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@xxxxxxxxxx +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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