My new distro "Glacies"

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	josuah Offline \| 18-01-2020, 12:12 PM \| #16

(15-01-2020, 04:31 PM)eadwardus Wrote: calling realloc with just the required space is not optimal

I think I express poorly. So I will express myself in term of other people's code. :)

Here is the mechanism I am talking about in DJB code (skalibs):

Code:
int stralloc_ready_tuned (stralloc *sa, size_t n, size_t base, size_t a, size_t b)

{

  [...]

  t = n + base + a * n / b ;

  [...]

}

#define stralloc_ready(sa, n) stralloc_ready_tuned(sa, (n), 8, 1, 8)

int stralloc_catb (stralloc *sa, char const *s, size_t n)

{

  if (!stralloc_readyplus(sa, n)) return 0 ;

  [...]

}

https://github.com/skarnet/skalibs/blob/...uned.c#L12
https://github.com/skarnet/skalibs/blob/...lloc.h#L21
https://github.com/skarnet/skalibs/blob/...tb.c#L6-L8

Here is how it is already done in a few libcs.

Musl:

Code:
static int adjust_size(size_t *n)

{

    /* Result of pointer difference must fit in ptrdiff_t. */

    if (*n-1 > PTRDIFF_MAX - SIZE_ALIGN - PAGE_SIZE) {

        if (*n) {

            errno = ENOMEM;

            return -1;

        } else {

            *n = SIZE_ALIGN;

            return 0;

        }

    }

    *n = (*n + OVERHEAD + SIZE_ALIGN - 1) & SIZE_MASK;

    return 0;

}

void *realloc(void *p, size_t n)

{

    [...]

    if (adjust_size(&n) < 0) return 0;

    [...]

}

http://git.musl-libc.org/cgit/musl/tree/...loc.c#n377

OpenBSD:

Code:
#define PAGEROUND(x)  (((x) + (MALLOC_PAGEMASK)) & ~MALLOC_PAGEMASK)

static void *

orealloc(struct dir_info **argpool, void *p, size_t newsz, void *f)

{

    [...]

        size_t rnewsz = PAGEROUND(gnewsz);

    [...]

}

https://cvsweb.openbsd.org/src/lib/libc/...web-markup

Glibc:

Code:
/* pad request bytes into a usable size -- internal version */

#define request2size(req)                                         \

  (((req) + SIZE_SZ + MALLOC_ALIGN_MASK < MINSIZE)  ?             \

   MINSIZE :                                                      \

   ((req) + SIZE_SZ + MALLOC_ALIGN_MASK) & ~MALLOC_ALIGN_MASK)

void *

__libc_realloc (void *oldmem, size_t bytes)

{

  [...]

    if (!checked_request2size (bytes, &nb))

    {

      __set_errno (ENOMEM);

      return NULL;

    }

  [...]

}

https://sourceware.org/git/?p=glibc.git;...HEAD#l3130

But then, there still might be something I missed about that, which makes stralloc's (->a) size_t still useful. This happen often to me. I read some other code, and then I get what I read a day ago was there...

In any way, there is not much benefit besides a single function removed (in case of skarnet's code), and no performance benefit! Just me choosing the color of the bicycle shedding...