Re: [PATCH] riscv: hweight: relax assembly constraints

[Qingfang Deng]

Hi,

On Fri, May 24, 2024 at 9:02 AM Wang, Xiao W <xiao.w.wang@xxxxxxxxx> wrote:
>
>
>
> > -----Original Message-----
> > From: Qingfang Deng <dqfext@xxxxxxxxx>
> > Sent: Thursday, May 23, 2024 5:43 PM
> > To: Paul Walmsley <paul.walmsley@xxxxxxxxxx>; Palmer Dabbelt
> > <palmer@xxxxxxxxxxx>; Albert Ou <aou@xxxxxxxxxxxxxxxxx>; linux-
> > riscv@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > Cc: Wang, Xiao W <xiao.w.wang@xxxxxxxxx>; Qingfang Deng
> > <qingfang.deng@xxxxxxxxxxxxxxx>
> > Subject: [PATCH] riscv: hweight: relax assembly constraints
> >
> > From: Qingfang Deng <qingfang.deng@xxxxxxxxxxxxxxx>
> >
> > rd and rs don't have to be the same.
> >
> > Signed-off-by: Qingfang Deng <qingfang.deng@xxxxxxxxxxxxxxx>
> > ---
> >  arch/riscv/include/asm/arch_hweight.h | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/arch_hweight.h
> > b/arch/riscv/include/asm/arch_hweight.h
> > index 85b2c443823e..613769b9cdc9 100644
> > --- a/arch/riscv/include/asm/arch_hweight.h
> > +++ b/arch/riscv/include/asm/arch_hweight.h
> > @@ -26,9 +26,9 @@ static __always_inline unsigned int
> > __arch_hweight32(unsigned int w)
> >
> >       asm (".option push\n"
> >            ".option arch,+zbb\n"
> > -          CPOPW "%0, %0\n"
> > +          CPOPW "%0, %1\n"
> >            ".option pop\n"
> > -          : "+r" (w) : :);
> > +          : "=r" (w) : "r" (w) :);
>
> The above code piece takes variable "w" as both input and output, so intuitively, the previous
> patch made rs and rd the same.
> Though rs and rd can be different, do you see performance difference with this change?
> Or any analysis from assembly dump?

By making rs and rd different, we can save some `mv` instructions.

>
> BRs,
> Xiao
>
> >
> >       return w;
> >
> > @@ -57,9 +57,9 @@ static __always_inline unsigned long
> > __arch_hweight64(__u64 w)
> >
> >       asm (".option push\n"
> >            ".option arch,+zbb\n"
> > -          "cpop %0, %0\n"
> > +          "cpop %0, %1\n"
> >            ".option pop\n"
> > -          : "+r" (w) : :);
> > +          : "=r" (w) : "r" (w) :);
> >
> >       return w;
> >
> > --
> > 2.34.1
>