14 ms·

Writing memory efficient C structs

203 points by aragonite 1y ago

jakey_bakey 1y ago
Loved this, thanks!
sim7c00 1y ago
definitely interesting points for C structs. regarding CPU it might be interesting to look a little further still regarding optimal sizes for structs regarding cachcing, figuring out hot and cold areas and either including padding or putting cold values behind pointer to a substruct. like the examples though, very clear what the changes are and the specific impact. always a good reminder to review for these things after a session hacking in new things!
uticus 1y ago
for another good intro to the fun with structs, Beej's guide is always a good goto: https://beej.us/guide/bgc/html/split-wide/structs-ii-more-fun-with-structs.html https://beej.us/guide/bgc/html/split-wide/structs-ii-more-fu... ...goes into offsets, padding bytes, bit-field, unions, etc etc.
- degenoah 1y ago
  I feel like this page would have definitely helped me when I used to try reverse engineering things I had lying around on my computer.
warmwaffles 1y ago
This is also applicable outside of C as well.
- mcraiha 1y ago
  "The x_position and y_position can unfortunately not get a smaller type nor can a float be unsigned." If you have latest C++, you can use float16_t or bfloat16_t to get smaller type https://en.cppreference.com/w/cpp/types/floating-point.html https://en.cppreference.com/w/cpp/types/floating-point.html
  - david2ndaccount 1y ago
    C23 has _Float16
rwmj 1y ago
No mention of pahole / dwarves? https://lwn.net/Articles/335942/ https://lwn.net/Articles/335942/ It's the standard tool used by the Linux kernel developers.
- uticus 1y ago
  for somebody starting out, where do you go for a list of tools like this?
- petergeoghegan 1y ago
  Recent versions of clangd show pahole-like struct size/alignment padding overhead annotations. I find this built-in support a lot more convenient than using pahole.
- ryao 1y ago
  pahole is an awesome tool.
_rend 1y ago
For completeness, this description of alignment is misleading: > Well, dear reader, this padding is added because the CPU needs memory to be aligned in sets of 4 bytes because it’s optimized in that fashion. > ... > Remember: since structs are aligned to 4 bytes, any padding is therefore unnecessary if the size of the struct is a multiple of 4 without the padding. Individual data types have their own alignment (e.g., `bool`/`char` may be 1, `short` may be 2, `int` may be 4, `long` may be 8, etc.), and the alignment of a compound type (like a struct) defaults to the maximum alignment of its constituent types. In this article, `struct Monster` has an alignment of 4 because `int` and `float` have an alignment of 4 for the author's configuration. Expanding one of the `int`s to a `long` could increase the alignment to 8 on some CPUs, and removing the `int` and `float` fields would decrease the alignment to 1 for most CPUs.
- sumtechguy 1y ago
  Also keep in mind that is also all very CPU and compiler specific. Had one compiler where it packed everything at 4/8, usually 8. Not the 1/2/4/8 you would expect. That was because the CPU would just seg fault if you didnt play nice with the data access. The compiler hid a lot of it if you set the packing with offsets and mem moves and shifting. It was clever but slow. So they by default picked a wide enough packing that removed the extra instructions at the cost of using more memory. x86 was by far the most forgiving while at the time I was doing it. ARM was the least forgiving (at least on the platform I was using). With MIPS being OK in some cases but not others.
  - gavinsyancey 1y ago
    Um ... isn't alignment generally dictated by the platform ABI so that programs compiled by different compilers can be linked together?
    - plorkyeran 1y ago
      The widely used platforms with multiple compilers generally have one or more written down ABIs that the compilers all follow, but more niche platforms frequently have exactly one compiler (often a very out of date fork of gcc) that just does whatever they felt like implementing and may not even support linking together things built by different versions of that one compiler.
david2ndaccount 1y ago
The author relies on the compiler fitting the bitfield in unused padding bytes after “speed”, but that is implementation-defined behavior (almost everything about bitfields is implementation defined). MSVC will align the unsigned bitfield to alignof(unsigned), whereas GCC and clang will use the padding bytes. To portably pack the struct, use unsigned integers and use flags (monster.flags & CAN_FLY instead of monster.can_fly). See https://c.godbolt.org/z/K7oY77KGj https://c.godbolt.org/z/K7oY77KGj
- arjvik 1y ago
  Why can't there be a standard defined for bitfields in future C releases? This is a long-discussed drawback of a feature that I really really want to be able to use in production code.
  - ajross 1y ago
    There is, but it's part of the platform ABI and not the C language standard. The latter specifies syntax and behavior, the former is what's concerned with interoperability details like memory layout. I happen to have a copy of AAPCS64 in front of me, and you can find the specification of bit packing in section 10.1.8. The sysv ABI for x86/x86_64 has its own wording (but I'm pretty sure is compatible). MSVC does something else I believe, etc...
  - gizmo686 1y ago
    There is. I'm not sure when it was added, but looking at https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf, page 106, note 13 > An implementation may allocate any addressable storage unit large enough to hold a bit-field. If enough space remains, a bit-field that immediately follows another bit-field in a structure shall be packed into adjacent bits of the same unit. If insufficient space remains, whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is implementation-defined. The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation-defined. The alignment of the addressable storage unit is unspecified. In practice, I've been using this feature for ages (along with __attribute__((packed))). There comes a point where you can start depending on compiler specific features instead of relying solely on the standard.
bullen 1y ago
Adding to this you want the struct to be exactly 64 bytes, even on Apple MX processors which have 128 bytes of cache.
signa11 1y ago
and i raise you this: https://www.catb.org/esr/structure-packing/ https://www.catb.org/esr/structure-packing/
- rzzzt 1y ago
  Hmm, the certificate for this site has a billion SANs but catb .org is not one of them...
variadix 1y ago
Some other things you can do to improve packing are using integer indices/offsets/relative pointers instead of absolute pointers (especially on 64-bit machines), overlapping fields for unions (as an example you may have a tag that can share some bits with another field in each member of the union), using a structs position in memory to encode information (for example, if you have a binary tree of array allocated nodes, you can specify that the right child always follows the parent in memory, then you only need to store the index of the left child), and of course structure packing pragmas/attributes.
westurner 1y ago
Is it possible to apply these optimizations to Arrow? Arrow's struct is called StructArray. Fields of StructArray have a StructType. StructType has .bit_width and .byte_width attrs in Python and probably the other implementations too: https://arrow.apache.org/docs/python/generated/pyarrow.StructType.html#pyarrow.StructType.bit_width https://arrow.apache.org/docs/python/generated/pyarrow.Struc... Arrow supports bitfields with BooleanArray, and enums with categoricals but "BUG: Categorical columns using the PyArrow backend requires 4x more memory" (open) https://github.com/pandas-dev/pandas/issues/58062 https://github.com/pandas-dev/pandas/issues/58062 : > On disk Parquet appears to store the category data as logical type String which is compressed with snappy and encoded Arrow Flight RPC handles nested structs with enums over the wire somehow too FWIU
Retr0id 1y ago
I don't think I've ever seen struct bitfields used in real code, it's almost always manual bit-masks against a numeric type, often via macros. (I assume due to portability)
- abnercoimbre 1y ago
  Yep. If I need a boolean for a struct I know I’ll want more than one. So I’ll stick an integer type that lets me OR multiple enum values (and so on).
- gizmo686 1y ago
  I see it decently often in embedded code for memory mapped IO registers. Other than that, I don't think I've ever seen it either. I'm not sure portability is the reason though. I've seen it in places that have no problem spamming #define __GNU_SOURCE
  - nomel 1y ago
    This is the only place I've seen it used, and the only placed I've used it. It's absolutely wonderful for IO.
  - mort96 1y ago
    I would be scared of using it for IO... With a struct bit field, I don't know what bit offset a field ends up with, I don't know how it gets padded, and I don't know how large the generated load and store becomes. I'd rather do the bit operations and store myself.
    - gizmo686 1y ago
      __attribute__((packed)) is pretty much required for this use case.
      - mort96 1y ago
        That gets rid of the padding issue, but still does little to tell me which bit corresponds to which field.
miguel_martin 1y ago
>“… Note that I'm not an expert, in anything, so take everything I write about with a grain of salt I guess :).” >”I’m by far an expert and I could be wrong in some specifications” You’re missing a “not” on that 2nd sentence ;) In all seriousness, you don’t need this disclaimer, you’re selling yourself short. There’s nothing wrong with your post to make the disclaimer. And if there is something wrong with the post, let the comments flood in and tell you that rather than yourself.
cogman10 1y ago
Nice little article. The next thing to really explore is how the struct is being used. Smaller is always nice, but sometimes you need it to be fast. To do that, you'll want to group together fields that are commonly read. For example, x, y, and speed are likely always read together. So, it'd make sense to group those fields together to optimize for cache reads. The next thing to question is if your Monster is being interacted with as an individual thing or as a group of things. If it's as a group of things then it might make sense to create a "Struct of arrays" of monsters. Imagine, for example, you have a system which removes all the dead monsters. It'd be faster to iterate through an `isAlive` array rather than an array of `Monster` structs. And now you are on your way to reinventing an ECS :)
jesse__ 1y ago
Minor nitpick, 32bit max value is ~4 billion, not 4 million
zzo38computer 1y ago
I would expect many C programmers would know these things, because I do like that when writing programs in C, too (although for bit fields, I will use a single field and then named bits by macros). There are some other considerations as well though, such as static vs dynamic allocation, and the requirements of how the specific program is working.
andrewmcwatters 1y ago
The article casually ignores the fact that more production code uses bitmasking than bit fields.
- 1718627440 1y ago
  Why, though?
cjensen 1y ago
This is good stuff to have "in the toolbox," however habitually organizing structs this way is premature optimization. Much better to sensibly organize the fields and ignore alignment entirely unless you have a really good reason to do this. Good reasons could be that you know you will create a lot of these objects, or it has become a performance issue due to cache misses this would improve.
4gotunameagain 1y ago
Unpacked structs are also very useful for parsing network data. You take a package, cast it to an unpacked struct, and can access everything nicely. Many a spacecraft telemetry stacks work like that.
- JonChesterfield 1y ago
  The alias analysis pass in your C or C++ compiler will savage you for doing that unless you're careful with annotations or compiler flags though.
  - 4gotunameagain 1y ago
    Yeap, can definitely be a foot gun ! Even worse, sometimes you won't even get warnings about it, and then scratch your head why the binary seemingly has a mind of its own.. don't ask me how I know !
- rkangel 1y ago
  I think you mean "packed" structs. Otherwise you've got padding in your communication protocol. If you do this you also need to be careful of byte order.
  - 4gotunameagain 1y ago
    Yeah you're right, I mixed it up with unpadded ! And you're right about the second point as well, typically there will be network order to cpu order macros to ensure portability
foota 1y ago
Some related thoughts (more for C++ than C). You can eliminate the cost of a wrapping class with no members by taking advantage of the empty base class optimization (or rather: by avoiding members in a base class you will be taking advantage of EBCO). You can use an untagged union in C++ (taking care to avoid undefined behavior) to implement small object optimization. In it's general form, the idea is you can store some smaller struct in a union (as a char[], since C++ doesn't have native unions) with a larger structure, and use a sentinel in the larger structure to determine which it is. E.g., if large struct has some field that must not be zero, then make it zero in the smaller struct and you can distinguish between the two without a tag (like would be required with std::variant). I'm also a huge fan in general of "make invalid states unrepresentable" and following this can often reduce redundancy and eliminate extra fields.
kazinator 1y ago
Generally speaking, sort the members in descending order by alignment, if you want the tightest packing without resorting to nonportable compiler extensions. This descending order eliminates padding between members producing the smallest offset for the last byte of the last member, and that then minimizes the padding required at the end of the struct. In C, alignment for basic types is linked to size. A type may have an alignment requirement that is as much as its size: e.g. an 8 byte data type might be aligned as strictly as addresses divisible by 8. The alignment of an array aggregate is that of its element type. The alignment of a struct is that of its most strictly aligned member. In portable coding, we don't exactly know, but we can make the worst-case assumption that every basic type has the strictest possible alignment---i.e. its size. We also don't know the sizes of all the types, but we know that integers of higher ranks are at least as large as lower ranks; i.e. sizeof(char) <= sizeof(short) <= sizeof(int) <= sizeof(long), etc. Things are tricky when you have a mixture of floating-point, pointers and integers. E.g. pointer could be the same size as a float, or bigger. Here is a trick you use: combine together clumps of like types, and try to make combinations that are powers of two. For instance, if your structure has two or four pointers to things, try to make them adjacent. E.g.: struct s { void *p1, *p2, *p3, p4; short s1, s2, s3, s4; float f1, f2, f3, f4; }; Even though float is almost certainly larger than short and may have stricter alignment, its moot because I have four of everything. Even if this is on a tiny system where pointers are 16 bits wide, four of them will make 8 bytes. So when the f1 member is being placed into the struct, the offset is at least 8 byte aligned: we don't expect padding. If you aggregate enough members of the same type in a power-of-two sized clump, you can easily create alignment which meets or exceeds the strictest alignment in the system. So those are the main principles: group together members of the same type. Then keeping those groups together, sort by descending alignment strictness.
TinkersW 1y ago
Could improve it by using enum MonsterName :u8, moving it down near the other byte sized item, and specifying the exact type of the bitfield(u8). Then it would be 16 bytes.
_mocha 1y ago
I'm somewhat surprised to see this on the front page of HN. I distinctly remember this material being covered in a single lecture during our freshman CS course 20 years ago. I doubt this would've reached the front page in HN's early days, and it makes me wonder if we've moved away from teaching these fundamentals due to the heavy reliance on abstraction and ai agents in today's industry.
- jerf 1y ago
  One of the neat things about programming is that you can develop a high degree of skill in it without any formal education at all. But it does mean that we've always had those programmers who have had characteristic holes in their education as a result. Personally I enjoy having a mix of self-taught and formally-taught around; I think there quite a few things the self-taught are often better at than the formally educated as a group. I respect both sides. I'm just saying that there has always been a market for this sort of thing on HN because the self-taught have always been around.
  - nomel 1y ago
    > I think there quite a few things the self-taught are often better at than the formally educated as a group. Would you mind expanding on this a bit?
    - deleted 1y ago
      [deleted]
    - jerf 1y ago
      First, there are people who manage the best of both worlds, so bear in mind I'm talking about the ones who only manage the one side or the other. The self-taught ones are often much better at diagnosis of problems in my experience. "Hey, the servers are blowing up, own the process from diagnosis to deploying the fix" is something I can generally throw at a self-taught dev faster than a school-taught dev. Often because even if they've never owned a server they've already been through the "my graphics driver is crashing my desktop when I plug in my USB drive" dance on Linux a number of times and know how do go through diagnosis processes. They're also often better out-of-the-gate at being handed a large task and knowing how to break it down quickly without much direction. Sometimes they should be getting more direction than they do; this is often the source of bits of the code base that seem to be completely foreign in style, library usage, etc. Gotta watch the PRs. On the other hand, the self-taught will sometimes blunder into some critical O() mistakes because they haven't been taught to think that way, and may never have worked at any real scale before. They can also end up doing too much without using a library because they're used to just doing it themselves. (Not that school teaches you to "use libraries" exactly, but the school-taught are just generally less fluent and as such more likely to look for easier solutions... which is often exactly what we want!) Where the holes really develop is when we get into heavy data structures, or a compiler-like code base (though sadly many school-taught people escape without compilers), or heavily recursive code, which doesn't just include "recursion" per se but also things like working with DOMs that are recursive tree structures. A lot of time the self-taught are great at straight-line "do this, then do that, then do that" code bases but struggle to build longer-term maintainable code that has good reuse. And I'll say it again, these are all absolutely, positively, generalities and they do not apply to everyone. Some self-taught people are great at creating high quality, well-architected code bases. Some people escape school not just without compilers, but without any discernible skills whatsoever. (Those are some sad interviews.) These are just broad patterns I've observed.
o11c 1y ago
Note that although alignment and size are platform-specific, in practice padding can be minimized simply by using a fixed order: (various vector types) int128_t long double long long, int64_t, int_fast64_t, double off_t time_t (can be 64 bits on 32-bit platforms only if off_t also is) register_t, int_fast16_t, int_fast32_t (64-bit on e.g. x32) void *, void(*)(), intptr_t ptrdiff_t (theoretically wrong if fully aligned and bigger than pointer) size_t, ssize_t long (32-bit on win64; generally has 16-bit alignment if bigger than pointer) int32_t, float int (can be 16-bit) short, int16_t char, bool, int8_t, int_fast8_t (a trailing array member must come last; note when calculating combined size you should use offsetof instead of sizeof to not waste padding) Besides this, `int_leastN_t` goes with `intN_t`, and all unsigned types go with their signed types. Note that types like `id_t` are hard to place however. Although there do exist many platforms where this is not in order of size, the bigger types generally have smaller alignment on such platforms. Despite this I have split out rows occasionally despite not knowing of any such platform.
teo_zero 1y ago
Neither TFA nor any comment mention using a struct of arrays instead of an array of structs. This technique alone eliminates all padding bytes!
herf 1y ago
Sometimes struct-of-arrays (in-memory column store) is the right solution, and you get automatic packing that way. C makes it so much easier to store an array of structs that people don't do this often enough, but column stores prove it has its uses.
munificent 1y ago
This is a good article, but it bugs me that it sort of only tells half the story. Why are you making your structs smaller. Probably for "performance". If your goal is literally just reducing memory usage, then this is all fine. Maybe you're running on a small microcontroller and you just straight up don't have much RAM. But these days, in most areas, memory is cheap and plentiful. What's expensive is CPU cache. If you're optimizing memory use for that, it's really about runtime speed and less about total memory usage. You're trying to make things small so you have fewer cache misses and the CPU doesn't get stuck waiting as much. In that case, the advice here is only part of the story. Using bitfields and smaller integers saves memory. But in order to access those fields and do anything with them, they need to get loaded into registers. I'm not an expert here, but my understanding is that loading a bitfield or small int into a word-sized register can sometimes have some bit of overhead, so you have to pay attention to the trade-off. If I was optimizing for overall runtime speed and considering packing structs to do that, I'd really want to have good profiling and benchmarks in place first. Otherwise it's easy to think you're making progress when you're actually going backwards.
- zeusk 1y ago
  The real issue with packed structs and bitfields happens when concurrency gets involved. Majority of modern CPU caches are private and only allow one core to hold the cache line - so it creates more false dependencies when cores are trying to alter information that was compacted into a cache line on another core.
  - charleslmunger 1y ago
    Avoiding false sharing is a separate problem best solved by explicitly aligning the struct or relevant members to std::hardware_destructive_interference_size.
- wbl 1y ago
  Everything is microarch dependant but typically the memory subsystem can handle subword loads and writes. What it usually can't do is bits.
- 1y ago
iamcreasy 1y ago
This reminds of an amazing talk by Mike Acton on Data oriented design: https://www.youtube.com/watch?v=rX0ItVEVjHc https://www.youtube.com/watch?v=rX0ItVEVjHc And there is a related talk by Andrew Kelley implementing these ideas on Zig toolchain: https://www.youtube.com/watch?v=IroPQ150F6c https://www.youtube.com/watch?v=IroPQ150F6c
- anitil 1y ago
  I just watched that Andrew Kelly one yesterday, and really enjoyed it. I think Zig needs to be my next language to play with.
fattah25 1y ago
Remind me about Andrew Kelley conf "data-oriented design" He had explained about this topic.
bhawks 1y ago
Optimizing struct layouts is cool. but if your motivation is to pack more monsters in your game definitely consider transposing your row oriented monster struct into a column oriented entity using Entity Component System. Example framework: https://github.com/skypjack/entt?tab=readme-ov-file https://github.com/skypjack/entt?tab=readme-ov-file It has benefits besides memory/cache optimizations. It organizes logic in a much more composable way that is friendly for reuse.
skylabmelody 1y ago
I would suggest to never use unsigned int for values that will involve any sort of calculations on them (health, damage, speed). This is considered bad practice due to overflow and should be avoided, with the exception of values that are used for UIDs, indexes, bitfields, mask and flags.
- ChrisRR 1y ago
  Nah it's just whatever is most appropriate for your implementation. Overflow and wrap around is possible whether you use signed or unsigned. As I say to some more junior devs, check whether the maths is correct and then do it, don't do it and then check if it was wrong
  - skylabmelody 1y ago
    My suggestion is straight from the C++ guidelines by Bjarne Stroustrup and Herb Sutter (ES.102: Use signed types for arithmetic), for reasons that you don't seem to grasp.
    - ChrisRR 1y ago
      There's a reason they're guidelines and not rules. Use whatever is most appropriate for your use case
Rayhem 1y ago
> The health of a monster is currently represented by a 4 byte signed integer, meaning that the health can range from -2^31 to 2^31-1 (2^31 ~= 2 million). We can already see that half of the integers potential is unused because the health of a monster should never be negative, so an unsigned int would fit way better. If you're strictly optimizing for memory size out of necessity, then yes. It's dangerous to use an unsigned type in any context where you might even consider subtraction, though, and it's pretty effortless to see that new_health = health - damage is going to be used somewhere. Congrats! Your 100hp lvl3 monster just got resurrected with sixteen thousand hp.
Kilonzus 1y ago
There seems to be a typo in the 4th paragraph (directly under the struct memory table). I'm counting a padding of 7 bytes. 96 (size of struct) - 89 (size of primitive types) = 7. The first padding consist of 3 bytes to correctly align the 4 byte int after a 1 byte bool. Second padding is 2 bytes due to a 4 bytes int following two 1 byte bools and third padding is 2 bytes as described in the blog, 3+2+2=7 bytes of padding.

203 points by aragonite 1y ago

Loved this, thanks!

definitely interesting points for C structs. regarding CPU it might be interesting to look a little further still regarding optimal sizes for structs regarding cachcing, figuring out hot and cold areas and either including padding or putting cold values behind pointer to a substruct. like the examples though, very clear what the changes are and the specific impact. always a good reminder to review for these things after a session hacking in new things!

uticus 1y ago

for another good intro to the fun with structs, Beej's guide is always a good goto: https://beej.us/guide/bgc/html/split-wide/structs-ii-more-fun-with-structs.html https://beej.us/guide/bgc/html/split-wide/structs-ii-more-fu... ...goes into offsets, padding bytes, bit-field, unions, etc etc.

degenoah 1y ago
I feel like this page would have definitely helped me when I used to try reverse engineering things I had lying around on my computer.

warmwaffles 1y ago

This is also applicable outside of C as well.

mcraiha 1y ago
"The x_position and y_position can unfortunately not get a smaller type nor can a float be unsigned." If you have latest C++, you can use float16_t or bfloat16_t to get smaller type https://en.cppreference.com/w/cpp/types/floating-point.html https://en.cppreference.com/w/cpp/types/floating-point.html
- david2ndaccount 1y ago
  C23 has _Float16

rwmj 1y ago

No mention of pahole / dwarves? https://lwn.net/Articles/335942/ https://lwn.net/Articles/335942/ It's the standard tool used by the Linux kernel developers.

uticus 1y ago
for somebody starting out, where do you go for a list of tools like this?
petergeoghegan 1y ago
Recent versions of clangd show pahole-like struct size/alignment padding overhead annotations. I find this built-in support a lot more convenient than using pahole.
ryao 1y ago
pahole is an awesome tool.

_rend 1y ago

For completeness, this description of alignment is misleading: > Well, dear reader, this padding is added because the CPU needs memory to be aligned in sets of 4 bytes because it’s optimized in that fashion. > ... > Remember: since structs are aligned to 4 bytes, any padding is therefore unnecessary if the size of the struct is a multiple of 4 without the padding. Individual data types have their own alignment (e.g., `bool`/`char` may be 1, `short` may be 2, `int` may be 4, `long` may be 8, etc.), and the alignment of a compound type (like a struct) defaults to the maximum alignment of its constituent types. In this article, `struct Monster` has an alignment of 4 because `int` and `float` have an alignment of 4 for the author's configuration. Expanding one of the `int`s to a `long` could increase the alignment to 8 on some CPUs, and removing the `int` and `float` fields would decrease the alignment to 1 for most CPUs.

sumtechguy 1y ago
Also keep in mind that is also all very CPU and compiler specific. Had one compiler where it packed everything at 4/8, usually 8. Not the 1/2/4/8 you would expect. That was because the CPU would just seg fault if you didnt play nice with the data access. The compiler hid a lot of it if you set the packing with offsets and mem moves and shifting. It was clever but slow. So they by default picked a wide enough packing that removed the extra instructions at the cost of using more memory. x86 was by far the most forgiving while at the time I was doing it. ARM was the least forgiving (at least on the platform I was using). With MIPS being OK in some cases but not others.
- gavinsyancey 1y ago
  Um ... isn't alignment generally dictated by the platform ABI so that programs compiled by different compilers can be linked together?
  - plorkyeran 1y ago
    The widely used platforms with multiple compilers generally have one or more written down ABIs that the compilers all follow, but more niche platforms frequently have exactly one compiler (often a very out of date fork of gcc) that just does whatever they felt like implementing and may not even support linking together things built by different versions of that one compiler.

david2ndaccount 1y ago

The author relies on the compiler fitting the bitfield in unused padding bytes after “speed”, but that is implementation-defined behavior (almost everything about bitfields is implementation defined). MSVC will align the unsigned bitfield to alignof(unsigned), whereas GCC and clang will use the padding bytes. To portably pack the struct, use unsigned integers and use flags (monster.flags & CAN_FLY instead of monster.can_fly). See https://c.godbolt.org/z/K7oY77KGj https://c.godbolt.org/z/K7oY77KGj

arjvik 1y ago
Why can't there be a standard defined for bitfields in future C releases? This is a long-discussed drawback of a feature that I really really want to be able to use in production code.
- ajross 1y ago
  There is, but it's part of the platform ABI and not the C language standard. The latter specifies syntax and behavior, the former is what's concerned with interoperability details like memory layout. I happen to have a copy of AAPCS64 in front of me, and you can find the specification of bit packing in section 10.1.8. The sysv ABI for x86/x86_64 has its own wording (but I'm pretty sure is compatible). MSVC does something else I believe, etc...
- gizmo686 1y ago
  There is. I'm not sure when it was added, but looking at https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf, page 106, note 13 > An implementation may allocate any addressable storage unit large enough to hold a bit-field. If enough space remains, a bit-field that immediately follows another bit-field in a structure shall be packed into adjacent bits of the same unit. If insufficient space remains, whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is implementation-defined. The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation-defined. The alignment of the addressable storage unit is unspecified. In practice, I've been using this feature for ages (along with __attribute__((packed))). There comes a point where you can start depending on compiler specific features instead of relying solely on the standard.

bullen 1y ago

Adding to this you want the struct to be exactly 64 bytes, even on Apple MX processors which have 128 bytes of cache.

signa11 1y ago

and i raise you this: https://www.catb.org/esr/structure-packing/ https://www.catb.org/esr/structure-packing/

rzzzt 1y ago
Hmm, the certificate for this site has a billion SANs but catb .org is not one of them...

variadix 1y ago

Some other things you can do to improve packing are using integer indices/offsets/relative pointers instead of absolute pointers (especially on 64-bit machines), overlapping fields for unions (as an example you may have a tag that can share some bits with another field in each member of the union), using a structs position in memory to encode information (for example, if you have a binary tree of array allocated nodes, you can specify that the right child always follows the parent in memory, then you only need to store the index of the left child), and of course structure packing pragmas/attributes.

westurner 1y ago

Is it possible to apply these optimizations to Arrow? Arrow's struct is called StructArray. Fields of StructArray have a StructType. StructType has .bit_width and .byte_width attrs in Python and probably the other implementations too: https://arrow.apache.org/docs/python/generated/pyarrow.StructType.html#pyarrow.StructType.bit_width https://arrow.apache.org/docs/python/generated/pyarrow.Struc... Arrow supports bitfields with BooleanArray, and enums with categoricals but "BUG: Categorical columns using the PyArrow backend requires 4x more memory" (open) https://github.com/pandas-dev/pandas/issues/58062 https://github.com/pandas-dev/pandas/issues/58062 : > On disk Parquet appears to store the category data as logical type String which is compressed with snappy and encoded Arrow Flight RPC handles nested structs with enums over the wire somehow too FWIU

Retr0id 1y ago

I don't think I've ever seen struct bitfields used in real code, it's almost always manual bit-masks against a numeric type, often via macros. (I assume due to portability)

abnercoimbre 1y ago
Yep. If I need a boolean for a struct I know I’ll want more than one. So I’ll stick an integer type that lets me OR multiple enum values (and so on).
gizmo686 1y ago
I see it decently often in embedded code for memory mapped IO registers. Other than that, I don't think I've ever seen it either. I'm not sure portability is the reason though. I've seen it in places that have no problem spamming #define __GNU_SOURCE
- nomel 1y ago
  This is the only place I've seen it used, and the only placed I've used it. It's absolutely wonderful for IO.
- mort96 1y ago
  I would be scared of using it for IO... With a struct bit field, I don't know what bit offset a field ends up with, I don't know how it gets padded, and I don't know how large the generated load and store becomes. I'd rather do the bit operations and store myself.
  - gizmo686 1y ago
    __attribute__((packed)) is pretty much required for this use case.
    - mort96 1y ago
      That gets rid of the padding issue, but still does little to tell me which bit corresponds to which field.

miguel_martin 1y ago

>“… Note that I'm not an expert, in anything, so take everything I write about with a grain of salt I guess :).” >”I’m by far an expert and I could be wrong in some specifications” You’re missing a “not” on that 2nd sentence ;) In all seriousness, you don’t need this disclaimer, you’re selling yourself short. There’s nothing wrong with your post to make the disclaimer. And if there is something wrong with the post, let the comments flood in and tell you that rather than yourself.

cogman10 1y ago

Nice little article. The next thing to really explore is how the struct is being used. Smaller is always nice, but sometimes you need it to be fast. To do that, you'll want to group together fields that are commonly read. For example, x, y, and speed are likely always read together. So, it'd make sense to group those fields together to optimize for cache reads. The next thing to question is if your Monster is being interacted with as an individual thing or as a group of things. If it's as a group of things then it might make sense to create a "Struct of arrays" of monsters. Imagine, for example, you have a system which removes all the dead monsters. It'd be faster to iterate through an `isAlive` array rather than an array of `Monster` structs. And now you are on your way to reinventing an ECS :)

jesse__ 1y ago

Minor nitpick, 32bit max value is ~4 billion, not 4 million

zzo38computer 1y ago

I would expect many C programmers would know these things, because I do like that when writing programs in C, too (although for bit fields, I will use a single field and then named bits by macros). There are some other considerations as well though, such as static vs dynamic allocation, and the requirements of how the specific program is working.

andrewmcwatters 1y ago

The article casually ignores the fact that more production code uses bitmasking than bit fields.

1718627440 1y ago
Why, though?

cjensen 1y ago

This is good stuff to have "in the toolbox," however habitually organizing structs this way is premature optimization. Much better to sensibly organize the fields and ignore alignment entirely unless you have a really good reason to do this. Good reasons could be that you know you will create a lot of these objects, or it has become a performance issue due to cache misses this would improve.

4gotunameagain 1y ago

Unpacked structs are also very useful for parsing network data. You take a package, cast it to an unpacked struct, and can access everything nicely. Many a spacecraft telemetry stacks work like that.

JonChesterfield 1y ago
The alias analysis pass in your C or C++ compiler will savage you for doing that unless you're careful with annotations or compiler flags though.
- 4gotunameagain 1y ago
  Yeap, can definitely be a foot gun ! Even worse, sometimes you won't even get warnings about it, and then scratch your head why the binary seemingly has a mind of its own.. don't ask me how I know !
rkangel 1y ago
I think you mean "packed" structs. Otherwise you've got padding in your communication protocol. If you do this you also need to be careful of byte order.
- 4gotunameagain 1y ago
  Yeah you're right, I mixed it up with unpadded ! And you're right about the second point as well, typically there will be network order to cpu order macros to ensure portability

foota 1y ago

Some related thoughts (more for C++ than C). You can eliminate the cost of a wrapping class with no members by taking advantage of the empty base class optimization (or rather: by avoiding members in a base class you will be taking advantage of EBCO). You can use an untagged union in C++ (taking care to avoid undefined behavior) to implement small object optimization. In it's general form, the idea is you can store some smaller struct in a union (as a char[], since C++ doesn't have native unions) with a larger structure, and use a sentinel in the larger structure to determine which it is. E.g., if large struct has some field that must not be zero, then make it zero in the smaller struct and you can distinguish between the two without a tag (like would be required with std::variant). I'm also a huge fan in general of "make invalid states unrepresentable" and following this can often reduce redundancy and eliminate extra fields.

kazinator 1y ago

Generally speaking, sort the members in descending order by alignment, if you want the tightest packing without resorting to nonportable compiler extensions. This descending order eliminates padding between members producing the smallest offset for the last byte of the last member, and that then minimizes the padding required at the end of the struct. In C, alignment for basic types is linked to size. A type may have an alignment requirement that is as much as its size: e.g. an 8 byte data type might be aligned as strictly as addresses divisible by 8. The alignment of an array aggregate is that of its element type. The alignment of a struct is that of its most strictly aligned member. In portable coding, we don't exactly know, but we can make the worst-case assumption that every basic type has the strictest possible alignment---i.e. its size. We also don't know the sizes of all the types, but we know that integers of higher ranks are at least as large as lower ranks; i.e. sizeof(char) <= sizeof(short) <= sizeof(int) <= sizeof(long), etc. Things are tricky when you have a mixture of floating-point, pointers and integers. E.g. pointer could be the same size as a float, or bigger. Here is a trick you use: combine together clumps of like types, and try to make combinations that are powers of two. For instance, if your structure has two or four pointers to things, try to make them adjacent. E.g.: struct s { void *p1, *p2, *p3, p4; short s1, s2, s3, s4; float f1, f2, f3, f4; }; Even though float is almost certainly larger than short and may have stricter alignment, its moot because I have four of everything. Even if this is on a tiny system where pointers are 16 bits wide, four of them will make 8 bytes. So when the f1 member is being placed into the struct, the offset is at least 8 byte aligned: we don't expect padding. If you aggregate enough members of the same type in a power-of-two sized clump, you can easily create alignment which meets or exceeds the strictest alignment in the system. So those are the main principles: group together members of the same type. Then keeping those groups together, sort by descending alignment strictness.

TinkersW 1y ago

Could improve it by using enum MonsterName :u8, moving it down near the other byte sized item, and specifying the exact type of the bitfield(u8). Then it would be 16 bytes.

_mocha 1y ago

I'm somewhat surprised to see this on the front page of HN. I distinctly remember this material being covered in a single lecture during our freshman CS course 20 years ago. I doubt this would've reached the front page in HN's early days, and it makes me wonder if we've moved away from teaching these fundamentals due to the heavy reliance on abstraction and ai agents in today's industry.

jerf 1y ago
One of the neat things about programming is that you can develop a high degree of skill in it without any formal education at all. But it does mean that we've always had those programmers who have had characteristic holes in their education as a result. Personally I enjoy having a mix of self-taught and formally-taught around; I think there quite a few things the self-taught are often better at than the formally educated as a group. I respect both sides. I'm just saying that there has always been a market for this sort of thing on HN because the self-taught have always been around.
- nomel 1y ago
  > I think there quite a few things the self-taught are often better at than the formally educated as a group. Would you mind expanding on this a bit?
  - deleted 1y ago
    [deleted]
  - jerf 1y ago
    First, there are people who manage the best of both worlds, so bear in mind I'm talking about the ones who only manage the one side or the other. The self-taught ones are often much better at diagnosis of problems in my experience. "Hey, the servers are blowing up, own the process from diagnosis to deploying the fix" is something I can generally throw at a self-taught dev faster than a school-taught dev. Often because even if they've never owned a server they've already been through the "my graphics driver is crashing my desktop when I plug in my USB drive" dance on Linux a number of times and know how do go through diagnosis processes. They're also often better out-of-the-gate at being handed a large task and knowing how to break it down quickly without much direction. Sometimes they should be getting more direction than they do; this is often the source of bits of the code base that seem to be completely foreign in style, library usage, etc. Gotta watch the PRs. On the other hand, the self-taught will sometimes blunder into some critical O() mistakes because they haven't been taught to think that way, and may never have worked at any real scale before. They can also end up doing too much without using a library because they're used to just doing it themselves. (Not that school teaches you to "use libraries" exactly, but the school-taught are just generally less fluent and as such more likely to look for easier solutions... which is often exactly what we want!) Where the holes really develop is when we get into heavy data structures, or a compiler-like code base (though sadly many school-taught people escape without compilers), or heavily recursive code, which doesn't just include "recursion" per se but also things like working with DOMs that are recursive tree structures. A lot of time the self-taught are great at straight-line "do this, then do that, then do that" code bases but struggle to build longer-term maintainable code that has good reuse. And I'll say it again, these are all absolutely, positively, generalities and they do not apply to everyone. Some self-taught people are great at creating high quality, well-architected code bases. Some people escape school not just without compilers, but without any discernible skills whatsoever. (Those are some sad interviews.) These are just broad patterns I've observed.

o11c 1y ago

Note that although alignment and size are platform-specific, in practice padding can be minimized simply by using a fixed order: (various vector types) int128_t long double long long, int64_t, int_fast64_t, double off_t time_t (can be 64 bits on 32-bit platforms only if off_t also is) register_t, int_fast16_t, int_fast32_t (64-bit on e.g. x32) void *, void(*)(), intptr_t ptrdiff_t (theoretically wrong if fully aligned and bigger than pointer) size_t, ssize_t long (32-bit on win64; generally has 16-bit alignment if bigger than pointer) int32_t, float int (can be 16-bit) short, int16_t char, bool, int8_t, int_fast8_t (a trailing array member must come last; note when calculating combined size you should use offsetof instead of sizeof to not waste padding) Besides this, `int_leastN_t` goes with `intN_t`, and all unsigned types go with their signed types. Note that types like `id_t` are hard to place however. Although there do exist many platforms where this is not in order of size, the bigger types generally have smaller alignment on such platforms. Despite this I have split out rows occasionally despite not knowing of any such platform.

teo_zero 1y ago

Neither TFA nor any comment mention using a struct of arrays instead of an array of structs. This technique alone eliminates all padding bytes!

herf 1y ago

Sometimes struct-of-arrays (in-memory column store) is the right solution, and you get automatic packing that way. C makes it so much easier to store an array of structs that people don't do this often enough, but column stores prove it has its uses.

munificent 1y ago

This is a good article, but it bugs me that it sort of only tells half the story. Why are you making your structs smaller. Probably for "performance". If your goal is literally just reducing memory usage, then this is all fine. Maybe you're running on a small microcontroller and you just straight up don't have much RAM. But these days, in most areas, memory is cheap and plentiful. What's expensive is CPU cache. If you're optimizing memory use for that, it's really about runtime speed and less about total memory usage. You're trying to make things small so you have fewer cache misses and the CPU doesn't get stuck waiting as much. In that case, the advice here is only part of the story. Using bitfields and smaller integers saves memory. But in order to access those fields and do anything with them, they need to get loaded into registers. I'm not an expert here, but my understanding is that loading a bitfield or small int into a word-sized register can sometimes have some bit of overhead, so you have to pay attention to the trade-off. If I was optimizing for overall runtime speed and considering packing structs to do that, I'd really want to have good profiling and benchmarks in place first. Otherwise it's easy to think you're making progress when you're actually going backwards.

zeusk 1y ago
The real issue with packed structs and bitfields happens when concurrency gets involved. Majority of modern CPU caches are private and only allow one core to hold the cache line - so it creates more false dependencies when cores are trying to alter information that was compacted into a cache line on another core.
- charleslmunger 1y ago
  Avoiding false sharing is a separate problem best solved by explicitly aligning the struct or relevant members to std::hardware_destructive_interference_size.
wbl 1y ago
Everything is microarch dependant but typically the memory subsystem can handle subword loads and writes. What it usually can't do is bits.
1y ago

iamcreasy 1y ago

This reminds of an amazing talk by Mike Acton on Data oriented design: https://www.youtube.com/watch?v=rX0ItVEVjHc https://www.youtube.com/watch?v=rX0ItVEVjHc And there is a related talk by Andrew Kelley implementing these ideas on Zig toolchain: https://www.youtube.com/watch?v=IroPQ150F6c https://www.youtube.com/watch?v=IroPQ150F6c

anitil 1y ago
I just watched that Andrew Kelly one yesterday, and really enjoyed it. I think Zig needs to be my next language to play with.

fattah25 1y ago

Remind me about Andrew Kelley conf "data-oriented design" He had explained about this topic.

bhawks 1y ago

Optimizing struct layouts is cool. but if your motivation is to pack more monsters in your game definitely consider transposing your row oriented monster struct into a column oriented entity using Entity Component System. Example framework: https://github.com/skypjack/entt?tab=readme-ov-file https://github.com/skypjack/entt?tab=readme-ov-file It has benefits besides memory/cache optimizations. It organizes logic in a much more composable way that is friendly for reuse.

skylabmelody 1y ago

I would suggest to never use unsigned int for values that will involve any sort of calculations on them (health, damage, speed). This is considered bad practice due to overflow and should be avoided, with the exception of values that are used for UIDs, indexes, bitfields, mask and flags.

ChrisRR 1y ago
Nah it's just whatever is most appropriate for your implementation. Overflow and wrap around is possible whether you use signed or unsigned. As I say to some more junior devs, check whether the maths is correct and then do it, don't do it and then check if it was wrong
- skylabmelody 1y ago
  My suggestion is straight from the C++ guidelines by Bjarne Stroustrup and Herb Sutter (ES.102: Use signed types for arithmetic), for reasons that you don't seem to grasp.
  - ChrisRR 1y ago
    There's a reason they're guidelines and not rules. Use whatever is most appropriate for your use case

Rayhem 1y ago

> The health of a monster is currently represented by a 4 byte signed integer, meaning that the health can range from -2^31 to 2^31-1 (2^31 ~= 2 million). We can already see that half of the integers potential is unused because the health of a monster should never be negative, so an unsigned int would fit way better. If you're strictly optimizing for memory size out of necessity, then yes. It's dangerous to use an unsigned type in any context where you might even consider subtraction, though, and it's pretty effortless to see that new_health = health - damage is going to be used somewhere. Congrats! Your 100hp lvl3 monster just got resurrected with sixteen thousand hp.

Kilonzus 1y ago

There seems to be a typo in the 4th paragraph (directly under the struct memory table). I'm counting a padding of 7 bytes. 96 (size of struct) - 89 (size of primitive types) = 7. The first padding consist of 3 bytes to correctly align the 4 byte int after a 1 byte bool. Second padding is 2 bytes due to a 4 bytes int following two 1 byte bools and third padding is 2 bytes as described in the blog, 3+2+2=7 bytes of padding.