From: "Xiao Zeng" <zengxiao@eswincomputing.com>
To: "Torbjorn SVENSSON" <torbjorn.svensson@foss.st.com>,
"newlib@sourceware.org" <newlib@sourceware.org>
Cc: "vapier@gentoo.org" <vapier@gentoo.org>,
"palmer@rivosinc.com" <palmer@rivosinc.com>,
jeffreyalaw <jeffreyalaw@gmail.com>
Subject: Re: Re: [PATCH] newlib: libc: Improved the readability of strcspn with minor optimization
Date: Sat, 16 Dec 2023 17:30:50 +0800 [thread overview]
Message-ID: <202312161730500900107@eswincomputing.com> (raw)
In-Reply-To: <70b09ef7-fe4a-4669-ad27-2bd6c8bdca61@foss.st.com>
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
next prev parent reply other threads:[~2023-12-16 9:30 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-15 8:31 Xiao Zeng
2023-12-15 10:28 ` Torbjorn SVENSSON
2023-12-16 4:45 ` Xiao Zeng
2023-12-16 4:56 ` Xiao Zeng
2023-12-20 15:08 ` Torbjorn SVENSSON
2023-12-21 1:22 ` Xiao Zeng
2023-12-16 9:30 ` Xiao Zeng [this message]
2023-12-20 4:24 ` Jeff Johnston
2023-12-20 5:51 ` Xiao Zeng
2023-12-20 22:24 ` Brian Inglis
2023-12-21 8:00 ` Stefan Tauner
2023-12-21 17:47 ` Brian Inglis
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