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

[Tiezhu Yang <yangtiezhu@loongson.cn>]

On 03/15/2021 04:49 AM, Maciej W. Rozycki wrote:
> On Tue, 9 Mar 2021, Tiezhu Yang wrote:
>
>> 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
>   This looks much better to me, but I'd keep the variable names unchanged
> as `sum_in' isn't used beyond the initial assignment anyway (you'll have
> to update the references with asm operands accordingly of course).
>
>   Have you verified that code produced by GCC remains the same with your
> change in place as it used to be up to commit 198688edbf77?  I can see no
> such information in the commit description whether here or in the said
> commit.
>
>    Maciej

Hi Maciej,

Thanks for your reply.

gcc --version
gcc (Debian 10.2.1-6) 10.2.1 20210110

net/ipv4/tcp_ipv4.c
tcp_v4_send_reset()
   csum_tcpudp_nofold()

objdump -d vmlinux > vmlinux.dump

(1) Before commit 198688edbf77
("MIPS: Fix inline asm input/output type mismatch in checksum.h used 
with Clang"):

ffffffff80aa835c:       00004025        move    a4,zero
ffffffff80aa8360:       92020012        lbu     v0,18(s0)
ffffffff80aa8364:       de140030        ld      s4,48(s0)
ffffffff80aa8368:       0064182d        daddu   v1,v1,a0
ffffffff80aa836c:       304200ff        andi    v0,v0,0xff
ffffffff80aa8370:       9c64000c        lwu     a0,12(v1)
ffffffff80aa8374:       9c660010        lwu     a2,16(v1)
ffffffff80aa8378:       afa70038        sw      a3,56(sp)
ffffffff80aa837c:       24071a00        li      a3,6656
ffffffff80aa8380:       0086202d        daddu   a0,a0,a2
ffffffff80aa8384:       0087202d        daddu   a0,a0,a3
ffffffff80aa8388:       0088202d        daddu   a0,a0,a4
ffffffff80aa838c:       0004083c        dsll32  at,a0,0x0
ffffffff80aa8390:       0081202d        daddu   a0,a0,at
ffffffff80aa8394:       0081082b        sltu    at,a0,at
ffffffff80aa8398:       0004203f        dsra32  a0,a0,0x0
ffffffff80aa839c:       00812021        addu    a0,a0,at

(2) After commit 198688edbf77
("MIPS: Fix inline asm input/output type mismatch in checksum.h used 
with Clang"):

ffffffff80aa836c:       00004025        move    a4,zero
ffffffff80aa8370:       92040012        lbu     a0,18(s0)
ffffffff80aa8374:       de140030        ld      s4,48(s0)
ffffffff80aa8378:       0062182d        daddu   v1,v1,v0
ffffffff80aa837c:       308400ff        andi    a0,a0,0xff
ffffffff80aa8380:       9c62000c        lwu     v0,12(v1)
ffffffff80aa8384:       9c660010        lwu     a2,16(v1)
ffffffff80aa8388:       afa70038        sw      a3,56(sp)
ffffffff80aa838c:       24071a00        li      a3,6656
ffffffff80aa8390:       0046102d        daddu   v0,v0,a2
ffffffff80aa8394:       0047102d        daddu   v0,v0,a3
ffffffff80aa8398:       0048102d        daddu   v0,v0,a4
ffffffff80aa839c:       0002083c        dsll32  at,v0,0x0
ffffffff80aa83a0:       0041102d        daddu   v0,v0,at
ffffffff80aa83a4:       0041082b        sltu    at,v0,at
ffffffff80aa83a8:       0002103f        dsra32  v0,v0,0x0
ffffffff80aa83ac:       00411021        addu    v0,v0,at

(3) With this patch:

ffffffff80aa835c:       00004025        move    a4,zero
ffffffff80aa8360:       92020012        lbu     v0,18(s0)
ffffffff80aa8364:       de140030        ld      s4,48(s0)
ffffffff80aa8368:       0064182d        daddu   v1,v1,a0
ffffffff80aa836c:       304200ff        andi    v0,v0,0xff
ffffffff80aa8370:       9c64000c        lwu     a0,12(v1)
ffffffff80aa8374:       9c660010        lwu     a2,16(v1)
ffffffff80aa8378:       afa70038        sw      a3,56(sp)
ffffffff80aa837c:       24071a00        li      a3,6656
ffffffff80aa8380:       0086202d        daddu   a0,a0,a2
ffffffff80aa8384:       0087202d        daddu   a0,a0,a3
ffffffff80aa8388:       0088202d        daddu   a0,a0,a4
ffffffff80aa838c:       0004083c        dsll32  at,a0,0x0
ffffffff80aa8390:       0081202d        daddu   a0,a0,at
ffffffff80aa8394:       0081082b        sltu    at,a0,at
ffffffff80aa8398:       0004203f        dsra32  a0,a0,0x0
ffffffff80aa839c:       00812021        addu    a0,a0,at

(4) With the following changes based on commit 198688edbf77
("MIPS: Fix inline asm input/output type mismatch in checksum.h used 
with Clang"):

diff --git a/arch/mips/include/asm/checksum.h 
b/arch/mips/include/asm/checksum.h
index 1e6c135..e1f80407 100644
--- a/arch/mips/include/asm/checksum.h
+++ b/arch/mips/include/asm/checksum.h
@@ -130,7 +130,11 @@ 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;
+#else
+    __wsum tmp = sum;
+#endif

      __asm__(
      "    .set    push        # csum_tcpudp_nofold\n"

ffffffff80aa835c:       00004025        move    a4,zero
ffffffff80aa8360:       92020012        lbu     v0,18(s0)
ffffffff80aa8364:       de140030        ld      s4,48(s0)
ffffffff80aa8368:       0064182d        daddu   v1,v1,a0
ffffffff80aa836c:       304200ff        andi    v0,v0,0xff
ffffffff80aa8370:       9c64000c        lwu     a0,12(v1)
ffffffff80aa8374:       9c660010        lwu     a2,16(v1)
ffffffff80aa8378:       afa70038        sw      a3,56(sp)
ffffffff80aa837c:       24071a00        li      a3,6656
ffffffff80aa8380:       0086202d        daddu   a0,a0,a2
ffffffff80aa8384:       0087202d        daddu   a0,a0,a3
ffffffff80aa8388:       0088202d        daddu   a0,a0,a4
ffffffff80aa838c:       0004083c        dsll32  at,a0,0x0
ffffffff80aa8390:       0081202d        daddu   a0,a0,at
ffffffff80aa8394:       0081082b        sltu    at,a0,at
ffffffff80aa8398:       0004203f        dsra32  a0,a0,0x0
ffffffff80aa839c:       00812021        addu    a0,a0,at

The code produced by GCC remains the same between (1), (3) and (4),
the last changes looks like better (with less changes based on commit
198688edbf77), so I will send v3 later.

Thanks,
Tiezhu