Re: [PATCH 1/1] string: Add stpecpy(3)

Re: [PATCH 1/1] string: Add stpecpy(3)

[Wilco Dijkstra]

Hi Alex,

> For that, we'd first need to discuss what is a typical scenario.

Like copying/concatenating strings that fit within the buffer.

> And also, it depends a lot on what the compiler can optimize.  If I call 
> strlcat(3) in a loop, I know that stpecpy(3) is going to be orders of magnitude 
> faster.

If you're trying to say that the 'strcat' variant is bad then yes absolutely -
it's better to inline in the compiler or avoid the 'strcat' versions altogether
(that's also why I would strongly suggest never to add more 'cat' variants).
But that doesn't say anything about whether stpecpy is better than strlcpy.

> If I call strlcpy(3) in a loop, doing what an ideal compiler might do, that 
> might be something to benchmark, but we'd also need to discuss what is a good 
> input for the benchmark.

The typical case would be copying or concatenating smallish strings to a buffer.

> In the OpenBSD definition of strlcpy(), I count 4 branches, and one of them is 
> inside a while loop.  So, I'd find it very surprising if strlcpy(3) outperformed 
> stpecpy(3).

If that really is the OpenBSD implementation then this proves my point that
non-standard string functions are often totally unoptimized.

A basic implementation of strlcpy would use strlen and memcpy so it is fast
on every system without requiring any optimization:

size_t
strlcpy (char *dst, const char *src, size_t size)
{
  size_t len = strlen (src);

  if (size == 0)
    return len;
  size = len >= size ? size - 1 : len;
  dst[size] = 0;
  memcpy (dst, src, size);
  return len;
}

> Well, with the current memccpy(3) I already suspect it's going to be faster than 
> strlcpy(3).  If you optimize it, it would increase the chances that it's faster :)

I don't see why it would be any faster given memccpy might also not be
optimized.

> I find it _way_ more readable than the strlcpy(3)/cat(3) code.  Oh, and did I 
> say it has less branches? :)

I'm not so sure about that - you've got 3 call/returns plus at least 4 branches
for each stpecpy (besides whatever memcpy/memchr do). strlcpy has 2 calls/
returns plus one branch. So needing an extra branch in case you need to do
something special for the buffer full case doesn't seem like a major problem.

>> In contrast we can be pretty sure that the standard strlen, memcpy etc are both
>> correct and efficient on all targets/libc's.
>
> Sure, but memcpy(3) is not usable in code that needs to truncate.  We need to 
> compare against stpncpy(3) (ughhh) and strlcpy(3).

The idea is that if we add new string functions, their implementation should use
other string functions that are known to be well optimized for most targets.

Cheers,
Wilco

Re: [PATCH 1/1] string: Add stpecpy(3)

[Alejandro Colomar]

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On 12/24/22 00:24, Wilco Dijkstra wrote:
> Hi Alex,
> 
>> For that, we'd first need to discuss what is a typical scenario.
> 
> Like copying/concatenating strings that fit within the buffer. >
>> And also, it depends a lot on what the compiler can optimize.  If I call
>> strlcat(3) in a loop, I know that stpecpy(3) is going to be orders of magnitude
>> faster.
> 
> If you're trying to say that the 'strcat' variant is bad then yes absolutely -

I must admit that it has good things, if you have a compiler that does the magic 
for you.  GCC optimizes strcat(3) into stpcpy(3), so if you know that it will be 
optimized, it's not so bad, and the source code is cleaner to the eye.

> it's better to inline in the compiler or avoid the 'strcat' versions altogether
> (that's also why I would strongly suggest never to add more 'cat' variants).

inlining in the compiler is a good solution.  And yes, I agree on not adding cat 
variants, but at the same time, str functions are problematic in that they can't 
be chained, as opposed to stp functions.  That problem shows itself in the 
snprintf(3) bugs I mentioned.  Users need a way to catenate easily, even if not 
with a 'cat' function.

> But that doesn't say anything about whether stpecpy is better than strlcpy.
> 
>> If I call strlcpy(3) in a loop, doing what an ideal compiler might do, that
>> might be something to benchmark, but we'd also need to discuss what is a good
>> input for the benchmark.
> 
> The typical case would be copying or concatenating smallish strings to a buffer.

Okay, I'll try to prepare a benchmark.

> 
>> In the OpenBSD definition of strlcpy(), I count 4 branches, and one of them is
>> inside a while loop.  So, I'd find it very surprising if strlcpy(3) outperformed
>> stpecpy(3).
> 
> If that really is the OpenBSD implementation then this proves my point that
> non-standard string functions are often totally unoptimized.

And not only that, but I find your version much more readable.  I don't 
understand how the OpenBSD version was written that way, and hasn't been fixed 
so far.

> 
> A basic implementation of strlcpy would use strlen and memcpy so it is fast
> on every system without requiring any optimization:
> 
> size_t
> strlcpy (char *dst, const char *src, size_t size)
> {
>    size_t len = strlen (src);
> 
>    if (size == 0)
>      return len;
>    size = len >= size ? size - 1 : len;

I'd use a separate variable dlen, to differentiate it from size.  Otherwise, it 
looks like an off-by-one bug just below, since writing at a [size] usually means 
writing past the array.

>    dst[size] = 0;
>    memcpy (dst, src, size);
>    return len;
> }

Then, to compare oranges to oranges, I'll provide the equivalently optimized 
stpecpy(3):

char *
stpecpy (char *dst, char *end, const char *restrict src)
{
   size_t dsize;
   size_t dlen;
   size_t slen;

   slen = strlen(src);

   if (dst == end)
     return NULL;
   if (unlikely(dst == NULL))
     return NULL;
   if (dst > end)
     unreachable();
   dsize = end - dst;
   dlen = slen >= dsize ? dsize - 1 : slen;
   dst[dlen] = 0;
   return mempcpy(dst, src, dlen);
}

Now we can really compare them.  (unlikely() is the obvious wrapper over 
__builtin_expect(), and unreachable() is C23's equivalent of 
__builtin_unreachable();  they're just extra optimizations, but can be ignored.)

There are various decissions to take here:

-  We could leave NULL as UB, but I want to handle it for being able to combine 
with stpeprintf().  Although, we could implement stpeprintf() so that it never 
fails (we would need to implement it without the INT_MAX limitation of snprintf(3)).

-  We could call strnlen(3) instead, but strlen(3) is probably faster in the 
average use case, and has the benefit of crashing on invalid input.

The differences with your strlcpy(3) implementation are:

-  NULL check.
-  dsize = end - dst; calculation

Considering that strlcpy(3) chained calls need extra boilerplate at call site 
(remember):

               n = strlcpy(buf, "Hello ", sizeof(buf));
               if (n >= sizeof(buf))
                   goto toolong;
               n += strlcpy(buf + n, "world", sizeof(buf) - n);
               if (n >= sizeof(buf))
                   goto toolong;
               n += strlcpy(buf + n, "!", sizeof(buf) - n);
               if (n >= sizeof(buf))
                   goto toolong;
               puts(buf);

we see that there are in reality more calculations in the case of strlcpy(3); I 
see 2 '+'s and 1 '-' at strlcpy(3) call site, while we only had an extra '-' in 
the stpecpy(3) internals.  The number of conditionals seems to be the same after 
all, except for one single conditional after all the chained stpecpy(3) calls.

So, for equally optimized code, stpecpy(3) seems to win.  It's not hard to 
believe, since they perform the same operation, with the difference that 
strlcpy(3) forces the user to recalculate the buffer size (or really, the 
compiler on behalf of the user, since the intention is that users don't write 
this code, and instead write calls to strlcat(3)).  While it seems very obvious, 
since the source code is so similar that we can see the differences, I'll still 
try to write a benchmark.

And remember that performance is only second to usability, which is the main 
selling point of stpe*() functions.

[...]

>>> In contrast we can be pretty sure that the standard strlen, memcpy etc are both
>>> correct and efficient on all targets/libc's.
>>
>> Sure, but memcpy(3) is not usable in code that needs to truncate.  We need to
>> compare against stpncpy(3) (ughhh) and strlcpy(3).
> 
> The idea is that if we add new string functions, their implementation should use
> other string functions that are known to be well optimized for most targets.

Fair.  Above is my optimized version of stpecpy(3).

> 
> Cheers,
> Wilco

Cheers,

Alex

-- 
<http://www.alejandro-colomar.es/>

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Re: [PATCH 1/1] string: Add stpecpy(3)

[Alejandro Colomar]

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On 12/24/22 01:05, Alejandro Colomar wrote:
> 
> 
> On 12/24/22 00:24, Wilco Dijkstra wrote:
>> Hi Alex,
>>
>>> For that, we'd first need to discuss what is a typical scenario.
>>
>> Like copying/concatenating strings that fit within the buffer. >
>>> And also, it depends a lot on what the compiler can optimize.  If I call
>>> strlcat(3) in a loop, I know that stpecpy(3) is going to be orders of magnitude
>>> faster.
>>
>> If you're trying to say that the 'strcat' variant is bad then yes absolutely -
> 
> I must admit that it has good things, if you have a compiler that does the magic 
> for you.  GCC optimizes strcat(3) into stpcpy(3), so if you know that it will be 
> optimized, it's not so bad, and the source code is cleaner to the eye.
> 
>> it's better to inline in the compiler or avoid the 'strcat' versions altogether
>> (that's also why I would strongly suggest never to add more 'cat' variants).
> 
> inlining in the compiler is a good solution.  And yes, I agree on not adding cat 
> variants, but at the same time, str functions are problematic in that they can't 
> be chained, as opposed to stp functions.  That problem shows itself in the 
> snprintf(3) bugs I mentioned.  Users need a way to catenate easily, even if not 
> with a 'cat' function.
> 
>> But that doesn't say anything about whether stpecpy is better than strlcpy.
>>
>>> If I call strlcpy(3) in a loop, doing what an ideal compiler might do, that
>>> might be something to benchmark, but we'd also need to discuss what is a good
>>> input for the benchmark.
>>
>> The typical case would be copying or concatenating smallish strings to a buffer.
> 
> Okay, I'll try to prepare a benchmark.
> 
>>
>>> In the OpenBSD definition of strlcpy(), I count 4 branches, and one of them is
>>> inside a while loop.  So, I'd find it very surprising if strlcpy(3) outperformed
>>> stpecpy(3).
>>
>> If that really is the OpenBSD implementation then this proves my point that
>> non-standard string functions are often totally unoptimized.
> 
> And not only that, but I find your version much more readable.  I don't 
> understand how the OpenBSD version was written that way, and hasn't been fixed 
> so far.
> 
>>
>> A basic implementation of strlcpy would use strlen and memcpy so it is fast
>> on every system without requiring any optimization:
>>
>> size_t
>> strlcpy (char *dst, const char *src, size_t size)
>> {
>>    size_t len = strlen (src);
>>
>>    if (size == 0)
>>      return len;
>>    size = len >= size ? size - 1 : len;
> 
> I'd use a separate variable dlen, to differentiate it from size.  Otherwise, it 
> looks like an off-by-one bug just below, since writing at a [size] usually means 
> writing past the array.
> 
>>    dst[size] = 0;
>>    memcpy (dst, src, size);
>>    return len;
>> }
> 
> Then, to compare oranges to oranges, I'll provide the equivalently optimized 
> stpecpy(3):
> 
> char *
> stpecpy (char *dst, char *end, const char *restrict src)
> {
>    size_t dsize;
>    size_t dlen;
>    size_t slen;
> 
>    slen = strlen(src);
> 
>    if (dst == end)
>      return NULL;
>    if (unlikely(dst == NULL))
>      return NULL;
>    if (dst > end)
>      unreachable();
>    dsize = end - dst;
>    dlen = slen >= dsize ? dsize - 1 : slen;
>    dst[dlen] = 0;
>    return mempcpy(dst, src, dlen);
> }

Sorry, I wrote a bug while optimizing: I forgot about the sentinel 'end' return. 
  Now I think it should be fine (anyway, I'll test it soon):

char *
stpecpy (char *dst, char *end, const char *restrict src)
{
   size_t dsize;
   size_t dlen;
   size_t slen = strlen (src);
   bool   trunc = false;

   if (dst == end)
     return NULL;
   if (dst == NULL)
     return NULL;
   dsize = end - dst;
   trunc = (slen >= dsize);
   dlen = trunc ? dsize - 1 : slen;
   dst[dlen] = 0;
   return mempcpy(dst, src, dlen) + trunc;
}

This adds a '+' operation, so the difference compared to your strlcpy(3) is 
smaller, but stpecpy() still wins, I think.


-- 
<http://www.alejandro-colomar.es/>

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stpecpy(3) vs strlcpy(3) benchmark (was: [PATCH 1/1] string: Add stpecpy(3))

[Alejandro Colomar]

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Hi Wilco,

Here goes benchmark code for comparing performance of stpecpy(3) against 
strlcpy(3).  I used my_strlcpy(3), with your definition, because I suspect 
libbsd's version won't be optimized.  stpecpy(3) was also updated to be as 
optimized as I can write it.

I won't be showing exact numbers, as that's probably very system- and 
version-dependent.  Test yourself for exact numbers.  However, I'll share the 
general results.

At first I was a little bit surprised (and disappointed) because strlcpy(3) 
seemed faster under GCC.  When forming the very short string "Hello, " "worlds" 
"!", in just 3 consecutive calls, my_strlcpy(3) outperforms (~10%) stpecpy() in 
GCC.  In Clang they had both similar times.

When I added a long string of 'x's, the results more or less were the same.

When I added a few calls by repeating "worlds" "!", that changed the balance 
very significantly for stpecpy(3).  When chaining more than just a couple calls, 
strlcpy(3) starts showing a performance decrease.  With the code below, I saw 
stpecpy(3) consistently outperform strlcpy(3) by around 3%.

I guess the advantage of strlcpy(3) over stpecpy(3) may be due to the overhead 
of setting 'end = buf + BUFSIZ;' for stpecpy(3), and to the fact that the first 
strlcpy(3) call doesn't have to do the usual '+=', ' + l', and ' - l', which can 
be significant given how negligible the internal difference between the two is.

Cheers,
Alex

---

alx@debian:~/tmp$ cat foo.c
#include <err.h>
#include <stdio.h>

#include <stp/stpe/stpecpy.h>

void
foo(char *buf, size_t *len)
{
	char    *p, *end;

	end = buf + BUFSIZ;
	p = buf;
	p = stpecpy(p, end, "Heylo, xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
	p = stpecpy(p, end, "worlds");
	p = stpecpy(p, end, "!");
	p = stpecpy(p, end, "worlds");
	p = stpecpy(p, end, "!");
	p = stpecpy(p, end, "worlds");
	p = stpecpy(p, end, "!");
	p = stpecpy(p, end, "worlds");
	p = stpecpy(p, end, "!");
	if (p == end) {
		p--;
		warnx("Truncated");
	}
	*len = p - buf;
}

void
bar(char *buf, size_t *len)
{
	size_t l;

	l = my_strlcpy(buf, "Hello, xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", BUFSIZ);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "!", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "!", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "!", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "!", BUFSIZ - l);
	if (l >= BUFSIZ) {
toolong:
		l = BUFSIZ - 1;
		warnx("Truncated");
	}

	*len = l;
}

alx@debian:~/tmp$ cat bench.c
#include <stdio.h>

int foo(char *buf, size_t *len);
int bar(char *buf, size_t *len);

int
main(void)
{
	char    buf[BUFSIZ];
	size_t  len;

	for (size_t i = 0; i < 10000000; i++)
#if 1
		foo(buf, &len);
#else
		bar(buf, &len);
#endif

	printf("%zu: %s\n", len, buf);
}


-- 
<http://www.alejandro-colomar.es/>

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Re: stpecpy(3) vs strlcpy(3) benchmark (was: [PATCH 1/1] string: Add stpecpy(3))

[Alejandro Colomar]

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Hi Wilco,

On 12/24/22 03:30, Alejandro Colomar wrote:
> Here goes benchmark code for comparing performance of stpecpy(3) against 
> strlcpy(3).  I used my_strlcpy(3), with your definition, because I suspect 
> libbsd's version won't be optimized.  stpecpy(3) was also updated to be as 
> optimized as I can write it.
> 
> I won't be showing exact numbers, as that's probably very system- and 
> version-dependent.  Test yourself for exact numbers.  However, I'll share the 
> general results.
> 
> At first I was a little bit surprised (and disappointed) because strlcpy(3) 
> seemed faster under GCC.  When forming the very short string "Hello, " "worlds" 
> "!", in just 3 consecutive calls, my_strlcpy(3) outperforms (~10%) stpecpy() in 
> GCC.  In Clang they had both similar times.

Small self correction:

While I compiled the benchmarked functions with -O3, I had forgotten to add -O3 
to the compilation of foo() and bar().  With that, stpecpy() is faster in every 
single case.  :)

Cheers,

Alex

> 
> When I added a long string of 'x's, the results more or less were the same.
> 
> When I added a few calls by repeating "worlds" "!", that changed the balance 
> very significantly for stpecpy(3).  When chaining more than just a couple calls, 
> strlcpy(3) starts showing a performance decrease.  With the code below, I saw 
> stpecpy(3) consistently outperform strlcpy(3) by around 3%.
> 
> I guess the advantage of strlcpy(3) over stpecpy(3) may be due to the overhead 
> of setting 'end = buf + BUFSIZ;' for stpecpy(3), and to the fact that the first 
> strlcpy(3) call doesn't have to do the usual '+=', ' + l', and ' - l', which can 
> be significant given how negligible the internal difference between the two is.
> 
> Cheers,
> Alex
> 
> ---
> 
> alx@debian:~/tmp$ cat foo.c
> #include <err.h>
> #include <stdio.h>
> 
> #include <stp/stpe/stpecpy.h>
> 
> void
> foo(char *buf, size_t *len)
> {
>      char    *p, *end;
> 
>      end = buf + BUFSIZ;
>      p = buf;
>      p = stpecpy(p, end, "Heylo, xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
>      p = stpecpy(p, end, "worlds");
>      p = stpecpy(p, end, "!");
>      p = stpecpy(p, end, "worlds");
>      p = stpecpy(p, end, "!");
>      p = stpecpy(p, end, "worlds");
>      p = stpecpy(p, end, "!");
>      p = stpecpy(p, end, "worlds");
>      p = stpecpy(p, end, "!");
>      if (p == end) {
>          p--;
>          warnx("Truncated");
>      }
>      *len = p - buf;
> }
> 
> void
> bar(char *buf, size_t *len)
> {
>      size_t l;
> 
>      l = my_strlcpy(buf, "Hello, xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", 
> BUFSIZ);
>      if (l >= BUFSIZ)
>          goto toolong;
>      l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
>      if (l >= BUFSIZ)
>          goto toolong;
>      l += my_strlcpy(buf + l, "!", BUFSIZ - l);
>      if (l >= BUFSIZ)
>          goto toolong;
>      l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
>      if (l >= BUFSIZ)
>          goto toolong;
>      l += my_strlcpy(buf + l, "!", BUFSIZ - l);
>      if (l >= BUFSIZ)
>          goto toolong;
>      l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
>      if (l >= BUFSIZ)
>          goto toolong;
>      l += my_strlcpy(buf + l, "!", BUFSIZ - l);
>      if (l >= BUFSIZ)
>          goto toolong;
>      l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
>      if (l >= BUFSIZ)
>          goto toolong;
>      l += my_strlcpy(buf + l, "!", BUFSIZ - l);
>      if (l >= BUFSIZ) {
> toolong:
>          l = BUFSIZ - 1;
>          warnx("Truncated");
>      }
> 
>      *len = l;
> }
> 
> alx@debian:~/tmp$ cat bench.c
> #include <stdio.h>
> 
> int foo(char *buf, size_t *len);
> int bar(char *buf, size_t *len);
> 
> int
> main(void)
> {
>      char    buf[BUFSIZ];
>      size_t  len;
> 
>      for (size_t i = 0; i < 10000000; i++)
> #if 1
>          foo(buf, &len);
> #else
>          bar(buf, &len);
> #endif
> 
>      printf("%zu: %s\n", len, buf);
> }
> 
> 

-- 
<http://www.alejandro-colomar.es/>

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Re: [PATCH 1/1] string: Add stpecpy(3)

[Noah Goldstein]

On Fri, Dec 23, 2022 at 4:26 PM Alejandro Colomar via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
>
>
> On 12/24/22 01:05, Alejandro Colomar wrote:
> >
> >
> > On 12/24/22 00:24, Wilco Dijkstra wrote:
> >> Hi Alex,
> >>
> >>> For that, we'd first need to discuss what is a typical scenario.
> >>
> >> Like copying/concatenating strings that fit within the buffer. >
> >>> And also, it depends a lot on what the compiler can optimize.  If I call
> >>> strlcat(3) in a loop, I know that stpecpy(3) is going to be orders of magnitude
> >>> faster.
> >>
> >> If you're trying to say that the 'strcat' variant is bad then yes absolutely -
> >
> > I must admit that it has good things, if you have a compiler that does the magic
> > for you.  GCC optimizes strcat(3) into stpcpy(3), so if you know that it will be
> > optimized, it's not so bad, and the source code is cleaner to the eye.
> >
> >> it's better to inline in the compiler or avoid the 'strcat' versions altogether
> >> (that's also why I would strongly suggest never to add more 'cat' variants).
> >
> > inlining in the compiler is a good solution.  And yes, I agree on not adding cat
> > variants, but at the same time, str functions are problematic in that they can't
> > be chained, as opposed to stp functions.  That problem shows itself in the
> > snprintf(3) bugs I mentioned.  Users need a way to catenate easily, even if not
> > with a 'cat' function.
> >
> >> But that doesn't say anything about whether stpecpy is better than strlcpy.
> >>
> >>> If I call strlcpy(3) in a loop, doing what an ideal compiler might do, that
> >>> might be something to benchmark, but we'd also need to discuss what is a good
> >>> input for the benchmark.
> >>
> >> The typical case would be copying or concatenating smallish strings to a buffer.
> >
> > Okay, I'll try to prepare a benchmark.
> >
> >>
> >>> In the OpenBSD definition of strlcpy(), I count 4 branches, and one of them is
> >>> inside a while loop.  So, I'd find it very surprising if strlcpy(3) outperformed
> >>> stpecpy(3).
> >>
> >> If that really is the OpenBSD implementation then this proves my point that
> >> non-standard string functions are often totally unoptimized.
> >
> > And not only that, but I find your version much more readable.  I don't
> > understand how the OpenBSD version was written that way, and hasn't been fixed
> > so far.
> >
> >>
> >> A basic implementation of strlcpy would use strlen and memcpy so it is fast
> >> on every system without requiring any optimization:
> >>
> >> size_t
> >> strlcpy (char *dst, const char *src, size_t size)
> >> {
> >>    size_t len = strlen (src);
> >>
> >>    if (size == 0)
> >>      return len;
> >>    size = len >= size ? size - 1 : len;
> >
> > I'd use a separate variable dlen, to differentiate it from size.  Otherwise, it
> > looks like an off-by-one bug just below, since writing at a [size] usually means
> > writing past the array.
> >
> >>    dst[size] = 0;
> >>    memcpy (dst, src, size);
> >>    return len;
> >> }
> >
> > Then, to compare oranges to oranges, I'll provide the equivalently optimized
> > stpecpy(3):
> >
> > char *
> > stpecpy (char *dst, char *end, const char *restrict src)
> > {
> >    size_t dsize;
> >    size_t dlen;
> >    size_t slen;
> >
> >    slen = strlen(src);
> >
> >    if (dst == end)
> >      return NULL;
> >    if (unlikely(dst == NULL))
> >      return NULL;
> >    if (dst > end)
> >      unreachable();
> >    dsize = end - dst;
> >    dlen = slen >= dsize ? dsize - 1 : slen;
> >    dst[dlen] = 0;
> >    return mempcpy(dst, src, dlen);
> > }
>
> Sorry, I wrote a bug while optimizing: I forgot about the sentinel 'end' return.
>   Now I think it should be fine (anyway, I'll test it soon):
>
> char *
> stpecpy (char *dst, char *end, const char *restrict src)
> {
>    size_t dsize;
>    size_t dlen;
>    size_t slen = strlen (src);

Imo move `dst == end` and `dst == NULL` check before strlen
and change strlen to `strnlen(src, dsize)`.
>    bool   trunc = false;
>
>    if (dst == end)

Out of curiosity what if `end < dst`?
>      return NULL;
>    if (dst == NULL)
>      return NULL;
>    dsize = end - dst;
>    trunc = (slen >= dsize);
>    dlen = trunc ? dsize - 1 : slen;
>    dst[dlen] = 0;
>    return mempcpy(dst, src, dlen) + trunc;
> }
>
> This adds a '+' operation, so the difference compared to your strlcpy(3) is
> smaller, but stpecpy() still wins, I think.
>
>
> --
> <http://www.alejandro-colomar.es/>

Re: [PATCH 1/1] string: Add stpecpy(3)

[Alejandro Colomar]

[-- Attachment #1.1: Type: text/plain, Size: 3160 bytes --]

Hi Noah,

On 12/25/22 02:52, Noah Goldstein wrote:

>> char *
>> stpecpy (char *dst, char *end, const char *restrict src)
>> {
>>     size_t dsize;
>>     size_t dlen;
>>     size_t slen = strlen (src);
> 
> Imo move `dst == end` and `dst == NULL` check before strlen


That's a valid optimization.  Having strlen(3) before has the advantage that you 
make sure that your strings are strings, as strlcpy(3) does.  But since we're 
inventing stpecpy(3), we can choose to be faster.  If anyone wants to instrument 
their code, they can add a temporary wrapper that does that.

> and change strlen to `strnlen(src, dsize)`.

About strnlen(3), I have doubts.  Isn't strlen(3) faster for the common case of 
no truncation or little truncation?  strnlen(3) would optimize for the case 
where you truncate by a large difference.

>>     bool   trunc = false;
>>
>>     if (dst == end)
> 
> Out of curiosity what if `end < dst`?

The behavior is undefined.  That's by design.  In the definition of stpecpy(3) I 
have currently in libstp, I even tell the compiler to optimize on that condition:
<http://www.alejandro-colomar.es/src/alx/alx/libstp.git/tree/include/stp/stpe/stpecpy.h#n33>


alx@asus5775:~/src/alx/libstp$ grepc -tfd stpecpy
./include/stp/stpe/stpecpy.h:21:
inline char *stp_nullable
stpecpy(char *stp_nullable dst, char *end, const char *restrict src)
{
	bool    trunc;
	size_t  dsize, dlen, slen;

	slen = strlen(src);

	if (dst == end)
		return end;
	if (stp_unlikely(dst == NULL))  // Allow chaining with stpeprintf().
		return NULL;
	stp_impossible(dst > end);

	dsize = end - dst;
	trunc = (slen >= dsize);
	dlen = trunc ? dsize - 1 : slen;
	dst[dlen] = '\0';

	return mempcpy(dst, src, dlen) + trunc;
}
alx@asus5775:~/src/alx/libstp$ grepc -tm stp_impossible
./include/stp/_compiler.h:14:
#define stp_impossible(e)   do                                                \
{                                                                             \
	if (e)                                                                \
		stp_unreachable();                                            \
} while (0)
alx@asus5775:~/src/alx/libstp$ grep -rnC1 define.stp_unreachable
include/stp/_compiler.h-28-#if defined(unreachable)
include/stp/_compiler.h:29:# define stp_unreachable()  unreachable()
include/stp/_compiler.h-30-#else
include/stp/_compiler.h:31:# define stp_unreachable()  __builtin_unreachable()
include/stp/_compiler.h-32-#endif


I'd do that for glibc, but I don't see any facility.  Maybe we should add an 
__impossible() macro to document UB, and help the compiler.

Cheers,

Alex

>>       return NULL;
>>     if (dst == NULL)
>>       return NULL;
>>     dsize = end - dst;
>>     trunc = (slen >= dsize);
>>     dlen = trunc ? dsize - 1 : slen;
>>     dst[dlen] = 0;
>>     return mempcpy(dst, src, dlen) + trunc;
>> }
>>
>> This adds a '+' operation, so the difference compared to your strlcpy(3) is
>> smaller, but stpecpy() still wins, I think.
>>
>>
>> --
>> <http://www.alejandro-colomar.es/>

-- 
<http://www.alejandro-colomar.es/>

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Re: [PATCH 1/1] string: Add stpecpy(3)

[Noah Goldstein]

On Sun, Dec 25, 2022 at 6:37 AM Alejandro Colomar
<alx.manpages@gmail.com> wrote:
>
> Hi Noah,
>
> On 12/25/22 02:52, Noah Goldstein wrote:
>
> >> char *
> >> stpecpy (char *dst, char *end, const char *restrict src)
> >> {
> >>     size_t dsize;
> >>     size_t dlen;
> >>     size_t slen = strlen (src);
> >
> > Imo move `dst == end` and `dst == NULL` check before strlen
>
>
> That's a valid optimization.  Having strlen(3) before has the advantage that you
> make sure that your strings are strings, as strlcpy(3) does.  But since we're
> inventing stpecpy(3), we can choose to be faster.  If anyone wants to instrument
> their code, they can add a temporary wrapper that does that.
>
> > and change strlen to `strnlen(src, dsize)`.
>
> About strnlen(3), I have doubts.  Isn't strlen(3) faster for the common case of
> no truncation or little truncation?  strnlen(3) would optimize for the case
> where you truncate by a large difference.

It's faster if strlen(s) <= strnlen(s, N) (maybe up to N + 32).

But generally I think of it like `qsort`. Most data gets n * log(n) behavior
but still it's worth preventing the worst case for minor constant cost.


>
> >>     bool   trunc = false;
> >>
> >>     if (dst == end)
> >
> > Out of curiosity what if `end < dst`?
>
> The behavior is undefined.  That's by design.  In the definition of stpecpy(3) I
> have currently in libstp, I even tell the compiler to optimize on that condition:
> <http://www.alejandro-colomar.es/src/alx/alx/libstp.git/tree/include/stp/stpe/stpecpy.h#n33>
>

You could probably optimize out one of the branches along the line of:
if((dst - 1UL) >= (end - 1UL)) {
    // if dst == NULL, then dst - 1UL -> SIZE_MAX and must be >= any value.
    // if dst == end, then (dst - 1UL) >= (end - 1UL) will be true.
    return NULL;
}
>
> alx@asus5775:~/src/alx/libstp$ grepc -tfd stpecpy
> ./include/stp/stpe/stpecpy.h:21:
> inline char *stp_nullable
> stpecpy(char *stp_nullable dst, char *end, const char *restrict src)
> {
>         bool    trunc;
>         size_t  dsize, dlen, slen;
>
>         slen = strlen(src);
>
>         if (dst == end)
>                 return end;
>         if (stp_unlikely(dst == NULL))  // Allow chaining with stpeprintf().
>                 return NULL;
>         stp_impossible(dst > end);
>
>         dsize = end - dst;
>         trunc = (slen >= dsize);
>         dlen = trunc ? dsize - 1 : slen;
>         dst[dlen] = '\0';
>
>         return mempcpy(dst, src, dlen) + trunc;
> }
> alx@asus5775:~/src/alx/libstp$ grepc -tm stp_impossible
> ./include/stp/_compiler.h:14:
> #define stp_impossible(e)   do                                                \
> {                                                                             \
>         if (e)                                                                \
>                 stp_unreachable();                                            \
> } while (0)
> alx@asus5775:~/src/alx/libstp$ grep -rnC1 define.stp_unreachable
> include/stp/_compiler.h-28-#if defined(unreachable)
> include/stp/_compiler.h:29:# define stp_unreachable()  unreachable()
> include/stp/_compiler.h-30-#else
> include/stp/_compiler.h:31:# define stp_unreachable()  __builtin_unreachable()
> include/stp/_compiler.h-32-#endif
>
>
> I'd do that for glibc, but I don't see any facility.  Maybe we should add an
> __impossible() macro to document UB, and help the compiler.

Does it result in any improved codegen? If not seems like
making it fail more noisily is always a win.
>
> Cheers,
>
> Alex
>
> >>       return NULL;
> >>     if (dst == NULL)
> >>       return NULL;
> >>     dsize = end - dst;
> >>     trunc = (slen >= dsize);
> >>     dlen = trunc ? dsize - 1 : slen;
> >>     dst[dlen] = 0;
> >>     return mempcpy(dst, src, dlen) + trunc;
> >> }
> >>
> >> This adds a '+' operation, so the difference compared to your strlcpy(3) is
> >> smaller, but stpecpy() still wins, I think.
> >>
> >>
> >> --
> >> <http://www.alejandro-colomar.es/>
>
> --
> <http://www.alejandro-colomar.es/>

Re: [PATCH 1/1] string: Add stpecpy(3)

[Alejandro Colomar]

[-- Attachment #1.1: Type: text/plain, Size: 6088 bytes --]



On 12/25/22 23:31, Noah Goldstein wrote:
> On Sun, Dec 25, 2022 at 6:37 AM Alejandro Colomar
> <alx.manpages@gmail.com> wrote:
>>
>> Hi Noah,
>>
>> On 12/25/22 02:52, Noah Goldstein wrote:
>>
>>>> char *
>>>> stpecpy (char *dst, char *end, const char *restrict src)
>>>> {
>>>>      size_t dsize;
>>>>      size_t dlen;
>>>>      size_t slen = strlen (src);
>>>
>>> Imo move `dst == end` and `dst == NULL` check before strlen
>>
>>
>> That's a valid optimization.  Having strlen(3) before has the advantage that you
>> make sure that your strings are strings, as strlcpy(3) does.  But since we're
>> inventing stpecpy(3), we can choose to be faster.  If anyone wants to instrument
>> their code, they can add a temporary wrapper that does that.
>>
>>> and change strlen to `strnlen(src, dsize)`.
>>
>> About strnlen(3), I have doubts.  Isn't strlen(3) faster for the common case of
>> no truncation or little truncation?  strnlen(3) would optimize for the case
>> where you truncate by a large difference.
> 
> It's faster if strlen(s) <= strnlen(s, N) (maybe up to N + 32).
> 
> But generally I think of it like `qsort`. Most data gets n * log(n) behavior
> but still it's worth preventing the worst case for minor constant cost.
> 

I.  Maybe it's a good thing.  Since it's a truncating API, I guess optimizing 
for truncation is reasonable.  For common strings, which will be short (size <= 
64), I guess the constant will really be negligible.

> 
>>
>>>>      bool   trunc = false;
>>>>
>>>>      if (dst == end)
>>>
>>> Out of curiosity what if `end < dst`?
>>
>> The behavior is undefined.  That's by design.  In the definition of stpecpy(3) I
>> have currently in libstp, I even tell the compiler to optimize on that condition:
>> <http://www.alejandro-colomar.es/src/alx/alx/libstp.git/tree/include/stp/stpe/stpecpy.h#n33>
>>
> 
> You could probably optimize out one of the branches along the line of:
> if((dst - 1UL) >= (end - 1UL)) {
>      // if dst == NULL, then dst - 1UL -> SIZE_MAX and must be >= any value.

You would need a cast, wouldn't you?  Otherwise, you'll get pointer arithmetic. 
Pointer arithmetic with NULL is UB.

>      // if dst == end, then (dst - 1UL) >= (end - 1UL) will be true.
>      return NULL;

Returning NULL on truncation would be a possibility, but then we'd need to use 
errno to tell the user if the error was truncation or an input NULL (which 
reports an error to a previous vsnprintf(3) call wrapped by [v]stpeprintf().

Using errno would probably counter any optimization, since you'd still need one 
more branch for setting errno, so I guess it's simpler to just use end for 
truncation.


Oooor, if we reimplement __vsnprintf_internal(3) to work on size_t and never 
fail, then we could add a [v]stpeprintf(3) that never fails, and then this 
function would only bail out on truncation.

Would it be possible to make __vsnprintf_internal() never fail?  What are the 
current failing conditions; only a size greater than INT_MAX, or are there more 
errors?

> }
>>
>> alx@asus5775:~/src/alx/libstp$ grepc -tfd stpecpy
>> ./include/stp/stpe/stpecpy.h:21:
>> inline char *stp_nullable
>> stpecpy(char *stp_nullable dst, char *end, const char *restrict src)
>> {
>>          bool    trunc;
>>          size_t  dsize, dlen, slen;
>>
>>          slen = strlen(src);
>>
>>          if (dst == end)
>>                  return end;
>>          if (stp_unlikely(dst == NULL))  // Allow chaining with stpeprintf().
>>                  return NULL;
>>          stp_impossible(dst > end);
>>
>>          dsize = end - dst;
>>          trunc = (slen >= dsize);
>>          dlen = trunc ? dsize - 1 : slen;
>>          dst[dlen] = '\0';
>>
>>          return mempcpy(dst, src, dlen) + trunc;
>> }
>> alx@asus5775:~/src/alx/libstp$ grepc -tm stp_impossible
>> ./include/stp/_compiler.h:14:
>> #define stp_impossible(e)   do                                                \
>> {                                                                             \
>>          if (e)                                                                \
>>                  stp_unreachable();                                            \
>> } while (0)
>> alx@asus5775:~/src/alx/libstp$ grep -rnC1 define.stp_unreachable
>> include/stp/_compiler.h-28-#if defined(unreachable)
>> include/stp/_compiler.h:29:# define stp_unreachable()  unreachable()
>> include/stp/_compiler.h-30-#else
>> include/stp/_compiler.h:31:# define stp_unreachable()  __builtin_unreachable()
>> include/stp/_compiler.h-32-#endif
>>
>>
>> I'd do that for glibc, but I don't see any facility.  Maybe we should add an
>> __impossible() macro to document UB, and help the compiler.
> 
> Does it result in any improved codegen? If not seems like
> making it fail more noisily is always a win.

Both Clang and GCC generate the same code with or without the hint that it's 
impossible.  Anyway, I'll keep it in my source code because it also helps tell 
the programmer that dst>end was taken into consideration and explicitly outlawed.

The 'end' pointer is expected to be always generated as 'buf + sizeof(buf)'. 
Doing something different is not what this API is designed for, and should be 
warned by compilers.  'end' should be a pointer to one after the last byte in an 
array.  Thus, no valid pointer can be greater than end.  If you use this API as 
expected, which is, only chain it with itself and with stpeprintf(3), then it is 
impossible to have dst>end.  But as always, GIGO.

As for the expected result, it would be akin calling strlcpy(3) with a negative 
size.  It would wrap around size_t, and give something close to 2^64.  Both 
would result in a buffer overrun, so writing at random memory, and later a 
crash, but I don't expect that libc should try to detect if the input to 
strlcpy(3) (or actually, any mem*() function) is huge, and neither if input to 
stpecpy(3) is similarly broken.

-- 
<http://www.alejandro-colomar.es/>

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Re: [PATCH 1/1] string: Add stpecpy(3)

[Noah Goldstein]

On Sun, Dec 25, 2022 at 4:26 PM Alejandro Colomar
<alx.manpages@gmail.com> wrote:
>
>
>
> On 12/25/22 23:31, Noah Goldstein wrote:
> > On Sun, Dec 25, 2022 at 6:37 AM Alejandro Colomar
> > <alx.manpages@gmail.com> wrote:
> >>
> >> Hi Noah,
> >>
> >> On 12/25/22 02:52, Noah Goldstein wrote:
> >>
> >>>> char *
> >>>> stpecpy (char *dst, char *end, const char *restrict src)
> >>>> {
> >>>>      size_t dsize;
> >>>>      size_t dlen;
> >>>>      size_t slen = strlen (src);
> >>>
> >>> Imo move `dst == end` and `dst == NULL` check before strlen
> >>
> >>
> >> That's a valid optimization.  Having strlen(3) before has the advantage that you
> >> make sure that your strings are strings, as strlcpy(3) does.  But since we're
> >> inventing stpecpy(3), we can choose to be faster.  If anyone wants to instrument
> >> their code, they can add a temporary wrapper that does that.
> >>
> >>> and change strlen to `strnlen(src, dsize)`.
> >>
> >> About strnlen(3), I have doubts.  Isn't strlen(3) faster for the common case of
> >> no truncation or little truncation?  strnlen(3) would optimize for the case
> >> where you truncate by a large difference.
> >
> > It's faster if strlen(s) <= strnlen(s, N) (maybe up to N + 32).
> >
> > But generally I think of it like `qsort`. Most data gets n * log(n) behavior
> > but still it's worth preventing the worst case for minor constant cost.
> >
>
> I.  Maybe it's a good thing.  Since it's a truncating API, I guess optimizing
> for truncation is reasonable.  For common strings, which will be short (size <=
> 64), I guess the constant will really be negligible.
>
> >
> >>
> >>>>      bool   trunc = false;
> >>>>
> >>>>      if (dst == end)
> >>>
> >>> Out of curiosity what if `end < dst`?
> >>
> >> The behavior is undefined.  That's by design.  In the definition of stpecpy(3) I
> >> have currently in libstp, I even tell the compiler to optimize on that condition:
> >> <http://www.alejandro-colomar.es/src/alx/alx/libstp.git/tree/include/stp/stpe/stpecpy.h#n33>
> >>
> >
> > You could probably optimize out one of the branches along the line of:
> > if((dst - 1UL) >= (end - 1UL)) {
> >      // if dst == NULL, then dst - 1UL -> SIZE_MAX and must be >= any value.
>
> You would need a cast, wouldn't you?  Otherwise, you'll get pointer arithmetic.
> Pointer arithmetic with NULL is UB.
>
> >      // if dst == end, then (dst - 1UL) >= (end - 1UL) will be true.
> >      return NULL;
>
> Returning NULL on truncation would be a possibility, but then we'd need to use
> errno to tell the user if the error was truncation or an input NULL (which
> reports an error to a previous vsnprintf(3) call wrapped by [v]stpeprintf().

I'm not sure I see what you mean. Your current logic is:
```
   if (dst == end)
     return NULL;
   if (dst == NULL)
     return NULL;
```
Equivalent (since dst >= end || dst == NULL is required) is:
```
if((dst - 1UL) >= (end - 1UL)) {
    return NULL;
}
```
May need to be cast to a `uintptr` or something but don't see
what you mean about needing to check errno and such.

>
> Using errno would probably counter any optimization, since you'd still need one
> more branch for setting errno, so I guess it's simpler to just use end for
> truncation.
>
>
> Oooor, if we reimplement __vsnprintf_internal(3) to work on size_t and never
> fail, then we could add a [v]stpeprintf(3) that never fails, and then this
> function would only bail out on truncation.
>
> Would it be possible to make __vsnprintf_internal() never fail?  What are the
> current failing conditions; only a size greater than INT_MAX, or are there more
> errors?

Don't think its worth reimplementing    __vsnprintf_internal to save a single
branch here.
>
> > }
> >>
> >> alx@asus5775:~/src/alx/libstp$ grepc -tfd stpecpy
> >> ./include/stp/stpe/stpecpy.h:21:
> >> inline char *stp_nullable
> >> stpecpy(char *stp_nullable dst, char *end, const char *restrict src)
> >> {
> >>          bool    trunc;
> >>          size_t  dsize, dlen, slen;
> >>
> >>          slen = strlen(src);
> >>
> >>          if (dst == end)
> >>                  return end;
> >>          if (stp_unlikely(dst == NULL))  // Allow chaining with stpeprintf().
> >>                  return NULL;
> >>          stp_impossible(dst > end);
> >>
> >>          dsize = end - dst;
> >>          trunc = (slen >= dsize);
> >>          dlen = trunc ? dsize - 1 : slen;
> >>          dst[dlen] = '\0';
> >>
> >>          return mempcpy(dst, src, dlen) + trunc;
> >> }
> >> alx@asus5775:~/src/alx/libstp$ grepc -tm stp_impossible
> >> ./include/stp/_compiler.h:14:
> >> #define stp_impossible(e)   do                                                \
> >> {                                                                             \
> >>          if (e)                                                                \
> >>                  stp_unreachable();                                            \
> >> } while (0)
> >> alx@asus5775:~/src/alx/libstp$ grep -rnC1 define.stp_unreachable
> >> include/stp/_compiler.h-28-#if defined(unreachable)
> >> include/stp/_compiler.h:29:# define stp_unreachable()  unreachable()
> >> include/stp/_compiler.h-30-#else
> >> include/stp/_compiler.h:31:# define stp_unreachable()  __builtin_unreachable()
> >> include/stp/_compiler.h-32-#endif
> >>
> >>
> >> I'd do that for glibc, but I don't see any facility.  Maybe we should add an
> >> __impossible() macro to document UB, and help the compiler.
> >
> > Does it result in any improved codegen? If not seems like
> > making it fail more noisily is always a win.
>
> Both Clang and GCC generate the same code with or without the hint that it's
> impossible.  Anyway, I'll keep it in my source code because it also helps tell
> the programmer that dst>end was taken into consideration and explicitly outlawed.
>
> The 'end' pointer is expected to be always generated as 'buf + sizeof(buf)'.
> Doing something different is not what this API is designed for, and should be
> warned by compilers.  'end' should be a pointer to one after the last byte in an
> array.  Thus, no valid pointer can be greater than end.  If you use this API as
> expected, which is, only chain it with itself and with stpeprintf(3), then it is
> impossible to have dst>end.  But as always, GIGO.
>
> As for the expected result, it would be akin calling strlcpy(3) with a negative
> size.  It would wrap around size_t, and give something close to 2^64.  Both
> would result in a buffer overrun, so writing at random memory, and later a
> crash, but I don't expect that libc should try to detect if the input to
> strlcpy(3) (or actually, any mem*() function) is huge, and neither if input to
> stpecpy(3) is similarly broken.
>
> --
> <http://www.alejandro-colomar.es/>

Re: [PATCH 1/1] string: Add stpecpy(3)

[Alejandro Colomar]

[-- Attachment #1.1: Type: text/plain, Size: 2443 bytes --]

On 12/26/22 01:32, Noah Goldstein wrote:
>>> You could probably optimize out one of the branches along the line of:
>>> if((dst - 1UL) >= (end - 1UL)) {
>>>       // if dst == NULL, then dst - 1UL -> SIZE_MAX and must be >= any value.
>>
>> You would need a cast, wouldn't you?  Otherwise, you'll get pointer arithmetic.
>> Pointer arithmetic with NULL is UB.
>>
>>>       // if dst == end, then (dst - 1UL) >= (end - 1UL) will be true.
>>>       return NULL;
>>
>> Returning NULL on truncation would be a possibility, but then we'd need to use
>> errno to tell the user if the error was truncation or an input NULL (which
>> reports an error to a previous vsnprintf(3) call wrapped by [v]stpeprintf().
> 
> I'm not sure I see what you mean. Your current logic is:
> ```
>     if (dst == end)
>       return NULL;
>     if (dst == NULL)
>       return NULL;

No; current code is:

     if (dst == end)
         return end;
     if (dst == NULL)
         return NULL;

NULL is an error (contents of string are undefined; per vsnprintf(3)'s spec), 
while 'end' is just truncation, and contents if the string are well defined.


> ```
> Equivalent (since dst >= end || dst == NULL is required) is:
> ```
> if((dst - 1UL) >= (end - 1UL)) {
>      return NULL;
> }
> ```
> May need to be cast to a `uintptr` or something but don't see
> what you mean about needing to check errno and such.
> 
>>
>> Using errno would probably counter any optimization, since you'd still need one
>> more branch for setting errno, so I guess it's simpler to just use end for
>> truncation.
>>
>>
>> Oooor, if we reimplement __vsnprintf_internal(3) to work on size_t and never
>> fail, then we could add a [v]stpeprintf(3) that never fails, and then this
>> function would only bail out on truncation.
>>
>> Would it be possible to make __vsnprintf_internal() never fail?  What are the
>> current failing conditions; only a size greater than INT_MAX, or are there more
>> errors?
> 
> Don't think its worth reimplementing    __vsnprintf_internal to save a single
> branch here.

It wouldn't be only for that, but also allowing to write size_t bytes of 
formatted output.  However, I question how useful that is, since you only need 
that many bytes when you're catenating strings with %s, for which stpecpy(3) can 
be used; so yes, probably it's not worth it.

-- 
<http://www.alejandro-colomar.es/>

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Re: [PATCH 1/1] string: Add stpecpy(3)

[Noah Goldstein]

On Sun, Dec 25, 2022 at 4:37 PM Alejandro Colomar
<alx.manpages@gmail.com> wrote:
>
> On 12/26/22 01:32, Noah Goldstein wrote:
> >>> You could probably optimize out one of the branches along the line of:
> >>> if((dst - 1UL) >= (end - 1UL)) {
> >>>       // if dst == NULL, then dst - 1UL -> SIZE_MAX and must be >= any value.
> >>
> >> You would need a cast, wouldn't you?  Otherwise, you'll get pointer arithmetic.
> >> Pointer arithmetic with NULL is UB.
> >>
> >>>       // if dst == end, then (dst - 1UL) >= (end - 1UL) will be true.
> >>>       return NULL;
> >>
> >> Returning NULL on truncation would be a possibility, but then we'd need to use
> >> errno to tell the user if the error was truncation or an input NULL (which
> >> reports an error to a previous vsnprintf(3) call wrapped by [v]stpeprintf().
> >
> > I'm not sure I see what you mean. Your current logic is:
> > ```
> >     if (dst == end)
> >       return NULL;
> >     if (dst == NULL)
> >       return NULL;
>
> No; current code is:
>
>      if (dst == end)
>          return end;
>      if (dst == NULL)
>          return NULL;

I see. The code you commented earlier was NULL for both.

Either way you can just make it:

```
if((dst - 1UL) >= (end - 1UL)) {
 return dst; // either dst == NULL or dst == end.
}
```

>
> NULL is an error (contents of string are undefined; per vsnprintf(3)'s spec),
> while 'end' is just truncation, and contents if the string are well defined.
>
>
> > ```
> > Equivalent (since dst >= end || dst == NULL is required) is:
> > ```
> > if((dst - 1UL) >= (end - 1UL)) {
> >      return NULL;
> > }
> > ```
> > May need to be cast to a `uintptr` or something but don't see
> > what you mean about needing to check errno and such.
> >
> >>
> >> Using errno would probably counter any optimization, since you'd still need one
> >> more branch for setting errno, so I guess it's simpler to just use end for
> >> truncation.
> >>
> >>
> >> Oooor, if we reimplement __vsnprintf_internal(3) to work on size_t and never
> >> fail, then we could add a [v]stpeprintf(3) that never fails, and then this
> >> function would only bail out on truncation.
> >>
> >> Would it be possible to make __vsnprintf_internal() never fail?  What are the
> >> current failing conditions; only a size greater than INT_MAX, or are there more
> >> errors?
> >
> > Don't think its worth reimplementing    __vsnprintf_internal to save a single
> > branch here.
>
> It wouldn't be only for that, but also allowing to write size_t bytes of
> formatted output.  However, I question how useful that is, since you only need
> that many bytes when you're catenating strings with %s, for which stpecpy(3) can
> be used; so yes, probably it's not worth it.
>
> --
> <http://www.alejandro-colomar.es/>

Re: [PATCH 1/1] string: Add stpecpy(3)

[Alejandro Colomar]

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Hi Noah,

On 12/26/22 03:43, Noah Goldstein wrote:

> I see. The code you commented earlier was NULL for both.

I don't remember; maybe there was a typo...

> 
> Either way you can just make it:
> 
> ```
> if((dst - 1UL) >= (end - 1UL)) {

I didn't expect that would be significantly better than `(dst == end || dst == 
NULL)`.  However, I compiled both just to see, and the assembly output for your 
code is shorter.  I'll benchmark both to see if there are performance differences.

I wonder why the compiler doesn't generate this code if it's better; it has all 
the information to decide that it can be transformed into that.

Both clang and GCC produce better code with your suggestion (although in the 
case of GCC, the difference is bigger.

Cheers,

Alex

>   return dst; // either dst == NULL or dst == end.
> }
> ```


-- 
<http://www.alejandro-colomar.es/>

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Re: [PATCH 1/1] string: Add stpecpy(3)

[Alejandro Colomar]

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On 12/26/22 23:25, Alejandro Colomar wrote:
> Hi Noah,
> 
> On 12/26/22 03:43, Noah Goldstein wrote:
> 
>> I see. The code you commented earlier was NULL for both.
> 
> I don't remember; maybe there was a typo...
> 
>>
>> Either way you can just make it:
>>
>> ```
>> if((dst - 1UL) >= (end - 1UL)) {
> 
> I didn't expect that would be significantly better than `(dst == end || dst == 
> NULL)`.  However, I compiled both just to see, and the assembly output for your 
> code is shorter.  I'll benchmark both to see if there are performance differences.
> 
> I wonder why the compiler doesn't generate this code if it's better; it has all 
> the information to decide that it can be transformed into that.
> 
> Both clang and GCC produce better code with your suggestion (although in the 
> case of GCC, the difference is bigger.

Self-correction:

They produce smaller code for your suggestion, but it seems to be slower code.

> 
> Cheers,
> 
> Alex
> 
>>   return dst; // either dst == NULL or dst == end.
>> }
>> ```
> 
> 

-- 
<http://www.alejandro-colomar.es/>

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Re: [PATCH 1/1] string: Add stpecpy(3)

[Alejandro Colomar]

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On 12/27/22 00:24, Alejandro Colomar wrote:
> 
> 
> On 12/26/22 23:25, Alejandro Colomar wrote:
>> Hi Noah,
>>
>> On 12/26/22 03:43, Noah Goldstein wrote:
>>
>>> I see. The code you commented earlier was NULL for both.
>>
>> I don't remember; maybe there was a typo...
>>
>>>
>>> Either way you can just make it:
>>>
>>> ```
>>> if((dst - 1UL) >= (end - 1UL)) {
>>
>> I didn't expect that would be significantly better than `(dst == end || dst == 
>> NULL)`.  However, I compiled both just to see, and the assembly output for 
>> your code is shorter.  I'll benchmark both to see if there are performance 
>> differences.
>>
>> I wonder why the compiler doesn't generate this code if it's better; it has 
>> all the information to decide that it can be transformed into that.
>>
>> Both clang and GCC produce better code with your suggestion (although in the 
>> case of GCC, the difference is bigger.
> 
> Self-correction:
> 
> They produce smaller code for your suggestion, but it seems to be slower code.

However, and this is interesting, calling strnlen(3) results in faster code even 
when no truncation occurs; at least for the short string I tested.

I suspect it might be because it is already heavily optimized in glibc, and it 
implicitly simplifies surrounding code:

-       slen = strlen(src);
         dsize = end - dst;
-       trunc = (slen >= dsize);
+       slen = strnlen(src, dsize);
+       trunc = (slen == dsize);

The generated assembly code is one line smaller (on my system, and phase of the 
moon), and some small percent faster.  :)

Cheers,

Alex


> 
>>
>> Cheers,
>>
>> Alex
>>
>>>   return dst; // either dst == NULL or dst == end.
>>> }
>>> ```
>>
>>
> 

-- 
<http://www.alejandro-colomar.es/>

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Re: [PATCH 1/1] string: Add stpecpy(3)

[Alejandro Colomar]

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On 12/27/22 00:52, Alejandro Colomar wrote:
> However, and this is interesting, calling strnlen(3) results in faster code even 
> when no truncation occurs; at least for the short string I tested.
> 
> I suspect it might be because it is already heavily optimized in glibc, and it 
> implicitly simplifies surrounding code:
> 
> -       slen = strlen(src);
>          dsize = end - dst;
> -       trunc = (slen >= dsize);
> +       slen = strnlen(src, dsize);
> +       trunc = (slen == dsize);
> 
> The generated assembly code is one line smaller (on my system, and phase of the 
> moon), and some small percent faster.  :)

I found the reason; it helps simplify considerably the code.  Here's the 
resulting optimized code:


char *stp_nullable
stpecpy(char *stp_nullable dst, char *end, const char *restrict src)
{
	bool    trunc;
	size_t  dsize, dlen, slen;

	if (dst == end)
		return end;
	if (stp_unlikely(dst == NULL))  // Allow chaining with stpeprintf().
		return NULL;
	stp_impossible(dst > end);

	dsize = end - dst;
	slen = strnlen(src, dsize);
	trunc = (slen == dsize);
	dlen = slen - trunc;
	dst[dlen] = '\0';

	return mempcpy(dst, src, dlen) + trunc;
}


See how using strnlen(3) removed the ternary operator.  That's a great 
optimization.  :)

-- 
<http://www.alejandro-colomar.es/>

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