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

[Jeff Johnston]

Patch merged to master.

-- 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
>
>
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