended the value to a long type.
If the simplified macro works, then the usage of macros will result in
a better assembly code, where zero-extends will be omitted.

Uros.