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Date:   Fri, 15 Sep 2023 11:45:19 -0400
From:   Charlie Jenkins <charlie@rivosinc.com>
To:     David Laight <David.Laight@aculab.com>
Cc:     Palmer Dabbelt <palmer@dabbelt.com>,
        Conor Dooley <conor@kernel.org>,
        Samuel Holland <samuel.holland@sifive.com>,
        "linux-riscv@lists.infradead.org" <linux-riscv@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>
Subject: Re: [PATCH v5 1/4] asm-generic: Improve csum_fold
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References: <20230914-optimize_checksum-v5-0-c95b82a2757e@rivosinc.com>
 <20230914-optimize_checksum-v5-1-c95b82a2757e@rivosinc.com>
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On Fri, Sep 15, 2023 at 07:29:23AM +0000, David Laight wrote:
> From: Charlie Jenkins
> > Sent: 15 September 2023 04:50
> > 
> > This csum_fold implementation introduced into arch/arc by Vineet Gupta
> > is better than the default implementation on at least arc, x86, arm, and
> > riscv. Using GCC trunk and compiling non-inlined version, this
> > implementation has 41.6667%, 25%, 16.6667% fewer instructions on
> > riscv64, x86-64, and arm64 respectively with -O3 optimization.
> 
> Nit-picking the commit message...
> Some of those architectures have their own asm implementation.
> The arm one is better than the C code below, the x86 ones aren't.
I can clean up the commit message to be more accurate.
> 
> I think that only sparc32 (carry flag but no rotate) and
> arm/arm64 (barrel shifter on every instruction) have versions
> that are better than the one here.
> 
> Since I suggested it to Charlie:
> 
> Reviewed-by: David Laight <david.laight@aculab.com>
> 
> > 
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
> >  include/asm-generic/checksum.h | 5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> > 
> > diff --git a/include/asm-generic/checksum.h b/include/asm-generic/checksum.h
> > index 43e18db89c14..adab9ac4312c 100644
> > --- a/include/asm-generic/checksum.h
> > +++ b/include/asm-generic/checksum.h
> > @@ -30,10 +30,7 @@ extern __sum16 ip_fast_csum(const void *iph, unsigned int ihl);
> >   */
> >  static inline __sum16 csum_fold(__wsum csum)
> >  {
> > -	u32 sum = (__force u32)csum;
> 
> You'll need to re-instate that line to stop sparse complaining.

I will add that back, thanks.

- Charlie
> 
> > -	sum = (sum & 0xffff) + (sum >> 16);
> > -	sum = (sum & 0xffff) + (sum >> 16);
> > -	return (__force __sum16)~sum;
> > +	return (__force __sum16)((~csum - ror32(csum, 16)) >> 16);
> >  }
> >  #endif
> > 
> > 
> > --
> > 2.42.0
> 
> -
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