Date: Tue, 5 May 2009 12:34:20 +0530 From: venki kaps <venkiece2005@gmail.com> To: Mark Tinguely <tinguely@casselton.net> Cc: channa.kad@gmail.com, freebsd-arm@freebsd.org Subject: Re: strncmp issue Message-ID: <6d53329e0905050004k27d957bve15d37113b5fedbb@mail.gmail.com> In-Reply-To: <200905041259.n44CxtV2077931@casselton.net> References: <515c64960905032352j25edfbcajb3d146fa769b97bb@mail.gmail.com> <200905041259.n44CxtV2077931@casselton.net>
next in thread | previous in thread | raw e-mail | index | archive | help
Hi,
I have tested strncmp("abcdefg", "abcdefh", 6) without beq 2f.
It returns zero not -1.
I have checked with conditional assembler but not normal assembler.
The beq 2f is required for normal assembler.
Right/Wrong?
However also checked with strncmp.c implementation:
int strnncmp(const char *s1, const char *s2, size_t n) {
if (n == 0)
return (0);
do {
if (*s1 != *s2++)
return (*(const unsigned char *)s1 -
*(const unsigned char *)--s2);
if (*s1++ == 0)
break;
} while (--n != 0);
return (0);
}
the equivalent assembly code:
cmp r2, #0
mov ip, r0
beq 2f
1:
ldrb r0, [ip, #0]
ldrb r3, [r1], #1
add ip, ip, #1
cmp r0, r3
bne 3f
cmp r0, #0
beq 2f
subs r2, r2, #1
bne 1b
2:
mov r0, #0
mov pc, lr
3:
ldrb r3, [r1, #-1]
rsb r0, r3, r0 /* operand2 - operand1 */
mov pc, lr
The results are same.
what could you suggest?
Thanks & Regards,
Venkappa
On Mon, May 4, 2009 at 6:29 PM, Mark Tinguely <tinguely@casselton.net>wrote:
>
> Hi, the propose code by both of you two:
>
> /* if (len == 0) return 0 */
> cmp r2, #0
> moveq r0, #0
> moveq pc, lr
>
> /* ip == last src address to compare */
> add ip, r0, r2
> 1:
> ldrb r2, [r0], #1
> ldrb r3, [r1], #1
> cmp ip, r0
> cmp r2, #1
> cmpcs r2, r3
> beq 1b
> sub r0, r2, r3
>
> That was one of my failed attempts. I to was hoping to not add the branch
> to cut down in cycles. A person has to test every possible call to strncmp.
> This will fail on a positive string length less than strlen length of the
> input strings:
>
> strncmp("abcdefg", "abcdefh", 6)
>
> Will return (-1) str1 < str2 at 6 characters which is wrong.
>
>
> /* if (len == 0) return 0 */
> cmp r2, #0
> moveq r0, #0
> moveq pc, lr
>
> /* ip == last src address to compare */
> add ip, r0, r2
> 1:
> ldrb r2, [r0], #1
> ldrb r3, [r1], #1
> cmp ip, r0
> beq 2f <- stops in the case where strlen(s1) > len
> cmp r2, #1 <- stops thea NULL case, we can't just
> change
> the comparison because below we loop on
> an equality and can end up in a big loop
>
> cmpcs r2, r3 <- compare characters.
> beq 1b
> 2:
> sub r0, r2, r3
>
>
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?6d53329e0905050004k27d957bve15d37113b5fedbb>