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Re: [libvirt] [PATCH v1 2/2] command: Change virCommandAddEnv so it replaces existing environment variables.



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.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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