In C++11 we are provided with fixed-width integer types, such as std::int32_tand std::int64_t, which are optional and therefore not optimal for writing cross-platform code. However, we also got non-optional variants for the types: e.g. the "fast" variants, e.g. std::int_fast32_tand std::int_fast64_t, as well as the "smallest-size" variants, e.g. std::int_least32_t, which both are at least the specified number of bits in size.
The code I am working on is part of a C++11-based cross-platform library, which supports compilation on the most popular Unix/Windows/Mac compilers. A question that now came up is if there is an advantage in replacing the existing integer types in the code by the C++11 fixed-width integer types.
A disadvantage of using variables like std::int16_t and std::int32_t is the lack of a guarantee that they are available, since they are only provided if the implementation directly supports the type (according to http://en.cppreference.com/w/cpp/types/integer).
However, since int is at least 16 bits and 16-bit are large enough for the integers used in the code, what about the usage of std::int_fast16_t over int? Does it provide a benefit to replace all int types by std::int_fast16_t and all unsigned int's by std::uint_fast16_t in that way or is this unnecessary?
Anologously, if knowing that all supported platforms and compilers feature an int of at least 32 bits size, does it make sense to replace them by std::int_fast32_t and std::uint_fast32_t respectively?
int can be 16, 32 or even 64 bit on current computers and compilers. In the future, it could be bigger (say, 128 bits).
If your code is ok with that, go with it.
If your code is only tested and working with 32 bit ints, then consider using int32_t. Then the code will fail at compile time instead of at run time when run on a system that doesn't have 32 bit ints (which is extremely rare today).
int_fast32_t is when you need at least 32 bits, but you care a lot about performance. On hardware that a 32 bit integer is loaded as a 64 bit integer, then bitshifted back down to a 32 bit integer in a cumbersome process, the int_fast_32_t may be a 64 bit integer. The cost of this is that on obscure platforms, your code behaves very differently.
If you are not testing on such platforms, I would advise against it.
Having things break at build time is usually better than having breaks at run time. If and when your code is actually run on some obscure processor needing these features, then fix it. The rule of "you probably won't need it" applies.
Be conservative, generate early errors on hardware you are not tested on, and when you need to port to said hardware do the work and testing required to be reliable.
In short:
Use int_fast##_t if and only if you have tested your code (and will continue to test it) on platforms where the int size varies, and you have shown that the performance improvement is worth that future maintenance.
Using int##_t with common ## sizes means that your code will fail to compile on platforms that you have not tested it on. This is good; untested code is not reliable, and unreliable code is usually worse than useless.
Without using int32_t, and using int, your code will sometimes have ints that are 32 and sometimes ints that are 64 (and in theory more), and sometimes ints that are 16. If you are willing to test and support every such case in every such int, go for it.
Note that arrays of int_fast##_t can have cache problems: they could be unreasonably big. As an example, int_fast16_t could be 64 bits. An array of a few thousand or million of them could be individually fast to work with, but the cache misses caused by their bulk could make them slower overall; and the risk that things get swapped out to slower storage grows.
int_least##_t can be faster in those cases.
The same applies, doubly so, to network-transmitted and file-stored data, on top of the obvious issue that network/file data usually has to follow formats that are stable over compiler/hardware changes. This, however, is a different question.
However, when using fixed width integer types you must pay special attention to the fact that int, long, etc. still have the same width as before. Integer promotion still happens based on the size of int, which depends on the compiler you are using. An integral number in your code will be of type int, with the associated width. This can lead to unwanted behaviour if you compile your code using a different compiler. For more detailed info: https://stackoverflow.com/a/13424208/3144964
I have just realised that the OP is just asking about int_fast##_t not int##_t since the later is optional. However, I will keep the answer hopping it may help someone.
I would add something. Fixed size integers are so important (or even a must) for building APIs for other languages. One example is when when you want to pInvoke functions and pass data to them in a native C++ DLL from a .NET managed code for example. In .NET, int is guaranteed to be a fixed size (I think it is 32bit). So, if you used int in C++ and it was considered as 64-bit rather than 32bit, this may cause problems and cuts down the sequence of wrapped structs.
Related
I just read this link: The difference of int8_t, int_least8_t and int_fast8_t? and now I know that int8_t is exactly 8 bits whereas int_fast8_t is the fastest int type that has at least 8 bits.
I'm a developer who develops backend processes with c++11 on Linux. Most of time I don't need to worry about the size of my processes. But I need always care about the sizes of integers in my project. For example, if I want to use an int to store the ID of user or to store a millisecond-timepoint, I can't simply use int because it may cause overflow, I must use int32_t or int64_t.
So I'm thinking if it's good to use int_fast8_t everywhere and stop using int8_t (same as int_fast32_t, int_fast64_t, uint_fast8_t etc).
Well, using int_fastN_t may change nothing because my program is always deployed on X86 or Arm64. But I still want to know if there is any drawback if I change all of intN_t into int_fastN_t. If there isn't any drawback, I think I would start to use int_fastN_t and stop using intN_t.
So I'm thinking if it's good to use int_fast8_t everywhere
No. It's not good to use it everywhere.
I still want to know if there is any drawback if I change all of intN_t into int_fastN_t
The main drawback is that the size of the integer won't be exactly N bits. In some use cases, this is crucial.
Another potential drawback is that it may be slower. Yes, "fast" type alias can be slower. The alias isn't magic; the efficiency depends on use case.
Using the "fast" alias is fine if:
The exact size doesn't matter, but only the minimum.
You have the option of changing the type later (no need for backward compatibility).
You don't have time to measure which type is actually fastest (which is often reasonable).
You didn't ask for drawbacks of using the fixed width integers. But for balance, I'll mention: they are not guaranteed to be provided on all systems. And of course, they may be slower in some other use cases (which is probably less surprising given the naming).
As far as I understand, the number of bytes used for int is system dependent. Usually, 2 or 4 bytes are used for int.
As per Microsoft's documentation, __int8, __int16, __int32 and __int64 are Microsoft Specific keywords. Furthermore, __int16 uses 16-bits (i.e. 2 bytes).
Question: What are advantage/disadvantage of using __int16 (or int16_t)? For example, if I am sure that the value of my integer variable will never need more than 16 bits then, will it be beneficial to declare the variable as __int16 var (or int16_t var)?
UPDATE: I see that several comments/answers suggest using int16_t instead of __int16, which is a good suggestion but not really an advantage/disadvantage of using __int16. Basically, my question is, what is the advantage/disadvantage of saving 2 bytes by using 16-bit version of an integer instead of int ?
Saving 2 bytes is almost never worth it. However, saving thousands of bytes is. If you have an large array containing integers, using a small integer type can save quite a lot of memory. This leads to faster code, because the less memory one uses the less cache misses one receives (cache misses are a major loss of performance).
TL;DR: this is beneficial to do in large arrays, but pointless for 1-off variables.
The second use of these is if for dealing with binary files and messages. If you are reading a binary file that uses 16-bit integers, well, it's pretty convenient if you can represent that type exactly in your code.
BTW, don't use microsoft's versions. Use the standard versions (std::int16_t)
It depends.
On x86, primitive types are generally aligned on their size. So 2-byte types would be aligned on a 2-byte boundary. This is useful when you have more than one of these short variables, because you will be saving 50% of space. That directly translates to better memory and cache utilization and thus theoretically, better performance.
On the other hand, doing arithmetic on shorter-than-int types usually involves widening conversion to int. So if you do a lot of arithmetic on these types, using int types might result in better performance (contrived example).
So if you care about performance of a critical section of code, profile it to find out for sure if using a certain data type is faster or slower.
A possible rule of thumb would be - if you're memory-bound (i.e. you have lots of variables and especially arrays), use as short a data types as possible. If not - don't worry about it and use int types.
If you for some reason just need a shorter integer type it's already have that in the language - called short - unless you know you need exactly 16 bits there's really no good reason not to just stick with the agnostic short and int types. The broad idea is that these types should align well the target architecture (for example see word ).
That being said, theres no need to use the platform specific type (__int16), you can just use the standard one:
int16_t
See https://en.cppreference.com/w/cpp/types/integer for more information and standard types
Even if you still insist on __int16 you probably want a typedef something ala.:
using my_short = __int16;
Update
Your main question is:
What is the advantage/disadvantage of
saving 2 bytes by using 16-bit version of an integer instead of int ?
If you have a lot of data (In the ballpark of at least some 100.000-1.000.000 elements as a rule of thumb) - then there could be an overall performance saving in terms of using less cpu-cache. Overall there's no disadvantage of using a smaller type - except for the obvious one - and possible conversions as explained in this answer
The main reason for using these types is to make sure about the size of your variable in different architectures and compilers. we call it "code reusability" and "portability"
in higher-level modern languages, all this will handle with compiler/interpreter/virtual machine/etc. that you don't need to worry about, but it has some performance and memory usage costs.
When you have some kind of limitation you may need to optimize everything. The best example is embedded systems that have a very limited size of memory and work at low frequency. In the other hand, there are lots of compilers out there with different implementations. Some of them interpret "int" as a "16bit" value and some as a "32bit".
for example, you receive and specific stream of values over a communication system, you want to save them in a buffer or array and you want to make sure the input data is always interpreted as a 16bit noting else.
A c++ specific question. So i read a question about what makes a program 32 bit/64 bit, and the anwser it got was something like this (sorry i cant find the question, was somedays ago i looked at it and i cant find it again:( ): As long as you dont make any "pointer assumptions", you only need to recompile it. So my question is, what are pointer assumtions ? To my understanding there is 32 bit pointer and 64 bit pointers so i figure it is something to do with that . Please show the diffrence in code between them. Any other good habits to keep in mind while writing code, that helps it making it easy to convert between the to are also welcome :) tho please share examples with them
Ps. I know there is this post:
How do you write code that is both 32 bit and 64 bit compatible?
but i tougth it was kind of to generall with no good examples, for new programmers like myself. Like what is a 32 bit storage unit ect. Kinda hopping to break it down a bit more (no pun intended ^^ ) ds.
In general it means that your program behavior should never depend on the sizeof() of any types (that are not made to be of some exact size), neither explicitly nor implicitly (this includes possible struct alignments as well).
Pointers are just a subset of them, and it probably also means that you should not try to rely on being able to convert between unrelated pointer types and/or integers, unless they are specifically made for this (e.g. intptr_t).
In the same way you need to take care of things written to disk, where you should also never rely on the size of e.g. built in types, being the same everywhere.
Whenever you have to (because of e.g. external data formats) use explicitly sized types like uint32_t.
For a well-formed program (that is, a program written according to syntax and semantic rules of C++ with no undefined behaviour), the C++ standard guarantees that your program will have one of a set of observable behaviours. The observable behaviours vary due to unspecified behaviour (including implementation-defined behaviour) within your program. If you avoid unspecified behaviour or resolve it, your program will be guaranteed to have a specific and certain output. If you write your program in this way, you will witness no differences between your program on a 32-bit or 64-bit machine.
A simple (forced) example of a program that will have different possible outputs is as follows:
int main()
{
std::cout << sizeof(void*) << std::endl;
return 0;
}
This program will likely have different output on 32- and 64-bit machines (but not necessarily). The result of sizeof(void*) is implementation-defined. However, it is certainly possible to have a program that contains implementation-defined behaviour but is resolved to be well-defined:
int main()
{
int size = sizeof(void*);
if (size != 4) {
size = 4;
}
std::cout << size << std::endl;
return 0;
}
This program will always print out 4, despite the fact it uses implementation-defined behaviour. This is a silly example because we could have just done int size = 4;, but there are cases when this does appear in writing platform-independent code.
So the rule for writing portable code is: aim to avoid or resolve unspecified behaviour.
Here are some tips for avoiding unspecified behaviour:
Do not assume anything about the size of the fundamental types beyond that which the C++ standard specifies. That is, a char is at least 8 bit, both short and int are at least 16 bits, and so on.
Don't try to do pointer magic (casting between pointer types or storing pointers in integral types).
Don't use a unsigned char* to read the value representation of a non-char object (for serialisation or related tasks).
Avoid reinterpret_cast.
Be careful when performing operations that may over or underflow. Think carefully when doing bit-shift operations.
Be careful when doing arithmetic on pointer types.
Don't use void*.
There are many more occurrences of unspecified or undefined behaviour in the standard. It's well worth looking them up. There are some great articles online that cover some of the more common differences that you'll experience between 32- and 64-bit platforms.
"Pointer assumptions" is when you write code that relies on pointers fitting in other data types, e.g. int copy_of_pointer = ptr; - if int is a 32-bit type, then this code will break on 64-bit machines, because only part of the pointer will be stored.
So long as pointers are only stored in pointer types, it should be no problem at all.
Typically, pointers are the size of the "machine word", so on a 32-bit architecture, 32 bits, and on a 64-bit architecture, all pointers are 64-bit. However, there are SOME architectures where this is not true. I have never worked on such machines myself [other than x86 with it's "far" and "near" pointers - but lets ignore that for now].
Most compilers will tell you when you convert pointers to integers that the pointer doesn't fit into, so if you enable warnings, MOST of the problems will become apparent - fix the warnings, and chances are pretty decent that your code will work straight away.
There will be no difference between 32bit code and 64bit code, the goal of C/C++ and other programming languages are their portability, instead of the assembly language.
The only difference will be the distrib you'll compile your code on, all the work is automatically done by your compiler/linker, so just don't think about that.
But: if you are programming on a 64bit distrib, and you need to use an external library for example SDL, the external library will have to also be compiled in 64bit if you want your code to compile.
One thing to know is that your ELF file will be bigger on a 64bit distrib than on a 32bit one, it's just logic.
What's the point with pointer? when you increment/change a pointer, the compiler will increment your pointer from the size of the pointing type.
The contained type size is defined by your processor's register size/the distrib your working on.
But you just don't have to care about this, the compilation will do everything for you.
Sum: That's why you can't execute a 64bit ELF file on a 32bit distrib.
Typical pitfalls for 32bit/64bit porting are:
The implicit assumption by the programmer that sizeof(void*) == 4 * sizeof(char).
If you're making this assumption and e.g. allocate arrays that way ("I need 20 pointers so I allocate 80 bytes"), your code breaks on 64bit because it'll cause buffer overruns.
The "kitten-killer" , int x = (int)&something; (and the reverse, void* ptr = (void*)some_int). Again an assumption of sizeof(int) == sizeof(void*). This doesn't cause overflows but looses data - the higher 32bit of the pointer, namely.
Both of these issues are of a class called type aliasing (assuming identity / interchangability / equivalence on a binary representation level between two types), and such assumptions are common; like on UN*X, assuming time_t, size_t, off_t being int, or on Windows, HANDLE, void* and long being interchangeable, etc...
Assumptions about data structure / stack space usage (See 5. below as well). In C/C++ code, local variables are allocated on the stack, and the space used there is different between 32bit and 64bit mode due to the point below, and due to the different rules for passing arguments (32bit x86 usually on the stack, 64bit x86 in part in registers). Code that just about gets away with the default stacksize on 32bit might cause stack overflow crashes on 64bit.
This is relatively easy to spot as a cause of the crash but depending on the configurability of the application possibly hard to fix.
Timing differences between 32bit and 64bit code (due to different code sizes / cache footprints, or different memory access characteristics / patterns, or different calling conventions ) might break "calibrations". Say, for (int i = 0; i < 1000000; ++i) sleep(0); is likely going to have different timings for 32bit and 64bit ...
Finally, the ABI (Application Binary Interface). There's usually bigger differences between 64bit and 32bit environments than the size of pointers...
Currently, two main "branches" of 64bit environments exist, IL32P64 (what Win64 uses - int and long are int32_t, only uintptr_t/void* is uint64_t, talking in terms of the sized integers from ) and LP64 (what UN*X uses - int is int32_t, long is int64_t and uintptr_t/void* is uint64_t), but there's the "subdivisions" of different alignment rules as well - some environments assume long, float or double align at their respective sizes, while others assume they align at multiples of four bytes. In 32bit Linux, they align all at four bytes, while in 64bit Linux, float aligns at four, long and double at eight-byte multiples.
The consequence of these rules is that in many cases, bith sizeof(struct { ...}) and the offset of structure/class members are different between 32bit and 64bit environments even if the data type declaration is completely identical.
Beyond impacting array/vector allocations, these issues also affect data in/output e.g. through files - if a 32bit app writes e.g. struct { char a; int b; char c, long d; double e } to a file that the same app recompiled for 64bit reads in, the result will not be quite what's hoped for.
The examples just given are only about language primitives (char, int, long etc.) but of course affect all sorts of platform-dependent / runtime library data types, whether size_t, off_t, time_t, HANDLE, essentially any nontrivial struct/union/class ... - so the space for error here is large,
And then there's the lower-level differences, which come into play e.g. for hand-optimized assembly (SSE/SSE2/...); 32bit and 64bit have different (numbers of) registers, different argument passing rules; all of this affects strongly how such optimizations perform and it's very likely that e.g. SSE2 code which gives best performance in 32bit mode will need to be rewritten / needs to be enhanced to give best performance 64bit mode.
There's also code design constraints which are very different for 32bit and 64bit, particularly around memory allocation / management; an application that's been carefully coded to "maximize the hell out of the mem it can get in 32bit" will have complex logic on how / when to allocate/free memory, memory-mapped file usage, internal caching, etc - much of which will be detrimental in 64bit where you could "simply" take advantage of the huge available address space. Such an app might recompile for 64bit just fine, but perform worse there than some "ancient simple deprecated version" which didn't have all the maximize-32bit peephole optimizations.
So, ultimately, it's also about enhancements / gains, and that's where more work, partly in programming, partly in design/requirements comes in. Even if your app cleanly recompiles both on 32bit and 64bit environments and is verified on both, is it actually benefitting from 64bit ? Are there changes that can/should be done to the code logic to make it do more / run faster in 64bit ? Can you do those changes without breaking 32bit backward compatibility ? Without negative impacts on the 32bit target ? Where will the enhancements be, and how much can you gain ?
For a large commercial project, answers to these questions are often important markers on the roadmap because your starting point is some existing "money maker"...
I have a matrix that is over 17,000 x 14,000 that I'm storing in memory in C++. The values will never get over 255 so I'm thinking I should store this matrix as a uint8_t type instead of a regular int type. Will the regular int type will assume the native word size (64 bit so 8 bytes per cell) even with an optimizing compiler? I'm assuming I'll use 8x less memory if I store the array as uint8_t?
If you doubt this, you could have just tried it.
Of course it will be smaller.
However, it wholly depends on your usage patterns which will be faster. Profile! Profile! Profile!
Reasons for unexpected performance considerations:
alignment issues
elements sharing cache lines (could be positive on sequential access; negative in multicore scenarios)
increased need for locking on atomic reads/writes (in case of threading)
reduced applicability of certain optimized MIPS instructions (? - I'm not up-to-date with details here; also a very good optimizing compiler might simply register-allocate temporaries of the right size)
other, unrelated border conditions, originating from the surrounding code
The standard doesn't specify the exact size of int other than it's at least the size of short. On some 64-bit architectures (for example many Linux and Solaris x86 systems I work with) int is 32 bits and long is 64 bits. The exact size of each type will of course vary by compiler/hardware.
The best way to find out is to use sizeof(int) on your system and see how big it is. If you have enough RAM using the native type may in fact be significantly faster than the uint8_t.
Even the best optimizing compiler is not going to do an analysis of the values of the data that you put into your matrix and assume (anthropomorphizing here) "Hmmm. He said int but everything is between 0 and 255. I'm going to make that an array of uint8_t."
The compiler can interpret some keywords such as register and inline as suggestions rather than mandates. Types on the other hand are mandates. You told the compiler to use int so the compiler must use int. So switching to a uint8_t matrix will save you a considerable amount of memory here.
On what exactly does the size of a primitive data type like int depend on?
Compiler
Processor
Development Environment
Or is it a combination of these or other factors?
An explanation on the reason of the same will be really helpful.
EDIT: Sorry for the confusion..I meant to ask about Primitive data type like int and not regarding PODs, I do understand PODs can include structure and with structure it is a whole different ball game with padding coming in to the picture.
I have corrected the Q, the edit note here should ensure the answers regarding POD don't look irrelevant.
I think there are two parts to this question:
What sizes primitive types are allowed to be.
This is specified by the C and C++ standards: the types have allowed minimum value ranges they must have, which implicitly places a lower bound on their size in bits (e.g. long must be at least 32 bit to comply with the standard).
The standards do not specify the size in bytes, because the definition of the byte is up to the implementation, e.g. char is byte, but byte size (CHAR_BIT macro) may be 16 bit.
The actual size as defined by the implementation.
This, as other answers have already pointed out, is dependent on the implementation: the compiler. And the compiler implementation, in turn, is heavily influenced by the target architecture. So it's plausible to have two compilers running on the same OS and architecture, but having different size of int. The only assumption you can make is the one stated by the standard (given that the compiler implements it).
There also may be additional ABI requirements (e.g. fixed size of enums).
First of all, it depends on Compiler. Compiler in turns usually depends on the architecture, processor, development environment etc because it takes them into account. So you may say it's a combination of all. But I would NOT say that. I would say, Compiler, since on the same machine you may have different sizes of POD and built-in types, if you use different compilers. Also note that your source code is input to the compiler, so it's the compiler which makes final decision of the sizes of POD and built-in types. However, it's also true that this decision is influenced by the underlying architecture of the target machine. After all, the real useful compiler has to emit efficient code that eventually runs on the machine you target.
Compilers provides options too. Few of them might effect sizes also!
EDIT: What Standards say,
Size of char, signed char and unsigned char is defined by C++ Standard itself! Sizes of all other types are defined by the compiler.
C++03 Standard $5.3.3/1 says,
sizeof(char), sizeof(signed char) and
sizeof(unsigned char) are 1; the
result of sizeof applied to any other
fundamental type (3.9.1) is
implementation-defined. [Note: in
particular,sizeof(bool) and
sizeof(wchar_t) are
implementation-defined.69)
C99 Standard ($6.5.3.4) also itself defines the size of char, signed char and unsigned char to be 1, but leaves the size of other types to be defined by the compiler!
EDIT:
I found this C++ FAQ chapter really good. The entire chapter. It's very tiny chapter though. :-)
http://www.parashift.com/c++-faq-lite/intrinsic-types.html
Also read the comments below, there are some good arguments!
If you're asking about the size of a primitive type like int, I'd say it depends on the factor you cited.
The compiler/environment couple (where environment often means OS) is surely a part of it, since the compiler can map the various "sensible" sizes on the builtin types in different ways for various reasons: for example, compilers on x86_64 Windows will usually have a 32 bit long and a 64 bit long long to avoid breaking code thought for plain x86; on x86_64 Linux, instead, long is usually 64 bit because it's a more "natural" choice and apps developed for Linux are generally more architecture-neutral (because Linux runs on a much greater variety of architectures).
The processor surely matters in the decision: int should be the "natural size" of the processor, usually the size of the general-purpose registers of the processor. This means that it's the type that will work faster on the current architecture. long instead is often thought as a type which trades performance for an extended range (this is rarely true on regular PCs, but on microcontrollers it's normal).
If in instead you're also talking about structs & co. (which, if they respect some rules, are POD), again the compiler and the processor influence their size, since they are made of builtin types and of the appropriate padding chosen by the compiler to achieve the best performance on the target architecture.
As I commented under #Nawaz's answer, it technically depends solely on the compiler.
The compiler is just tasked with taking valid C++ code, and outputting valid machine code (or whatever language it targets).
So a C++ compiler could decide to make an int have a size of 15, and require it to be aligned on 5-byte boundaries, and it could decide to insert arbitrary padding between the variables in a POD. Nothing in the standard prohibits this, and it could still generate working code.
It'd just be much slower.
So in practice, compilers take some hints from the system they're running on, in two ways:
- the CPU has certain preferences: for example, it may have 32-bit wide registers, so making an int 32 bits wide would be a good idea, and it usually requires variables to be naturally aligned (a 4-byte wide variable must be aligned on an address divisible by 4, for example), so a sensible compiler respects these preferences because it yields faster code.
- the OS may have some influence too, in that if it uses another ABI than the compiler, making system calls is going to be needlessly difficult.
But those are just practical considerations to make life a bit easier for the programmer or to generate faster code. They're not required.
The compiler has the final word, and it can choose to completely ignore both the CPU and the OS. As long as it generates a working executable with the semantics specified in the C++ standard.
It depends on the implementation (compiler).
Implementation-defined behavior means unspecified behavior where each implementation documents how the choice is made.
A struct can also be POD, in which case you can explicity control potential padding between members with #pragma pack on some compilers.