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