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Date: Tue, 13 Jul 2021 11:22:36 -0300
From: √Črico Nogueira <ericonr@...root.org>
To: <musl@...ts.openwall.com>
Subject: Re: Changes for strcspn(), strspn(), strtok() and strtok_r()

On Tue Jul 13, 2021 at 9:02 AM -03, Stefan Kanthak wrote:
> <https://git.musl-libc.org/cgit/musl/plain/src/string/strcspn.c>
>
> #include <string.h>
>  
> #define BITOP(a,b,op) \
> ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof
> *(a))))
>  
> -size_t strcspn(const char *s, const char *c)
> +size_t strcspn(const char *restrict s, const char *c)
> {
> - const char *a = s;
> + const char *a;
> - size_t byteset[32/sizeof(size_t)];
> + size_t byteset[32/sizeof(size_t)] = { 0 };
>  
> - if (!c[0] || !c[1]) return __strchrnul(s, *c)-a;
> + if (!c[0] || !c[1]) return __strchrnul(a=s, *c)-a;
>  
> - memset(byteset, 0, sizeof byteset);
> for (; *c && BITOP(byteset, *(unsigned char *)c, |=); c++);
> - for (; *s && !BITOP(byteset, *(unsigned char *)s, &); s++);
> - return s-a;
> + for (a=s; *a && !BITOP(byteset, *(unsigned char *)a, &); a++);
> + return a-s;
> }

I don't know if it makes sense to give such a hint to the compiler, but
doing something like:

[...]
- size_t byteset[32/sizeof(size_t)];

- if (!c[0] || !c[1]) return __strchrnul(s, *c)-a;
+ if (!c[0] || !c[1]) return __strchrnul(a=s, *c)-a;

- memset(byteset, 0, sizeof byteset);
+ size_t byteset[32/sizeof(size_t)] = { 0 };
[...]

would possibly leave the idea about initialization order clearer, at the
expense of not following the code style.

>
> After this change, musl's favorite compiler (GCC) will generate code
> like the following (here for x86-64), just like the code it already
> generates for strspn(), where the initialization of byteset[] is NOT
> done via memset():
>
> strcspn:
> ...
> xor %eax, %eax
> movq %rax, byteset(%rsp)
> movq %rax, byteset+8(%rsp)
> movq %rax, byteset+16(%rsp)
> movq %rax, byteset+24(%rsp)
> ...
>
> <https://git.musl-libc.org/cgit/musl/plain/src/string/strspn.c>
>
> #include <string.h>
>  
> #define BITOP(a,b,op) \
> ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof
> *(a))))
>  
> -size_t strspn(const char *s, const char *c)
> +size_t strspn(const char *restrict s, const char *c)
> {
> - const char *a = s;
> + const char *a;
> size_t byteset[32/sizeof(size_t)] = { 0 };
>  
> if (!c[0]) return 0;
> if (!c[1]) {
> - for (; *s == *c; s++);
> - return s-a;
> + for (a=s; *a == *c; a++);
> + return a-s;
> }
>  
> for (; *c && BITOP(byteset, *(unsigned char *)c, |=); c++);
> - for (; *s && BITOP(byteset, *(unsigned char *)s, &); s++);
> - return s-a;
> + for (a=s; *a && BITOP(byteset, *(unsigned char *)a, &); a++);
> + return a-s;
> }

Does this improve register utilization? It would be nice to explain why
moving the assignment and changing the loop variable is an improvement.
(Sending full git patches helps with that ;)

This question applies to the previous diff as well.

>
> <https://git.musl-libc.org/cgit/musl/plain/src/string/strtok.c>
>
> #include <string.h>
>  
> char *strtok(char *restrict s, const char *restrict sep)
> {
> static char *p;
> + return strtok_r(s, sep, &p);
> - if (!s && !(s = p)) return NULL;
> - s += strspn(s, sep);
> - if (!*s) return p = 0;
> - p = s + strcspn(s, sep);
> - if (*p) *p++ = 0;
> - else p = 0;
> - return s;
> }

It's been this way since the initial check-in commit, I wonder if it was
intentional. Your way should save some bytes, though :)

>
> <https://git.musl-libc.org/cgit/musl/plain/src/string/strtok_r.c>
>
> #include <string.h>
>  
> char *strtok_r(char *restrict s, const char *restrict sep, char
> **restrict p)
> {
> if (!s && !(s = *p)) return NULL;
> s += strspn(s, sep);
> - if (!*s) return *p = 0;
> + if (!*s) return *p = NULL;
> *p = s + strcspn(s, sep);
> if (**p) *(*p)++ = 0;
> - else *p = 0;
> + else *p = NULL;
> return s;
> }

AFAIK musl style doesn't like using NULL, so 0 would always be
correct... Unless using NULL here is intentional to not confuse with the
string's null terminator?

>
> If you want to go a step further, avoid to build the same byteset twice:
>
> <https://git.musl-libc.org/cgit/musl/plain/src/string/strtok_r.c>
>
> #include <string.h>
>  
> char *strtok_r(char *restrict s, const char *restrict sep, char
> **restrict p)
> {
> + size_t byteset[32/sizeof(size_t)] = { 0 };
> +
> if (!s && !(s = *p)) return NULL;
> + if (!*s) return *p = NULL;
> + if (!*sep) return *p = NULL, *s;
> - s += strspn(s, sep);
> + for (; *c && BITOP(byteset, *(unsigned char *)c, |=); c++);
> + for (; *s && BITOP(byteset, *(unsigned char *)s, &); s++);
> - if (!*s) return *p = 0;
> + if (!*s) return *p = NULL;
> - *p = s + strcspn(s, sep);
> - if (**p) *(*p)++ = 0;
> - else *p = 0;
> - return s;
> + sep = s;
> + for (; *s && !BITOP(byteset, *(unsigned char *)s, &); s++);
> + if (*s) *s++ = 0;
> + else *s = NULL;
> + *p = s;
> + return sep;
> }
>
> Stefan

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