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

[Tiezhu Yang]

On 03/09/2021 12:52 AM, David Laight wrote:
> From: Tiezhu Yang
> > Sent: 08 March 2021 12:50
> >
> > 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 | 10 ++++++++++
> >   1 file changed, 10 insertions(+)
> >
> > diff --git a/arch/mips/include/asm/checksum.h b/arch/mips/include/asm/checksum.h
> > index 1e6c135..64d353e 100644
> > --- a/arch/mips/include/asm/checksum.h
> > +++ b/arch/mips/include/asm/checksum.h
> > @@ -130,7 +130,9 @@ static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
> >   					__u32 len, __u8 proto,
> >   					__wsum sum)
> >   {
> > +#ifdef __clang__
> >   	unsigned long tmp = (__force unsigned long)sum;
> > +#endif
> What happens if you make the above:
> #ifdef __clang__
> 	unsigned long tmp = (__force unsigned long)sum;
> #else
> 	__wsum tmp = sum;
> #endif
> 	
> and then leave the rest of the function the same for both compilers.
> Maybe do s/sum/sum_in/,s/tmp/sum/ to reduce the changes.

Hi David,

Thank you very much.

As you suggested, the following changes looks much better,
I will test it and then send v2 later.

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__
+       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

Thanks,
Tiezhu

> 	David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)