Re: [PATCH v2] MIPS: Check __clang__ to avoid performance influence with GCC in csum_tcpudp_nofold()

[Tiezhu Yang]

On 03/15/2021 06:24 PM, Alexander Lobakin wrote:
> From: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
> Date: Tue, 9 Mar 2021 12:18:13 +0800
>
> > The asm code in csum_tcpudp_nofold() is performance-critical, I am sorry
> > for the poorly considered implementation about the performance influence
> > with GCC in the commit 198688edbf77 ("MIPS: Fix inline asm input/output
> > type mismatch in checksum.h used with Clang").
> >
> > With this patch, we can build successfully by both GCC and Clang,
> > at the same time, we can avoid the potential performance influence
> > with GCC.
> >
> > Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
> > ---
> >   arch/mips/include/asm/checksum.h | 12 ++++++++----
> >   1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/mips/include/asm/checksum.h b/arch/mips/include/asm/checksum.h
> > index 1e6c135..80eddd4 100644
> > --- a/arch/mips/include/asm/checksum.h
> > +++ b/arch/mips/include/asm/checksum.h
> > @@ -128,9 +128,13 @@ static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
> >
> >   static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
> >   					__u32 len, __u8 proto,
> > -					__wsum sum)
> > +					__wsum sum_in)
> >   {
> > -	unsigned long tmp = (__force unsigned long)sum;
> > +#ifdef __clang__
> Why not rely on CONFIG_CC_IS_CLANG here?

Hi,

Thanks for your suggestion, I once considered that way:
https://lore.kernel.org/patchwork/patch/1371666/#1587127

But it still occurs build error under CC_IS_GCC when
make M=samples/bpf which used with Clang compiler,
so use __clang__ is better.

Thanks,
Tiezhu

> > +	unsigned long sum = (__force unsigned long)sum_in;
> > +#else
> > +	__wsum sum = sum_in;
> > +#endif
> >
> >   	__asm__(
> >   	"	.set	push		# csum_tcpudp_nofold\n"
> > @@ -159,7 +163,7 @@ static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
> >   	"	addu	%0, $1		\n"
> >   #endif
> >   	"	.set	pop"
> > -	: "=r" (tmp)
> > +	: "=r" (sum)
> >   	: "0" ((__force unsigned long)daddr),
> >   	  "r" ((__force unsigned long)saddr),
> >   #ifdef __MIPSEL__
> > @@ -169,7 +173,7 @@ static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
> >   #endif
> >   	  "r" ((__force unsigned long)sum));
> >
> > -	return (__force __wsum)tmp;
> > +	return (__force __wsum)sum;
> >   }
> >   #define csum_tcpudp_nofold csum_tcpudp_nofold
> >
> > --
> > 2.1.0
> Al