On Mon, Sep 24, 2012 at 02:07:36PM -0600, Eric Blake wrote:
> On 09/24/2012 01:53 PM, Richard W.M. Jones wrote:
>
> >>> + /* Search for the name in the existing environment. */
> >>> + namelen = strcspn(env, "=");
> >>
> >> Would 'strchr(env, '=') - env' be any more efficient? But that's a
> >> micro-optimization, probably not worth worrying about.
> >
> > I guess I trust glibc or gcc to have these string primitives
> > optimized better than I could.
>
> Ah, but glibc is open source, so we can check for ourselves:
>
> The naive C fallback when no .S is present is highly unoptimized. From
> glibcc/string/strcspn.c:
>
> size_t
> strcspn (s, reject)
> const char *s;
> const char *reject;
> {
> size_t count = 0;
>
> while (*s != '\0')
> if (strchr (reject, *s++) == NULL)
> ++count;
> else
> return count;
>
> return count;
> }
>
> and even in the .S optimized versions, there's still no shortcuts taken
> for a one-character reject (possibly worth filing a BZ about the missed
> optimization, though). From glibc/sysdeps/x86_64/strcspn.S:
> /* First we create a table with flags for all possible characters.
> For the ASCII (7bit/8bit) or ISO-8859-X character sets which are
> supported by the C string functions we have 256 characters.
> Before inserting marks for the stop characters we clear the whole
> table. */
> movq %rdi, %r8 /* Save value. */
> subq $256, %rsp /* Make space for 256 bytes. */
> cfi_adjust_cfa_offset(256)
> movl $32, %ecx /* 32*8 bytes = 256 bytes. */
> movq %rsp, %rdi
> xorl %eax, %eax /* We store 0s. */
> ...
>
> That is, you are definitely wasting time pre-computing the reject table,
> compared to doing a strchr() for the one rejection.
Possibly, if we weren't just about to spend 100000 cycles doing a fork.
Now, a project to rewrite all Python utilities in Linux in a compiled
high-level language like OCaml, there's something I could get behind :-)
-*-
Unfortunately this patch does not fix the bug, but it now
fails in a different, and stranger way:
libvir: error : libvirtd quit during handshake: Input/output error
In this code:
if ((rv = saferead(cmd->handshakeNotify[0], &c,
sizeof(c))) != sizeof(c)) {
if (rv < 0)
virReportSystemError(errno, "%s",
_("Unable to wait on parent process"));
else
virReportSystemError(EIO, "%s",
_("libvirtd quit during handshake"));
return -1;
}
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines. Supports shell scripting,
bindings from many languages. http://libguestfs.org
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