Re: Re: [PATCH] newlib: libc: Improved the readability of strcspn with minor optimization

["Xiao Zeng" <zengxiao@eswincomputing.com>]

2023-12-20 12:24  Jeff Johnston <jjohnstn@redhat.com> wrote:
>
 
>Patch merged to master. 
Thanks, Jeff.
There will be some similar patches in the future, and I will
submit them to the master immediately.

>
>-- Jeff J.
>
>On Sat, Dec 16, 2023 at 4:31 AM Xiao Zeng <zengxiao@eswincomputing.com>
>wrote:
>
>> 2023-12-15 18:28  Torbjorn SVENSSON <torbjorn.svensson@foss.st.com> wrote:
>> >
>>
>> >Hello Xiao,
>> >
>> >On 2023-12-15 09:31, Xiao Zeng wrote:
>> >> Signed-off-by: Xiao Zeng <zengxiao@eswincomputing.com>
>> >> ---
>> >>   newlib/libc/string/strcspn.c | 6 ++----
>> >>   1 file changed, 2 insertions(+), 4 deletions(-)
>> >>
>> >> diff --git a/newlib/libc/string/strcspn.c b/newlib/libc/string/strcspn.c
>> >> index abaa93ad6..8ac0bf10c 100644
>> >> --- a/newlib/libc/string/strcspn.c
>> >> +++ b/newlib/libc/string/strcspn.c
>> >> @@ -37,12 +37,10 @@ strcspn (const char *s1,
>> >>         for (c = s2; *c; c++)
>> >>   {
>> >>     if (*s1 == *c)
>> >> -        break;
>> >> +        goto end;
>> >>   }
>> >> -      if (*c)
>> >> -    break;
>> >>         s1++;
>> >>       }
>> >> -
>> >> +end:
>> >>     return s1 - s;
>> >>   }
>> >
>> >Just looking at this small snippet of code, I would say that the
>> >previous code and your suggestion won't do the same thing.
>> >
>> >Do you have unit tests that confirm that the behavior is identical with
>> >the current implementation and your suggested change?
>> >
>> >When I run your suggestion, I get return value 0, but with the current
>> >implementation it's 3 for this call: strspn("129th", "1234567890").
>> >
>> >Kind regards,
>> >Torbjörn
>>
>> After applying this patch, provide a comparison of assembly code
>> under the risc-v architecture, with default compilation parameters
>> used in both of them:
>>
>> no-patch:
>> ---------------------------------------------------------
>> libc_a-strcspn.o:     file format elf64-littleriscv
>>
>>
>> Disassembly of section .text:
>>
>> 0000000000000000 <strcspn>:
>>    0:   00054683                lbu     a3,0(a0)
>>    4:   06068263                beqz    a3,68 <.L9>
>>    8:   0005c803                lbu     a6,0(a1)
>>    c:   00050613                mv      a2,a0
>>
>> 0000000000000010 <.LVL1>:
>>   10:   02080463                beqz    a6,38 <.L14>
>>
>> 0000000000000014 <.L6>:
>>   14:   00058793                mv      a5,a1
>>   18:   00080713                mv      a4,a6
>>   1c:   00c0006f                j       28 <.L5>
>>
>> 0000000000000020 <.L4>:
>>   20:   0007c703                lbu     a4,0(a5)
>>   24:   02070863                beqz    a4,54 <.L17>
>>
>> 0000000000000028 <.L5>:
>>   28:   00178793                addi    a5,a5,1
>>
>> 000000000000002c <.LVL5>:
>>   2c:   fee69ae3                bne     a3,a4,20 <.L4>
>>
>> 0000000000000030 <.L7>:
>>   30:   40a60533                sub     a0,a2,a0
>>
>> 0000000000000034 <.LVL7>:
>>   34:   00008067                ret
>>
>> 0000000000000038 <.L14>:
>>   38:   00164683                lbu     a3,1(a2)
>>   3c:   00160613                addi    a2,a2,1
>>   40:   fe0688e3                beqz    a3,30 <.L7>
>>   44:   00164683                lbu     a3,1(a2)
>>   48:   00160613                addi    a2,a2,1
>>   4c:   fe0696e3                bnez    a3,38 <.L14>
>>   50:   fe1ff06f                j       30 <.L7>
>>
>> 0000000000000054 <.L17>:
>>   54:   00164683                lbu     a3,1(a2)
>>   58:   00160613                addi    a2,a2,1
>>   5c:   fa069ce3                bnez    a3,14 <.L6>
>>   60:   40a60533                sub     a0,a2,a0
>>
>> 0000000000000064 <.LVL13>:
>>   64:   00008067                ret
>>
>> 0000000000000068 <.L9>:
>>   68:   00000513                li      a0,0
>>
>> 000000000000006c <.LVL15>:
>>   6c:   00008067                ret
>> ---------------------------------------------------------
>>
>> patch
>> ---------------------------------------------------------
>> libc_a-strcspn.o:     file format elf64-littleriscv
>>
>>
>> Disassembly of section .text:
>>
>> 0000000000000000 <strcspn>:
>>    0:   00054683                lbu     a3,0(a0)
>>    4:   04068a63                beqz    a3,58 <.L2>
>>    8:   0005c803                lbu     a6,0(a1)
>>    c:   00050613                mv      a2,a0
>>
>> 0000000000000010 <.LVL1>:
>>   10:   02080c63                beqz    a6,48 <.L14>
>>
>> 0000000000000014 <.L6>:
>>   14:   00058793                mv      a5,a1
>>   18:   00080713                mv      a4,a6
>>   1c:   00c0006f                j       28 <.L5>
>>
>> 0000000000000020 <.L4>:
>>   20:   0007c703                lbu     a4,0(a5)
>>   24:   00070a63                beqz    a4,38 <.L16>
>>
>> 0000000000000028 <.L5>:
>>   28:   00178793                addi    a5,a5,1
>>
>> 000000000000002c <.LVL5>:
>>   2c:   fee69ae3                bne     a3,a4,20 <.L4>
>>
>> 0000000000000030 <.L7>:
>>   30:   40a60533                sub     a0,a2,a0
>>
>> 0000000000000034 <.LVL7>:
>>   34:   00008067                ret
>>
>> 0000000000000038 <.L16>:
>>   38:   00164683                lbu     a3,1(a2)
>>   3c:   00160613                addi    a2,a2,1
>>   40:   fc069ae3                bnez    a3,14 <.L6>
>>   44:   fedff06f                j       30 <.L7>
>>
>> 0000000000000048 <.L14>:
>>   48:   00164683                lbu     a3,1(a2)
>>   4c:   00160613                addi    a2,a2,1
>>   50:   fe069ce3                bnez    a3,48 <.L14>
>>   54:   fddff06f                j       30 <.L7>
>>
>> 0000000000000058 <.L2>:
>>   58:   00000513                li      a0,0
>>
>> 000000000000005c <.LVL12>:
>>   5c:   00008067                ret
>> ---------------------------------------------------------
>> After careful comparison, it was found that there are fewer assembly
>> instructions after the patch.
>>
>> Thanks
>> Xiao Zeng
>>
>>
 
Thanks
Xiao Zeng