Re: 4x faster strlen with 4 char sentinel
On Tuesday, 28 June 2016 at 09:18:34 UTC, qznc wrote:
Did you also compare to strlen from libc? I'd guess GNU libc
uses a lot more tricks like vector instructions.
I did test with the libc strlen, although the D libraries did not
have a strlen for dchar or wchar. I'm currently using this for
comparison, and playing around with shorter string lengths:
nothrow pure size_t strlen(const(char)* c) {
if (c is null )
return 0;
const(char)* c_save = c;
while (*c) {
c++;
}
return c - c_save;
}
I'm also trying some tests on a PA device where I have tools to
look at cache hits, misses, branches mispredicted. Similar C
code.
Re: 4x faster strlen with 4 char sentinel
On Tuesday, 28 June 2016 at 09:31:46 UTC, Sebastiaan Koppe wrote: If we were in interview, I'd ask you "what does this returns if you pass it an empty string ?" Since no one is answering: It depends on the memory right before c. But if there is at least one 0 right before it - which is quite likely - then you get some crazy big number returned. Yes, the test checked for 0 length but not with a preceding 0. I posted the fix. if (c is null || *c==0)
Re: 4x faster strlen with 4 char sentinel
On Tuesday, 28 June 2016 at 01:53:22 UTC, deadalnix wrote:
On Sunday, 26 June 2016 at 16:40:08 UTC, Jay Norwood wrote:
After watching Andre's sentinel thing, I'm playing with strlen
on char strings with 4 terminating 0s instead of a single one.
Seems to work and is 4x faster compared to the runtime
version.
nothrow pure size_t strlen2(const(char)* c) {
if (c is null)
return 0;
size_t l=0;
while (*c){ c+=4; l+=4;}
while (*c==0){ c--; l--;}
return l+1;
}
This is the timing of my test case, which I can post if anyone
is interested.
strlen\Release>strlen
2738
681
If we were in interview, I'd ask you "what does this returns if
you pass it an empty string ?"
Since no one is answering:
It depends on the memory right before c. But if there is at least
one 0 right before it - which is quite likely - then you get some
crazy big number returned.
Re: 4x faster strlen with 4 char sentinel
On Sunday, 26 June 2016 at 16:40:08 UTC, Jay Norwood wrote:
After watching Andre's sentinel thing, I'm playing with strlen
on char strings with 4 terminating 0s instead of a single one.
Seems to work and is 4x faster compared to the runtime version.
nothrow pure size_t strlen2(const(char)* c) {
if (c is null)
return 0;
size_t l=0;
while (*c){ c+=4; l+=4;}
while (*c==0){ c--; l--;}
return l+1;
}
This is the timing of my test case, which I can post if anyone
is interested.
strlen\Release>strlen
2738
681
Did you also compare to strlen from libc? I'd guess GNU libc uses
a lot more tricks like vector instructions.
Re: 4x faster strlen with 4 char sentinel
On Tuesday, 28 June 2016 at 03:11:26 UTC, Jay Norwood wrote:
On Tuesday, 28 June 2016 at 01:53:22 UTC, deadalnix wrote:
If we were in interview, I'd ask you "what does this returns
if you pass it an empty string ?"
I'd say use this one instead, to avoid negative size_t. It is
also a little faster for the same measurement.
nothrow pure size_t strlen2(const(char)* c) {
if (c is null)
return 0;
const(char)* c_save = c;
while (*c){ c+=4; }
while (*c==0){ c--; }
c++;
return c - c_save;
}
2738
540
2744
If we were in an interview, I would insist.
Re: 4x faster strlen with 4 char sentinel
On Tuesday, 28 June 2016 at 03:58:23 UTC, Jay Norwood wrote:
On Tuesday, 28 June 2016 at 03:11:26 UTC, Jay Norwood wrote:
On Tuesday, 28 June 2016 at 01:53:22 UTC, deadalnix wrote:
If we were in interview, I'd ask you "what does this returns
if you pass it an empty string ?"
oops. I see ... need to test for empty string.
nothrow pure size_t strlen2(const(char)* c) {
if (c is null || *c==0)
return 0;
const(char)* c_save = c;
while (*c){ c+=4; }
while (*c==0){ c--; }
c++;
return c - c_save;
}
Why not just do
nothrow pure size_t strlen2(const(char)* c) {
if (!c)
return 0;
...
}
Re: 4x faster strlen with 4 char sentinel
On Tuesday, 28 June 2016 at 03:11:26 UTC, Jay Norwood wrote:
On Tuesday, 28 June 2016 at 01:53:22 UTC, deadalnix wrote:
If we were in interview, I'd ask you "what does this returns
if you pass it an empty string ?"
oops. I see ... need to test for empty string.
nothrow pure size_t strlen2(const(char)* c) {
if (c is null || *c==0)
return 0;
const(char)* c_save = c;
while (*c){ c+=4; }
while (*c==0){ c--; }
c++;
return c - c_save;
}
Re: 4x faster strlen with 4 char sentinel
On Tuesday, 28 June 2016 at 01:53:22 UTC, deadalnix wrote:
If we were in interview, I'd ask you "what does this returns if
you pass it an empty string ?"
I'd say use this one instead, to avoid negative size_t. It is
also a little faster for the same measurement.
nothrow pure size_t strlen2(const(char)* c) {
if (c is null)
return 0;
const(char)* c_save = c;
while (*c){ c+=4; }
while (*c==0){ c--; }
c++;
return c - c_save;
}
2738
540
2744
Re: 4x faster strlen with 4 char sentinel
On Sunday, 26 June 2016 at 16:40:08 UTC, Jay Norwood wrote:
After watching Andre's sentinel thing, I'm playing with strlen
on char strings with 4 terminating 0s instead of a single one.
Seems to work and is 4x faster compared to the runtime version.
nothrow pure size_t strlen2(const(char)* c) {
if (c is null)
return 0;
size_t l=0;
while (*c){ c+=4; l+=4;}
while (*c==0){ c--; l--;}
return l+1;
}
This is the timing of my test case, which I can post if anyone
is interested.
strlen\Release>strlen
2738
681
If we were in interview, I'd ask you "what does this returns if
you pass it an empty string ?"
Re: 4x faster strlen with 4 char sentinel
On Monday, 27 June 2016 at 19:51:48 UTC, Jay Norwood wrote: I also found it strange, the non-zero initialization values for char, dchar, wchar. I suppose there's some reason? int [100] to zeros. char [100] to 0xff; dchar [100] to 0x; wchar [100] to 0x; The same reason float and double are default initialized to nan. char, wchar and dchar are default initialized to invalid unicode values.
Re: 4x faster strlen with 4 char sentinel
On Monday, 27 June 2016 at 21:41:57 UTC, Jay Norwood wrote: measurements. I'm using a 100KB char array terminated by four zeros, and doing strlen on substring pointers into it incremented by 1 for 100K times. But this is a rather atypical use case for zero terminated strings? It would make more sense to test it on a massive amount of short strings stuffed into a very large hash-table. (filenames, keys, user names, email adresses etc)
Re: 4x faster strlen with 4 char sentinel
On Monday, 27 June 2016 at 20:43:40 UTC, Ola Fosheim Grøstad wrote: Just keep in mind that the major bottleneck now is loading 64 bytes from memory into cache. So if you test performance you have to make sure to invalidate the caches before you test and test with spurious reads over a very large memory area to get realistic results. But essentially, the operation is not heavy, so to speed it up you need to predict and prefetch from memory in time, meaning no library solution is sufficient. (you need to prefetch memory way before your library function is called) I doubt the external memory accesses are involved in these measurements. I'm using a 100KB char array terminated by four zeros, and doing strlen on substring pointers into it incremented by 1 for 100K times. The middle of the three timings is for strlen2, while the two outer timings are for strlen during the same program execution. I'm initializing the 100KB immediately prior to the measurement. The 100KB array should all be in L1 or L2 cache by the time I make even the first of the three time measurements. The prefetch shouldn't have a problem predicting this. 2749 688 2783 2741 683 2738
Re: 4x faster strlen with 4 char sentinel
On Monday, 27 June 2016 at 06:31:49 UTC, Ola Fosheim Grøstad wrote: On Monday, 27 June 2016 at 05:27:12 UTC, chmike wrote: Ending strings with a single null byte/char is to save space. It was critical in the 70´s when C was created and memory space was very limited. That's not the case anymore and I guess the Not only to save space, some CPUs also had cheap incrementing load/stores and branching on zero is faster than sacrificing another register for a counter. I incidentally just found my 1992 implementation for Motorola 68K, to illustrate: _mystrcpy move.l 4(sp),a1; pointer for destination move.l 8(sp),a0; pointer for source mystrcpymove.l a0,d0 1$ move.b (a0)+,(a1)+ ; copy bne.s 1$ ; jump back up if not zero rts As you can see it is a tight loop. Other CPUs are even tighter, and have single-instruction loops (even 8086?) So not only storage, also performance on specific CPUs. Which is a good reason for keeping datatypes in standard libraries abstract, different CPUs favour different representations. Even on very basic datatypes.
Re: 4x faster strlen with 4 char sentinel
On Monday, 27 June 2016 at 19:51:48 UTC, Jay Norwood wrote: Your link's use of padding pads out with a variable number of zeros, so that a larger data type can be used for the compare operations. This isn't the same as my example, which is simpler due to not having to fiddle with alignment and data type casting. That's true, and it is fun to think about different string implementations. Just keep in mind that prior to the 90s, text was the essential datatype for many programmers and inventing new ways to do strings is heavily explored. I remember the first exercise we got at the university when doing the OS course was to implement "strlen", "strcpy" and "strcmp" in C or machine language. It can be fun. Just keep in mind that the major bottleneck now is loading 64 bytes from memory into cache. So if you test performance you have to make sure to invalidate the caches before you test and test with spurious reads over a very large memory area to get realistic results. But essentially, the operation is not heavy, so to speed it up you need to predict and prefetch from memory in time, meaning no library solution is sufficient. (you need to prefetch memory way before your library function is called)
Re: 4x faster strlen with 4 char sentinel
On Monday, 27 June 2016 at 16:38:58 UTC, Ola Fosheim Grøstad wrote: Yes, and the idea of speeding up strings by padding out with zeros is not new. ;-) I recall suggesting it back in 1999 when discussing the benefits of having a big endian cpu when sorting strings. If it is big endian you can compare ascii as 32/64 bit integers, so if you align the string and pad out with zeros then you can speed up strcmp() by a significant factor. Oh, here it is: Your link's use of padding pads out with a variable number of zeros, so that a larger data type can be used for the compare operations. This isn't the same as my example, which is simpler due to not having to fiddle with alignment and data type casting. I didn't find a strlen implementation for dchar or wchar in the D libraries. I also found it strange, the non-zero initialization values for char, dchar, wchar. I suppose there's some reason? int [100] to zeros. char [100] to 0xff; dchar [100] to 0x; wchar [100] to 0x;
Re: 4x faster strlen with 4 char sentinel
On 6/26/2016 11:47 AM, Jay Norwood via Digitalmars-d-announce wrote: On Sunday, 26 June 2016 at 16:59:54 UTC, David Nadlinger wrote: Please keep general discussions like this off the announce list, which would e.g. be suitable for announcing a fleshed out collection of high-performance string handling routines. A couple of quick hints: - This is not a correct implementation of strlen, as it already assumes that the array is terminated by four zero bytes. That iterating memory with a stride of 4 instead of 1 will be faster is a self-evident truth. - You should be benchmarking against a "proper" SIMD-optimised strlen implementation. — David This is more of just an observation that the choice of the single zero sentinel for C string termination comes at a cost of 4x strlen speed vs using four terminating zeros. I don't see a SIMD strlen implementation in the D libraries. The strlen2 function I posted works on any string that is terminated by four zeros, and returns the same len as strlen in that case, but much faster. How to get strings initialized with four terminating zeros at compile time is a separate issue. I don't know the solution, else I might consider doing more with this. Yup.. there's a reason that many many hours have been spent optimizing strlen and other memory related length and comparison routines. They are used a lot and the number of ways of making them fast varies almost as much as the number of cpu's that exist. This effort is embedded in the code gen of compilers (other than dmd) and libc runtimes. Trying to re-invent it is noble, and very educational, but largely redundant.
Re: 4x faster strlen with 4 char sentinel
On Monday, 27 June 2016 at 16:22:56 UTC, Jay Norwood wrote: This strlen2 doesn't require special alignment or casting of char pointer types to some larger type. That keeps the strlen2 implementation fairly simple. Yes, and the idea of speeding up strings by padding out with zeros is not new. ;-) I recall suggesting it back in 1999 when discussing the benefits of having a big endian cpu when sorting strings. If it is big endian you can compare ascii as 32/64 bit integers, so if you align the string and pad out with zeros then you can speed up strcmp() by a significant factor. Oh, here it is: http://disinterest.org/resource/MUD-Dev/1999q1/009759.html Of course, this is all moot now, little endian + simd has made such tricks redundant. Simd probably makes your strlen2 redundant too. The bottle neck tends to be memory access/prefetching for simple algorithms.
Re: 4x faster strlen with 4 char sentinel
On Monday, 27 June 2016 at 06:31:49 UTC, Ola Fosheim Grøstad wrote: Besides there are plenty of other advantages to using a terminating sentinel depending on the use scenario. E.g. if you want many versions of the same tail or if you are splitting a string at white space (overwrite a white space char with a zero). This strlen2 doesn't require special alignment or casting of char pointer types to some larger type. That keeps the strlen2 implementation fairly simple. The implementation is only testing one char per increment. It doesn't require the extra xor processing used in some of the examples. I haven't checked if there is a strlen for dchar or wchar, but it should also speed up those.
Re: 4x faster strlen with 4 char sentinel
On Monday, 27 June 2016 at 05:27:12 UTC, chmike wrote: Ending strings with a single null byte/char is to save space. It was critical in the 70´s when C was created and memory space was very limited. That's not the case anymore and I guess the Not only to save space, some CPUs also had cheap incrementing load/stores and branching on zero is faster than sacrificing another register for a counter. IIRC Pascal has a short string with a single byte length. Besides there are plenty of other advantages to using a terminating sentinel depending on the use scenario. E.g. if you want many versions of the same tail or if you are splitting a string at white space (overwrite a white space char with a zero).
Re: 4x faster strlen with 4 char sentinel
On Monday, 27 June 2016 at 05:10:37 UTC, chmike wrote: If you store the string size in a four byte field, you get a hard to beat fast strlen. :) If you have the constrain to work with null terminated strings, then it could be interesting to look at SIMD as was suggested or use one of the following algorithms to test the presence of a 0 byte in a 4 byte or 8 byte integer when simd is not available. https://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord Ending strings with a single null byte/char is to save space. It was critical in the 70´s when C was created and memory space was very limited. That's not the case anymore and I guess the reason why you assume that ending a string with 4 null chars is OK.
Re: 4x faster strlen with 4 char sentinel
If you store the string size in a four byte field, you get a hard to beat fast strlen. :)
Re: 4x faster strlen with 4 char sentinel
On Sunday, 26 June 2016 at 16:59:54 UTC, David Nadlinger wrote: Please keep general discussions like this off the announce list, which would e.g. be suitable for announcing a fleshed out collection of high-performance string handling routines. A couple of quick hints: - This is not a correct implementation of strlen, as it already assumes that the array is terminated by four zero bytes. That iterating memory with a stride of 4 instead of 1 will be faster is a self-evident truth. - You should be benchmarking against a "proper" SIMD-optimised strlen implementation. — David This is more of just an observation that the choice of the single zero sentinel for C string termination comes at a cost of 4x strlen speed vs using four terminating zeros. I don't see a SIMD strlen implementation in the D libraries. The strlen2 function I posted works on any string that is terminated by four zeros, and returns the same len as strlen in that case, but much faster. How to get strings initialized with four terminating zeros at compile time is a separate issue. I don't know the solution, else I might consider doing more with this.
Re: 4x faster strlen with 4 char sentinel
Hi Jay,
On Sunday, 26 June 2016 at 16:40:08 UTC, Jay Norwood wrote:
After watching Andre's sentinel thing, I'm playing with strlen
on char strings with 4 terminating 0s instead of a single one.
Seems to work and is 4x faster compared to the runtime version.
Please keep general discussions like this off the announce list,
which would e.g. be suitable for announcing a fleshed out
collection of high-performance string handling routines.
nothrow pure size_t strlen2(const(char)* c) {
if (c is null)
return 0;
size_t l=0;
while (*c){ c+=4; l+=4;}
while (*c==0){ c--; l--;}
return l+1;
}
A couple of quick hints:
- This is not a correct implementation of strlen, as it already
assumes that the array is terminated by four zero bytes. That
iterating memory with a stride of 4 instead of 1 will be faster
is a self-evident truth.
- You should be benchmarking against a "proper" SIMD-optimised
strlen implementation.
— David
4x faster strlen with 4 char sentinel
After watching Andre's sentinel thing, I'm playing with strlen on
char strings with 4 terminating 0s instead of a single one.
Seems to work and is 4x faster compared to the runtime version.
nothrow pure size_t strlen2(const(char)* c) {
if (c is null)
return 0;
size_t l=0;
while (*c){ c+=4; l+=4;}
while (*c==0){ c--; l--;}
return l+1;
}
This is the timing of my test case, which I can post if anyone is
interested.
strlen\Release>strlen
2738
681
