musl - Re: [PATCH] improve strerror speed

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Message-ID: <alpine.LNX.2.20.13.2003041151460.17709@monopod.intra.ispras.ru>
Date: Wed, 4 Mar 2020 12:06:21 +0300 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
cc: Timo Teräs <timo.teras@....fi>
Subject: Re: [PATCH] improve strerror speed

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

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