lkml.org 
[lkml]   [2003]   [Aug]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: generic strncpy - off-by-one error
Date
> -----Original Message-----
> From: Willy Tarreau [mailto:willy@w.ods.org]
> Sent: Thursday, August 14, 2003 21:45
> To: Peter Kjellerstedt
> Cc: 'Timothy Miller'; linux-kernel mailing list
> Subject: Re: generic strncpy - off-by-one error
>
> On Thu, Aug 14, 2003 at 11:34:50AM +0200, Peter Kjellerstedt wrote:
>
> > char *strncpy(char *dest, const char *src, size_t count)
> > {
> > char *tmp = dest;
> >
> > while (count) {
> > if (*src == '\0')
> > break;
> >
> > *tmp++ = *src++;
> > count--;
> > }
> >
> > while (count) {
> > *tmp++ = '\0';
> > count--;
> > }
> >
> > return dest;
> > }
> >
> > Moving the check for *src == '\0' into the first loop seems
> > to let the compiler reuse the object code a little better
> > (may depend on the optimizer). Also note that your version
> > of the second loop needs an explicit comparison with -1,
> > whereas mine uses an implicit comparison with 0.
>
> Well, if you're at this level of comparison, then I may object that
> 'count' is evaluated twice when jumping from one loop to the
> other one.
>
> *Algorithmically* speaking, the most optimal code should be :
>
> char *strncpy(char *dest, const char *src, size_t count)
> {
> char *tmp = dest;
> if (unlikely(!count))
> goto end;
> loop1:
> if ((*tmp = *src++) == '\0')
> goto next;
> tmp++;
> if (likely(--count))
> goto loop1;
> else
> goto end;
> loop2:
> *tmp = '\0';
> next:
> tmp++;
> if (likely(--count))
> goto loop2;
> end:
> return dest;
> }

I hope we do not opt to go with this version. :)

> (sorry for those who hate gotos). Look at it : no test is
> performed twice, no byte is read nor written twice in any
> case. The assembly code produced is optimal on x86 (well,
> in fact we could delete exactly one conditionnal jump because
> the compiler doesn't know how to reuse them for several tests).
> 16 inlinable instructions (= excluding function in/out) which
> can be executed 2 at a time if your CPU has 2 pipelines. about
> 3 cycles/copied byte, 2 cycles/filled byte.

It does not give optimal code for CRIS for the second loop.
It can easily be fixed by combining loop2 and next (which
will cause tmp to be assigned twice when changing loop),
but I know this was not the intent of this exercise.

> Unfortunately, it fools branch predictors and prefetch
> mechanisms found in today's processors, so it results in
> slower code than yours (at least on athlon and P3). Perhaps
> it would be fast on older processors, I don't know.
>
> All that demonstrates that whatever your efforts in helping
> the optimizer, the only meaningful result is the benchmark.
> Number of bytes and speculations on the reusability of
> information between lines of codes are not what makes our
> programs fast anymore :-(

It was easier some years ago, wasn't it? ;)

> > Testing on the CRIS architecture, your version is 24 instructions,
> > whereas mine is 18. For comparison, Eric's one is 12 and the
> > currently used implementation is 26 (when corrected for the
> > off-by-one error by comparing with > 1 rather than != 0 in the
> > second loop).
>
> Just out of curiosity, can the CRIS architecture execute
> multiple instructions per cycle ? have you tried to determine

No it cannot. :( Our ETRAX 100LX processor which uses the
CRIS architecture is a 100 MIPS processor without most of
today's fancy processor optimizations like multiple instructions,
branch prediction etc. It is designed for embedded products.

> the dependencies between the instructions ? Did you time the
> different functions ? (yours is rather fast on x86 BTW).

No I didn't, but I have done it now.

I made a program that called the respective strncpy()
implementation a number of times copying a 100 byte string
into a 1000 byte buffer. For each function I tested with
copying 50, 100, 200 and 1000 bytes. I have attached the
source to this mail if anyone wants to make sure I did not
screw up the benchmarking too much... ;)

This table shows the times in seconds to call each function
500,000 times for CRIS:

50 100 200 1000
Original 3.123906 6.138616 9.146204 33.323051
Eric's 2.602632 5.105367 9.637701 45.910798
Timothy's 3.125003 6.128571 9.147905 33.325337
My first 2.868396 5.626149 8.138134 28.276760
My second (for) 2.622412 5.129988 7.636364 27.786745
Algorithmic 2.597808 5.119332 7.627300 27.785999

Going by these numbers, Eric's version is a good candidate
for the copying phase, but loses out badly when it comes to
the filling. The best versions here seem to be my version
with the for loops and your algorithmic version which both
give almost identical numbers (but look below for more on
the algorithmic version).

This table shows the times in seconds to call each function
20,000,000 times for my P4 2.53GHz:

50 100 200 1000
Original 2.900444 5.475311 9.095701 37.462796
Eric's 2.988404 5.637352 9.576857 37.580639
Timothy's 2.929508 5.534824 9.157147 36.836899
My first 2.951068 5.511169 8.321381 29.669017
My second (for) 2.921675 5.531486 8.296728 28.940855
Algorithmic 3.911937 7.343235 12.699275 54.288284

The interesting thing here is that the original code wins
the copying phase. But the numbers for the copying phase for
the first five versions are so similar, that I doubt one can
say one is better than the other. However, when it comes to
the filling phase my versions are back in the lead. These
numbers also shows very clearly what you stated above about
branch predictions, as the algorithmic version loses out
badly here.

All these numbers taken together, I would say that my version
with the for loops seems to be the overall winner. :)

> To conclude, I would say that if we want to implement really
> fast low-level primitives such as str*, mem*, ... there's
> nothing better than assembly associated with benchmarks on
> different CPU architectures and models.
>
> But I don't know if strncpy() is used enough to justify that...

I agree

> Regards,
> Willy

//Peter

[unhandled content-type:application/octet-stream]
\
 
 \ /
  Last update: 2005-03-22 13:47    [W:0.044 / U:0.424 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site