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

[Xiao Zeng]

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