Sujet : Re: Segments
De : terje.mathisen (at) *nospam* tmsw.no (Terje Mathisen)
Groupes : comp.archDate : 17. Jan 2025, 18:02:28
Autres entêtes
Organisation : A noiseless patient Spider
Message-ID : <vme2f5$4nes$1@dont-email.me>
References : 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
User-Agent : Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0 SeaMonkey/2.53.19
David Brown wrote:
On 17/01/2025 03:10, MitchAlsup1 wrote:
On Fri, 17 Jan 2025 1:04:03 +0000, Keith Thompson wrote:
>
mitchalsup@aol.com (MitchAlsup1) writes:
On Thu, 16 Jan 2025 23:18:22 +0000, Keith Thompson wrote:
Terje Mathisen <terje.mathisen@tmsw.no> writes:
[...]
I do know that several people have created fast string libraries,
where any string that is short enough ends up entirely inside the dope
vector, so no heap allocation.
>
Some implementations of C++ std::string do this. For example, the GNU
implementation appears to store up to 16 characters (including the
trailing null character) in the std::string object.
>
Why use an 8-byte pointer to store a string 16 or fewer bytes long ? !!
>
I don't understand. What pointer are you referring to?
>
The pointer which would have had to point elsewhere had the string
not been contained within.
>
There are a couple of ways you can do "small string optimisation". One would be to have a structure something like this :
struct String1 {
size_t capacity;
char * data;
char small_string[16];
}
Then "data" would point to "small_string" for a capacity of 16, and if that's not enough, use malloc to allocate more space.
An alternative would be to have something like this (I'm being /really/ sloppy with alignments, rules for unions, and so on - this is illustrative only, not real code!) :
struct String2 {
bool is_small;
union {
char small_string[31];
struct {
size_t capacity;
char * data;
}
}
}
This second version lets you put more characters in the local small_string area, reusing space that would otherwise be used for the pointer and capacity. But it has more runtime overhead when using the string :
void print1(String1 s) {
printf(s.data);
}
void print2(String2 s) {
if (s.is_small) {
printf(s.small_string);
} else {
printf(s.data);
}
}
There are, of course, many other ways to make string types (such as supporting copy-on-write), but I suspect that Mitch is thinking of style String2 while Keith is thinking of style String1.
All Vec<> types have a 3-word descriptor, with the first and second word being a pointer to the data and the current length, while the allocated size is stored in the third word.
This is a total of 24 bytes, so quite a bit of overhead if you just need a few bytes.
In the Rust Fast/Small vector type, they could use the top bit of the size field (no Vec<> object can be larger or equal to 2^63 bytes), then they need 4 or 5 bits for the actual length (but using 7 is easier), leaving 23 bytes for the embedded data. With little-endian storage this corresponds to the last byte of the 24-byte block.
Terje
-- - <Terje.Mathisen at tmsw.no>"almost all programming can be viewed as an exercise in caching"
Date | Sujet | # | | Auteur |
1 Oct 24 | Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress) | 387 | | MitchAlsup1 |
1 Oct 24 |  Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress) | 386 | | Thomas Koenig |
1 Oct 24 |   Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress) | 379 | | MitchAlsup1 |
2 Oct 24 |    Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress) | 377 | | Brett |
3 Oct 24 |     Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress) | 376 | | Lawrence D'Oliveiro |
3 Oct 24 |      Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress) | 1 | | Brett |
3 Oct 24 |      Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress) | 1 | | Anton Ertl |
3 Oct 24 |      Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress) | 373 | | David Brown |
3 Oct 24 |       Byte ordering (was: Whether something is RISC or not) | 372 | | Anton Ertl |
3 Oct 24 |        Re: Byte ordering (was: Whether something is RISC or not) | 1 | | David Brown |
3 Oct 24 |        Re: Byte ordering (was: Whether something is RISC or not) | 369 | | Lawrence D'Oliveiro |
4 Oct 24 |         Re: Byte ordering | 1 | | Lynn Wheeler |
4 Oct 24 |         Re: Byte ordering (was: Whether something is RISC or not) | 365 | | David Brown |
4 Oct 24 |          Re: Byte ordering (was: Whether something is RISC or not) | 364 | | Anton Ertl |
4 Oct 24 |           Re: Byte ordering | 5 | | BGB |
5 Oct 24 |            Re: Byte ordering | 4 | | MitchAlsup1 |
5 Oct 24 |             Re: Byte ordering | 2 | | BGB |
5 Oct 24 |              Re: Byte ordering | 1 | | Lawrence D'Oliveiro |
5 Oct 24 |             Re: Byte ordering | 1 | | Lawrence D'Oliveiro |
5 Oct 24 |           Re: Byte ordering (was: Whether something is RISC or not) | 13 | | Lawrence D'Oliveiro |
5 Oct 24 |            Re: Byte ordering (was: Whether something is RISC or not) | 12 | | Brett |
5 Oct 24 |             Re: Byte ordering (was: Whether something is RISC or not) | 11 | | Anton Ertl |
5 Oct 24 |              Re: Byte ordering (was: Whether something is RISC or not) | 10 | | Michael S |
6 Oct 24 |               Re: Byte ordering | 1 | | Terje Mathisen |
6 Oct 24 |               Re: Byte ordering (was: Whether something is RISC or not) | 8 | | Brett |
7 Oct 24 |                Re: Byte ordering (was: Whether something is RISC or not) | 7 | | Lawrence D'Oliveiro |
7 Oct 24 |                 Re: Byte ordering (was: Whether something is RISC or not) | 6 | | Brett |
7 Oct 24 |                  Re: Byte ordering (was: Whether something is RISC or not) | 5 | | Michael S |
7 Oct 24 |                   Re: Byte ordering | 2 | | Stefan Monnier |
7 Oct 24 |                    Re: Byte ordering | 1 | | Michael S |
7 Oct 24 |                   Re: Byte ordering (was: Whether something is RISC or not) | 2 | | Lawrence D'Oliveiro |
8 Oct 24 |                    Re: Byte ordering | 1 | | Terje Mathisen |
6 Oct 24 |           Re: Byte ordering | 345 | | David Brown |
6 Oct 24 |            Re: Byte ordering | 344 | | Anton Ertl |
6 Oct 24 |             Re: Byte ordering | 189 | | John Dallman |
7 Oct 24 |              Re: Byte ordering | 20 | | Lawrence D'Oliveiro |
8 Oct 24 |               Re: Byte ordering | 19 | | John Dallman |
9 Oct 24 |                VMS/NT memory management (was: Byte ordering) | 1 | | Stefan Monnier |
15 Oct 24 |                Re: Byte ordering | 2 | | Lawrence D'Oliveiro |
15 Oct 24 |                 Re: Byte ordering | 1 | | MitchAlsup1 |
15 Oct 24 |                Re: Byte ordering | 15 | | Lawrence D'Oliveiro |
15 Oct 24 |                 Re: Byte ordering | 3 | | Michael S |
15 Oct 24 |                  Re: Byte ordering | 1 | | John Dallman |
18 Oct 24 |                  Re: Byte ordering | 1 | | Lawrence D'Oliveiro |
15 Oct 24 |                 Re: Byte ordering | 9 | | John Dallman |
16 Oct 24 |                  Re: Byte ordering | 7 | | George Neuner |
16 Oct 24 |                   Re: Byte ordering | 6 | | Terje Mathisen |
16 Oct 24 |                    Re: Byte ordering | 5 | | David Brown |
17 Oct 24 |                     Re: Byte ordering | 2 | | George Neuner |
17 Oct 24 |                      Re: Byte ordering | 1 | | David Brown |
17 Oct 24 |                     Re: clouds, not Byte ordering | 2 | | John Levine |
17 Oct 24 |                      Re: clouds, not Byte ordering | 1 | | David Brown |
18 Oct 24 |                  Re: Byte ordering | 1 | | Lawrence D'Oliveiro |
16 Oct 24 |                 Re: Byte ordering | 2 | | Paul A. Clayton |
18 Oct 24 |                  Re: Microkernels & Capabilities (was Re: Byte ordering) | 1 | | Lawrence D'Oliveiro |
7 Oct 24 |              80286 protected mode | 168 | | Anton Ertl |
7 Oct 24 |               Re: 80286 protected mode | 5 | | Lars Poulsen |
7 Oct 24 |                Re: 80286 protected mode | 4 | | Terje Mathisen |
7 Oct 24 |                 Re: 80286 protected mode | 1 | | Michael S |
7 Oct 24 |                 Re: 80286 protected mode | 2 | | Lawrence D'Oliveiro |
8 Oct 24 |                  Re: 80286 protected mode | 1 | | Terje Mathisen |
7 Oct 24 |               Re: 80286 protected mode | 3 | | Brett |
7 Oct 24 |                Re: 80286 protected mode | 2 | | Michael S |
7 Oct 24 |                 Re: 80286 protected mode | 1 | | Brett |
7 Oct 24 |               Re: 80286 protected mode | 1 | | Lawrence D'Oliveiro |
8 Oct 24 |               Re: 80286 protected mode | 152 | | MitchAlsup1 |
8 Oct 24 |                Re: 80286 protected mode | 4 | | Lawrence D'Oliveiro |
8 Oct 24 |                 Re: 80286 protected mode | 3 | | MitchAlsup1 |
9 Oct 24 |                  Re: 80286 protected mode | 1 | | David Brown |
15 Oct 24 |                  Re: 80286 protected mode | 1 | | Lawrence D'Oliveiro |
8 Oct 24 |                Re: 80286 protected mode | 147 | | Anton Ertl |
8 Oct 24 |                 Re: 80286 protected mode | 1 | | Robert Finch |
9 Oct 24 |                 Re: 80286 protected mode | 145 | | David Brown |
9 Oct 24 |                  Re: 80286 protected mode | 79 | | MitchAlsup1 |
9 Oct 24 |                   Re: 80286 protected mode | 78 | | David Brown |
9 Oct 24 |                    Re: 80286 protected mode | 77 | | Stephen Fuld |
10 Oct 24 |                     Re: 80286 protected mode | 2 | | MitchAlsup1 |
10 Oct 24 |                      Re: 80286 protected mode | 1 | | David Brown |
10 Oct 24 |                     Re: 80286 protected mode | 1 | | David Brown |
11 Oct 24 |                     Re: 80286 protected mode | 73 | | Tim Rentsch |
15 Oct 24 |                      Re: 80286 protected mode | 72 | | Stefan Monnier |
15 Oct 24 |                       Re: 80286 protected mode | 30 | | MitchAlsup1 |
16 Oct 24 |                        Re: 80286 protected mode | 25 | | MitchAlsup1 |
16 Oct 24 |                         Re: C and turtles, 80286 protected mode | 13 | | John Levine |
16 Oct 24 |                          Re: C and turtles, 80286 protected mode | 7 | | MitchAlsup1 |
16 Oct 24 |                           Re: C and turtles, 80286 protected mode | 6 | | John Levine |
17 Oct 24 |                            Re: C and turtles, 80286 protected mode | 5 | | Thomas Koenig |
20 Oct 24 |                             Re: C and turtles, 80286 protected mode | 4 | | Lawrence D'Oliveiro |
20 Oct 24 |                              Re: C and turtles, 80286 protected mode | 3 | | George Neuner |
22 Oct 24 |                               Re: C and turtles, 80286 protected mode | 2 | | Tim Rentsch |
22 Oct 24 |                                Re: C and turtles, 80286 protected mode | 1 | | George Neuner |
16 Oct 24 |                          Re: C and turtles, 80286 protected mode | 1 | | David Brown |
16 Oct 24 |                          Re: C and turtles, 80286 protected mode | 4 | | Paul A. Clayton |
17 Oct 24 |                           Re: C and turtles, 80286 protected mode | 1 | | David Brown |
20 Oct 24 |                           Re: C and turtles, 80286 protected mode | 2 | | Lawrence D'Oliveiro |
20 Oct 24 |                            Re: C and turtles, 80286 protected mode | 1 | | Paul A. Clayton |
16 Oct 24 |                         Re: 80286 protected mode | 7 | | Thomas Koenig |
16 Oct 24 |                          Re: 80286 protected mode | 2 | | MitchAlsup1 |
17 Oct 24 |                           Re: 80286 protected mode | 1 | | Tim Rentsch |
17 Oct 24 |                          Re: 80286 protected mode | 4 | | Tim Rentsch |
17 Oct 24 |                           Re: fine points of dynamic memory allocation, not 80286 protected mode | 3 | | John Levine |
17 Oct 24 |                         Re: 80286 protected mode | 3 | | George Neuner |
17 Oct 24 |                         Re: 80286 protected mode | 1 | | Tim Rentsch |
16 Oct 24 |                        Re: 80286 protected mode | 3 | | David Brown |
17 Oct 24 |                        Re: 80286 protected mode | 1 | | Tim Rentsch |
16 Oct 24 |                       Re: 80286 protected mode | 41 | | David Brown |
9 Oct 24 |                  Re: 80286 protected mode | 51 | | Thomas Koenig |
13 Oct 24 |                  Re: 80286 protected mode | 14 | | Anton Ertl |
8 Oct 24 |               Re: 80286 protected mode | 6 | | John Levine |
3 Jan 25 |             Re: Byte ordering | 154 | | Waldek Hebisch |
6 Oct 24 |         Re: Byte ordering (was: Whether something is RISC or not) | 2 | | Michael S |
3 Oct 24 |        Re: Byte ordering (was: Whether something is RISC or not) | 1 | | John Dallman |
2 Oct 24 |    Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress) | 1 | | Thomas Koenig |
2 Oct 24 |   Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress) | 5 | | David Schultz |
3 Oct 24 |   Re: Whether something is RISC or not (Re: PDP-8 theology, not Concertina II Progress) | 1 | | Lawrence D'Oliveiro |