Re: [musl] [PATCH] improve strerror speed

[Alexander Monakov <amonakov@ispras.ru>]

[-- Attachment #1: Type: text/plain, Size: 1596 bytes --]

Hi,

On Tue, 3 Mar 2020, Timo Teräs wrote:

> change the current O(n) lookup to O(1) based on the machinery
> described in "How To Write Shared Libraries" (Appendix B).

I'm curious about the background of this change, did the inefficiency
came up in practice?

> --- a/src/errno/__strerror.h
> +++ b/src/errno/__strerror.h
> @@ -1,8 +1,9 @@
> -/* This file is sorted such that 'errors' which represent exceptional
> - * conditions under which a correct program may fail come first, followed
> - * by messages that indicate an incorrect program or system failure. The
> - * macro E() along with double-inclusion is used to ensure that ordering
> - * of the strings remains synchronized. */
> +/* The first '0' mapping will be used for error codes that
> + * are not explicitly mentioned here.
> + * This file is included multiple times to generate struct
> + * populate it's content and create a fast lookup index to it. */

The last sentence seems to have typos ("a struct,", "its").
I would write the comment like this:

/* The first entry is a catch-all for codes not enumerated here.
 * This file is included multiple times to declare and define a structure
 * with messages, and then to define a lookup table translating error codes
 * to offsets of corresponding fields in the structure. */

> +	if (e < 0 || e >= sizeof(errmsgidx)/sizeof(errmsgidx[0])) e = 0;

I think usually in musl such range checks are written in an easier-to-optimize
form that tests the argument in an unsigned type, e.g. like this:

  if ((size_t)e >= sizeof errmsgidx / sizeof *errmsgidx) e = 0;

Thanks.
Alexander