reinterpret_pointer_cast is not a member of std

I am assuming the only difference between the standard pointer casting and the intrinsic is that the intrinsic will immediately load the data onto a 64 byte register while the pointer casting will wait for a further instruction to do so. However, if you only have char*, and only one other pointer-type other than char* is used, then you can look at the memory as having whatever the other type is, and every char* access aliasing that. Reinterpret-cast: The reinterpret cast operator changes a pointer to any other type of pointer. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. cpprefjpMarkdown. Yes it's safe to point a char* at a uint16_t, but if you have an actual array of char buf[100], those objects are definitely char objects, and it's UB to access them through a uint16_t*. building jsoncpp (Linux) - an instruction for us mere mortals? Note that the null pointer constant nullptr or any other value of type std:: nullptr_t cannot be converted to a pointer: implicit conversion or static_cast can be used for this purpose. I assume that ARM load/store intrinsics are defined similar to memcpy, being able to read/write the bytes of any object. Just like memcpy between two int objects can be optimized away or done when it's convenient (as long as the result is as-if it were done in source order), so can store/load intrinsics depending on how you use them. when the result refers to the same object as the source glvalue. With the existence of intrinsics for gather and scatter, use-cases like using a 0 base with pointer elements for _mm256_i64gather_epi64 (e.g. The approach I present at the end can be used, if you only have non-static member function pointers. 9. Why does this code emit buffer overrun warnings(C6385/C6386) in code analysis on Visual Studio 2012? When would I give a checkpoint to my D&D party that they can return to if they die? So yes, it's safe to dereference a __m256* instead of using a _mm256_load_ps() aligned-load intrinsic. Why doesn't Stockfish announce when it solved a position as a book draw similar to how it announces a forced mate? 6. I've heard from gcc developers that they chose that implementation because it was required for compatibility with Intel. ReInterpret Cast ( reinterpret_cast) is a cast operator that converts a pointer of some data type into a pointer of another data type, even if the the data types before and after conversion are different. ), Intel's intrinsics API effectively requires compilers to support creating misaligned pointers as long as you don't deref them yourself. Intel's intrinsics API does define the behaviour of casting to __m128* and dereferencing: it's identical to _mm_load_ps on the same pointer. How is Jesus God when he sits at the right hand of the true God? Thus, vector data are likely not ensured to be 32-bytes aligned. IS 5.2.10 - Reinterpret cast -1- The result of the expression reinterpret_cast<T> (v) is the result of converting the expression v to type T. . Other things that are undefined behaviour but which you aren't doing: It's technically UB to form a pointer that isn't pointing inside an object, or one-past-end, but in practice mainstream implementations allow that as well. 2. Why won't this code compile and run in Visual Studio 2010? You can also use union type-punning between a vector and an array of some type, which is safe in ISO C99, and in GNU C++, but not in ISO C++. its operand, It is intended to be unsurprising to those who know the addressing Load/store intrinsics basically exist to communicate alignment guarantees to the compiler (via loadu/storeu), and to take care of types for you (at least for ps and pd load[u]/store[u]; integer still requires casting the pointer). And yes, deref of an __m256* is exactly equivalent to _mm256_load_ps, and is in fact how most compilers implement _mm256_load_ps. The exact aliasing semantics of Intel Intrinsics are not AFAIK documented anywhere. Reading/writing through a char* can alias anything, but when you have a char object, strict-aliasing does make it UB to read it through other types. Why does Visual Studio 2010 throw this error with Boost 1.42.0? Share Improve this answer Follow edited Mar 11, 2020 at 13:14 answered Mar 11, 2020 at 11:58 walnut 21.3k 4 22 58 be performed explicitly using, An expression of integral, enumeration, pointer, or pointer-to-member type Is it correct to say "The glue on the back of the sticker is dying down so I can not stick the sticker to the wall"? The resulting value is the same as the value of expression. [C++14: 5.2.10/2]: The reinterpret_cast operator shall not cast away constness (5.2.11). But the drafts are close enough for practical usage. So this . What is a smart pointer and when should I use one? Calculate the average of several values using a variadic-template function, OpenCV's Brute Force Matcher crashes on second iteration, Piecewise conversion of an MFC app to Unicode/MBCS, Implementations for event loop in C/C++ that's nice on the call stack, Modifying standard STL containers so support generic queue interface. "There's no Qt version assigned to this project for platform Win32" - visual studio plugin for Qt. 7. For float, there are no insert/extract intrinsics that you should use with scalar float. Nope, not at all. This is trivial for compilers targeting any normal modern CPU, including x86 with a flat memory model (no segmentation); pointers in asm are just integers kept in the same registers as data. reinterpret_cast is a very special and dangerous type of casting operator. How to read an intermittent hard drive consistently? I am learning C and C++. Demonstrates some uses of reinterpret_cast, // pointer to function to another and back, http://en.cppreference.com/w/cpp/language/reinterpret_cast. (Unfortunately even current GCC still has that bug; _mm_loadu_si32 was fixed in GCC11.3 but not the older _ss and _sd loads.) If you'd been using _mm_load_ss(arr) on a buggy GCC version that implements it as _mm_set_ss( *ptr ) instead using a may_alias typdef for float, then that would matter. You're not directly dereffing the float*, only passing it to _mm256_load_ps which does an aliasing-safe load. So a lot of Intel intrinsics stuff seemed pretty casual about C and C++ safety rules, like it was designed by people who thought of C as a portable assembler. In fact, it doesn't even compile in MSVC 2017: The problem is that you're using short* to access the elements of a __m128i* object. like int to pointer and pointer to int etc. Copyright 2022 www.appsloveworld.com. That's strange. 3) const_cast<Y*>(r.get()). A pointer converted to an integer of sufficient size (if any such exists on the implementation) and back to the same pointer type will have its original value; mappings between pointers and integers are otherwise implementation-defined. But in your case, you don't even need to depend on any de-facto guarantees here, beyond the fact that _mm256_load_ps itself is an aliasing-safe load. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? Because it's a may_alias type, the compiler can't infer that the underlying object is an __m256i; that's the whole point, and why it's safe to point it at an int arr[] or whatever. The actual standard is expensive to purchase. What is the difference between #include and #include "filename"? 3-4) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed. The consent submitted will only be used for data processing originating from this website. And _mm_storeu_ps to a location that isn't 4-byte aligned requires creating an under-aligned float*. (print a __m128i variable). Why does Visual Studio compiler allow violation of private inheritance in this example? 2. That just makes your C look more like asm if you want it to. Intel intrinsics are defined that way (e.g. - n. m. Dont do whatever this is. I am new in computer science and new in C++. Do non-Segwit nodes reject Segwit transactions with invalid signature? Since you are using reinterpret_cast on an integer literal (20000), the result of the conversion is not a safely-derived pointer value. (This makes it unlikely for a static analyzer to complain even if that wasn't the case, unless it could see something like (uint16_t*)(1 + (char*)&something_aligned) where you take an aligned address and offset it by an odd number, which would be guaranteed to produce a misaligned address.). Portably reinterpret_cast<> can freely convert between function pointer types and object pointer types - although any use of the result except conversion back to the original type is undefined in almost all cases. For C++11 it was defined as: . To learn more, see our tips on writing great answers. Does aliquot matter for final concentration? a reinterpret_cast is a conversion operator. MSVC defines vector types as a union with a .m128_f32[] member for per-element access. A C-style cast could evaluate to any of those automatically, thus it is considered safer if the programmer explicitly states which kind of cast is expected. You don't need a compile-time-visible guarantee of alignment, you just need to not ever actually create a uint16_t* that doesn't have alignof(uint16_t) alignment. The null pointer constant NULL or integer zero is not guaranteed to yield the null pointer value of the target type; static_cast or implicit conversion can be used for this purpose. the conversions that can be performed using a reinterpret_cast are limited; the permissible ones are specified by the standard. An expression of integral, enumeration, pointer, or pointer-to-member type can be explicitly converted to its own type; such a cast yields the value of its operand. [C++14: 5.2.10/2]: The reinterpret_cast operator shall not cast away constness (5.2.11). It is used when we want to work with bits. A pointer is valid if its a null pointer, if it points to something, or if it points to the one element past the end of an array. Explanation Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions (except when converting between integers and pointers or on obscure architectures where pointer representation depends on its type). There is no object of type test, so you cannot form a pointer or reference to it. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. So a load intrinsic is literally already doing this. 7-8) reinterpret_cast<Y*>(r.get()) The behavior of these functions is undefined unless the corresponding cast from U* to T* is well formed: 1-2) The behavior is undefined unless static_cast<T*>((U*)nullptr) is well formed. If _mm_load_ps() is supported, the implementation must also define the behaviour of the code in the question. How to get the address of the std::vector buffer start most elegantly? Conversion to the original type yields the original value, otherwise the resulting pointer cannot be used safely. And is suggested to use it using proper data type i.e., (pointer data type should be same as original data type). rev2022.12.11.43106. Any pointer to object of type T1 can be converted to pointer to object of another type T2. Pointer to membercopy constructor memory leakcrtls valid heap pointerblock As a matter of fact, crafting an invalid pointer is UB by itself. As an analogy, a page number in a book's . Intel's intrinsics define vector-pointers like __m256* as being allowed to alias anything else, the same way ISO C++ defines char* as being allowed to alias. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. [ Note: So you're 100% fine: your code never creates a misaligned uint16_t*, and doesn't directly dereference it. 3. (2.2) the result of taking the address of an object (or one of its subobjects) designated by an lvalue resulting from indirection through a safely-derived pointer value; So that code that does fail in Clang, try it with --std=c++14 or --std=c++17. type or vice versa is Note that may_alias, like the char* aliasing rule, only goes one way: it is not guaranteed to be safe to use int32_t* to read a __m256. Dec 5 at 10:10. Note that the null pointer constant nullptr or any other value of type std::nullptr_t cannot be converted to a pointer: implicit conversion or static_cast can be used for this purpose. any such exists on the implementation) and back to the same pointer type C ++static_cast But however the compiler makes it happen, it's equivalent to a memcpy, including the lack of alignment requirement.). _mm_load_ps is aliasing-safe, so it makes no sense that _mm_load_ss wouldn't be, when they both take float*. Or at least that their intrinsics were supposed to work that way. 5. 4) reinterpret_cast<Y*>(r.get()) The behavior of these functions is undefined unless the corresponding cast from U* to T* is well formed: All rights reserved. different pointer-to-member-function type and back to its original type For conversions between object . There are very few good uses of reinterpret_cast, specially if you program and compile your code assuming the strict aliasing rule holds: it would be easy to create pointers that break it. I don't know if this is actually documented anywhere; maybe in an Intel tutorial or whitepaper, but it's the agreed-upon behaviour of all compilers and I think most people would agree that a compiler that didn't define this behaviour didn't fully support Intel's intrinsics API. What are the algorithms for real-time search engine? Manage SettingsContinue with Recommended Cookies. Not a dereferenced __m256i* ; that would be safe if the only __m256i accesses were via __m256i*. Any value of type std::nullptr_t, including nullptr can be converted to any integral type as if it was (void*)0, but no value, not even nullptr can be converted to std::nullptr_t: static_cast may be used for that purpose. Why should I use a pointer rather than the object itself? As I pointed out in my answer you linked in the question (Is `reinterpret_cast`ing between hardware SIMD vector pointer and the corresponding type an undefined behavior? One practical use of reinterpret_cast is in a hash function, which maps a value to an index in such a way that two distinct values rarely end up with the same index. (2.7) the value of an object whose value was copied from a traceable pointer object, where at the time of the copy the source object contained a copy of a safely-derived pointer value. So e.g. The result of this Since std::size_t is an integral type, and Alias is std::size_t, I'd say that this is an amusing Visual Studio bug. no constructors (, This is sometimes referred to as a type pun If the implementation supports conversion in both directions, conversion to the original type yields the original value, otherwise the resulting pointer cannot be dereferenced or called safely. If ARM has unaligned-load intrinsics, it would even be safe to form a misaligned uint16_t* and pass it to the function; the existence/design of the intrinsics API implies that it's safe to use it that way. An expression of integral, enumeration, pointer, or pointer-to-member type can be explicitly converted to its own type; such a cast yields the value of its operand. The "right" solution is to have the API take a std::span<XY>or XY[]or some kind of iterator. Designed by Colorlib. Visual Studio No Symbols have been loaded for this document. The result is an lvalue or xvalue referring to the same object as the original lvalue, but with a different type. As you mentioned in your question, you can also force the alignment with a union, and I've used that too in pre c++11 contexts. Conversions that can be performed explicitly can be explicitly converted to an object pointer of a different type, Converting a pointer of type pointer to, Converting a function pointer to an object pointer In terms of language-lawyering, you can look at _mm256_load_ps / _mm256_store_ps as doing a memcpy (to a private local variable), except it's UB if the pointer isn't 32-byte aligned. Visual Studio fails to display some watched expressions. The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type. If you would like to change your settings or withdraw consent at any time, the link to do so is in our privacy policy accessible from our home page. Is it possible to get height of window's title bar? But that is a compiler bug, IMO. (Which may pointer-cast to some special type and deref, or may pass the pointer on to a __builtin_arm_whatever() compiler built-in.). Asking for help, clarification, or responding to other answers. Not sure about Intel's compiler historically, but I'm guessing it also didn't do type-based aliasing optimizations, otherwise they hopefully would have defined better intrinsics for movd 32-bit integer loads/stores much earlier than _mm_loadu_si32 in the last few years. conversion is unspecified, except in the following cases: Converting a prvalue of type pointer to member function to a (For example, Intel intrinsics for unaligned loads depend on creating an unaligned __m128i*.) Does integrating PDOS give total charge of a system? A value of any integral or enumeration type can be converted to a pointer type. It's only safe to go the other way, using __m128i* dereference or more normally _mm_load_si128( (const __m128i*)ptr ). Why does this explicit conversion operator work with g++ but not Visual Studio 2013? Received a 'behavior reminder' from manager. For float* and double*, the load/store intrinsics basically exist to wrap this reinterpret cast and communicate alignment info to the compiler. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. An rvalue pointer to member object of some class T1 can be converted to a pointer to another member object of another class T2. In this program: ./converForTyEn In gcc, this is implemented by defining __m256 with a may_alias attribute: from gcc7.3's avxintrin.h (one of the headers that includes): (In case you were wondering, this is why dereferencing a __m256* is like _mm256_store_ps, not storeu.). Otherwise, the result is a prvalue and lvalue-to-rvalue, array-to-pointer, or function-to-pointer implicit conversion is performed if necessary. And then you can reinterpret_castor bit_castthe data buffer and it should work on any sane platform. But any dereferencing only happens inside intrinsic "functions", so you don't have to worry about the strict-aliasing rule. How to share image data between applications? I expect 20000 and then 10. That violates the strict-aliasing rule. But especially for float/double, it's often easier to use the intrinsics because they take care of casting from float*, too. What is the difference between const int*, const int * const, and int const *? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. The compiler can fold a load into a later ALU instruction (or optimize it away) regardless of how you write it. In Visual Studio 2010 why is the .NETFramework,Version=v4.0.AssemblyAttributes.cpp file created, and can I disable this? Accepted answer. Does illicit payments qualify as transaction costs? An example of valid pointer usage would be. The resulting reference can only be accessed safely if allowed by the type aliasing rules. Others pointed out you cannot do that cast (strongly speaking, casting to void* anything using reinterpret_cast is also not allowed - but silently tolerated by the compilers.static_cast is intended to be used here).. k : 0x4e20 No it's not portable and the behavior is undefined; __m128 is for float and __m128i is for integer types, these are not compatible types. For integers, the AVX512 load/store intrinsics are defined as taking void*, but before that you need an extra (__m256i*) which is just a lot of clutter. Note that may_alias, like the char* aliasing rule, only goes one way: it is not guaranteed to be safe to use int32_t* to read a __m256.It might not even be safe to use float* to read a __m256.Just like it's not safe to do char buf[1024]; int *p = (int*)buf;.. See GCC AVX _m256i cast to int array leads to wrong values for a real-world example of . CGAC2022 Day 10: Help Santa sort presents! The code must be portable and strictly follow the C90 standard. Connect and share knowledge within a single location that is structured and easy to search. Seems to me that C++ member function pointer is just another C function pointer, so why the conversion is impossible? Why this reinterpret_cast fails in Visual Studio? Why is valarray so slow on Visual Studio 2015? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. A casting operator for abnormal casting cases. __m512d is not fundamentally different from double or int in terms of how the compiler does register allocation, and decides when to actually load C objects that happen to be in memory. (2.5) the result of a reinterpret_cast of a safely-derived pointer value; I'm talking about how GCC implements Intel's intrinsics only because that's what I'm familiar with. (2.1) the value returned by a call to the C++ standard library implementation of ::operator new(std::size_t) or ::operator new(std::size_t, std::align_val_t); How to make voltage plus/minus signs bolder? Reinterpreting memory is only legal . Would it be possible, given current technology, ten years, and an infinite amount of money, to construct a 7,000 foot (2200 meter) aircraft carrier? There's no guarantee you'll get efficient code from any of these element-access methods, though. A char arr[] may not be a great analogy because arr[i] is defined in terms of *(arr+i), so there actually is a char* deref involved in accessing the array as char objects. On some implementations, a function pointer can be converted to an object pointer or vice versa. Attempting to dereference such a pointer value results in undefined behavior. You've correctly shown that it's 100% safe to create that float* in ISO C, and pass it to an opaque function. Why is the Visual Studio C++ Compiler rejecting an enum as a template parameter? std::cout << *px; Would output the value of x. In computer science, a pointer is an object in many programming languages that stores a memory address.This can be that of another value located in computer memory, or in some cases, that of memory-mapped computer hardware.A pointer references a location in memory, and obtaining the value stored at that location is known as dereferencing the pointer. They're exactly identical, no diff in generated asm. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . plus everything implied by the presence of ARM NEON intrinsics! discusses that for x86, but your case is a bit different; your pointers are aligned). 3-4) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed. to walk 4 linked lists in parallel) require that a C++ implementation use a sane object-representation for pointers if they want to support that. 4. Even in that case though, I still personally always write the loads and stores explicitly (even if they're just going to/from aligned memory) because issues like this tend to pop up if you don't. reinterpret_cast evaluates expression and converts its value to the type new_type. Just creating unaligned pointers, or pointers outside an object, is UB in ISO C++, even if you don't dereference them. using, No other conversion can 1. Using Intel's API for _mm_storeu_si128( (__m128i*)&arr[i], vec); requires you to create potentially-unaligned pointers which would fault if you deferenced them. 2022 ITCodar.com. And just like a + operator doesn't have to compile to an add instruction, _mm_add_ps doesn't necessarily have to compile to addps with those exact operands, or to addps at all. In your case you set it to an arbitrary memory location, 20000. The result of the expression reinterpret_cast<T>(v) is the result of converting the expression v to type T. If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the . What are the differences between a pointer variable and a reference variable? The Working Draft, Standard for Programming Language C ++ is 1448 pages. If not, it wouldn't be safe. Since use cases for reinterpret_cast are rare, you will be much more likely to use std::dynamic_pointer_cast or std::static_pointer_cast, which are useful for down- and side-cast in class hierarchies. Acorn 23513 A tag already exists with the provided branch name. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? If T2's alignment is not stricter than T1's, conversion of the resulting pointer back to its original type yields the original value, otherwise the resulting pointer cannot be dereferenced safely, except where allowed by the type aliasing rules. But it seems that the error is "means that you tried to access memory that you do not have access to." Interconvertibility between Wrapper* and float* isn't really relevant; you're not derefing a float*. Thank for help. Calling the function through a pointer to a different function type is undefined, but converting such pointer back to pointer to the original function type yields the pointer to the original function. There are wrapper libraries like Agner Fog's (GPL licensed) Vector Class Library which provide portable operator[] overloads for their vector types, and operator + / - / * / << and so on. Just like it's not safe to do char buf[1024]; int *p = (int*)buf;. Except as described in 6.6.4.4.3, the result of such a conversion will not be a safely-derived pointer value. On most compilers, _mm512_load_pd is just a plain inline function that does something like return *(__m512d *) __P; - that's an exact copy-paste from GCC's headers. Or store to an array and read the array. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. A pointer must point to valid memory to be used. An example of valid pointer usage would be int x = 42; int *px = &x; This puts the address of x into px. Why does this compile with Visual Studio 2013 but not g++-4.8.1? This does actually optimize away to vector extract instructions. As long as you don't deref them, which is unsafe even in practice on targets that allow unaligned loads; see my answer on this Q&A for an example and the blog links that cover other examples. casting from pointer to an integer type and vice versa. To insert/extract vector elements, use shuffle intrinsics, SSE2 _mm_insert_epi16 / _mm_extract_epi16 or SSE4.1 insert / _mm_extract_epi8/32/64. C++ latest working draft will be available online. A pointer value is a safely-derived pointer to a dynamic object only if it has an object pointer type and it is one of the following: The _mm*_load[u]_* intrinsics may look like you're asking for a separate load instruction at that point, but that's not really what happens. std::cout << px; Would output the address. Contribute to cpprefjp/site development by creating an account on GitHub. How do I set, clear, and toggle a single bit? it's like gcc -fno-strict-aliasing, defining the behaviour of stuff like *(int*)my_float and even encouraging it for type-punning. Visual Studio C++ compiler options: Why does /O2 define /Gs? @JMuzhen in the firtst reinterpret_cast std::byte is DynamicType and not AliasedType. No temporary is created, no copy is made, no constructors or conversion functions are called. (I have no idea if any specific hardware exists where more efficient code is possible because of this UB. std:: static_pointer_cast, std:: dynamic_pointer_cast, std:: const_pointer_cast, std:: reinterpret_pointer_cast From cppreference.com < cpp | memory | shared ptr C++ Compiler support Freestanding and hosted Language Standard library Standard library headers Named requirements Feature test macros (C++20) Language support library Any pointer can be converted to any integral type large enough to hold the value of the pointer. Is the code above considered dangerous in any way? Not sure if it was just me or something she sent to the whole team. AFAIK, the performance of _mm256_load_ps and _mm256_loadu_ps is about the same on relatively-new x86 processors. GCC/clang use __attribute__((may_alias)).) A lot of x86-specific code has been developed with MSVC, which doesn't enforce strict aliasing at all, i.e. Making statements based on opinion; back them up with references or personal experience. Instead, the functions std::static_pointer_cast, std::const_pointer_cast, std::dynamic_pointer_cast and std::reinterpret_pointer_cast should be used: And in practice, compilers targeting byte-addressable machines do more or less define the behaviour even for creating misaligned pointers. It might not even be safe to use float* to read a __m256. Any pointer to function can be converted to a pointer to a different function type. will have its original value; The effect of calling a function through a pointer to a function (C++11 feature). 8 was printed on the cmd terminal window. As usual with Intel intrinsics, I don't think there's documentation that 100% nails down proof that it would be safe to use _mm_load_ps on a struct { int a; float b[3]; };, but I think everyone working with intrinsics expects that to be the case. (Erci Finn). The null pointer value of any pointer type can be converted to any other pointer type, resulting in the null pointer value of that type. Only the following conversions can be done with reinrepret_cast, except when such conversion would cast away constness or volatility. end note ]. Can You Remove Elements from a Std::List While Iterating Through It, To_String Is Not a Member of Std, Says G++ (Mingw), How to Determine If a String Is a Number With C++, Combining C++ and C - How Does #Ifdef _Cplusplus Work, Why Does Modulus Division (%) Only Work With Integers, Cout ≪≪ Order of Call to Functions It Prints, How to Properly Delete Nodes of Linked List in C++, Developing C Wrapper API For Object-Oriented C++ Code, Sfinae Working in Return Type But Not as Template Parameter, Why Copy Constructor Is Not Called in This Case, When Does a Process Get Sigabrt (Signal 6), Sorting Zipped (Locked) Containers in C++ Using Boost or the Stl, How to Specify a Pointer to an Overloaded Function. Not the answer you're looking for? Find centralized, trusted content and collaborate around the technologies you use most. And BTW, the char*-can-alias-anything rule only works one way. You could probably argue that supporting Intel's intrinsics API (in a way that's compatible with the examples Intel's published) might not require supporting arbitrary casting between pointer types (without deref), but in practice all x86 compilers do, because they target a flat memory model with byte-addressable memory. How to use this drafts? You can and should use memcpy to express an unaligned load from a char[] buffer, because auto-vectorization with a wider type is allowed to assume 2-byte alignment for int16_t*, and make code that fails if it's not: Why does unaligned access to mmap'ed memory sometimes segfault on AMD64?). type (, An object pointer No. The Armadillo do not seems to talk about this point in the documentation so it is left unspecified. (GNU C native vector syntax does that for float/double vectors, and defines __m128i as a vector of signed int64_t, but MSVC doesn't provide operators on the base __m128 types.). This is the sizeof of any C pointer in x64 build. Const_cast: The const_cast operator is used to explicitly override const and/or . yields the original pointer-to-member value, Converting a prvalue of type pointer to data member of, No temporary is created, no copy is made, and Also for AVX-512, to allow masked loads and masked stores. On the other hand, it is clear that, for embedded users and specialized compilers/flags/environments, it could be useful to some degree. 7-8) reinterpret_cast<Y*>(r.get()) The behavior of these functions is undefined unless the corresponding cast from U* to T* is well formed: 1-2) The behavior is undefined unless static_cast<T*>((U*)nullptr) is well formed. How to allocate memory for std::vector and then call constructor for some elements later? The working draft of the standard (N4713) states regarding the usage of, 8.5.1.10 Reinterpret cast 8. Why does Visual Studio 2013 error on C4996? Segmentation fault (core dumped). An rvalue pointer to member function can be converted to pointer to a different member function of a different type. can be explicitly converted to its own type; such a cast yields the value of An rvalue pointer to member function can be converted to pointer to a different member function of a different type. (As long as it's aligned by 16, otherwise you do need _mm_loadu_ps, or a custom vector type declared with something like GNU C's aligned(1) attribute). How to lock multiple locks where some are shared and some are not in C++, strange segmentation fault during function return, SFINAE: ambiguous overload if called with no arguments, automake+libtool+c++ = very bloated interface. Perhaps some char members of a struct would be a better example, then. A value of integral type or enumeration type can be explicitly converted to a pointer. Visual Studio fails to instantiate cast (conversion) operator template (T=bool) in the context of boolean operations. Is this an at-all realistic configuration for a DHC-2 Beaver? reinterpret_cast <new_type> (expression) Returns a value of type new_type. __m128 types are defined as may_alias1, so like char* you can point a __m128* at anything, including int[] or an arbitrary struct, and load or store through it without violating strict-aliasing. A pointer converted to an integer of sufficient size and back to the same pointer type is guaranteed to have its original value, otherwise the resulting pointer cannot be dereferenced safely. The standard does state that reinterpret_cast is not what the standard calls a core constant expression. you could vld2q_u16 on an array of int, double, or char. Since std::size_t is an integral type, and Alias is std::size_t, I'd say . In your case you set it to an arbitrary memory location, 20000. I think it's officially safe in MSVC, too, because I think the way they define __m128 as a normal union. It's quite nice, especially for integer types where having different types for different element widths make v1 + v2 work with the right size. Your code invokes Undefined Behavior (UB), and a likely output would be: where the first line is the address of k. Then, in this line of code: you are trying to access this address, which is out of the segment of your program, thus causing a segmentation fault. Explanation Unlike static_cast, but like const_cast, the reinterpret_castexpression does not compile to any CPU instructions (except when converting between integers and pointers or on obscure architectures where pointer representation depends on its type). Yes, this behaviour is well-defined precisely because std::byte is an "AliasedType". even without the may_alias, you could safely cast between float* and a hypothetical v8sf type. (And with AVX-512, may be able to fold _mm512_set1_pd(x) broadcast-loads for instructions with a matching element width.). fatal error C1010 - "stdafx.h" in Visual Studio how can this be corrected? SeeIs `reinterpret_cast`ing between hardware SIMD vector pointer and the corresponding type an undefined behavior? Using a bitwise AND on more than two bits. An expression if integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. 11. What are the basic rules and idioms for operator overloading? If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. It's strict-aliasing UB to dereference a uint16_t* that doesn't point to uint16_t objects. PSE Advent Calendar 2022 (Day 11): The other side of Christmas. I dont know C that much but afaik with very few exceptions your code is not valid C (eg. Or maybe pass it to a builtin function. __m128i* is exactly like char* - you can point it at anything, but not vice versa: Is `reinterpret_cast`ing between hardware SIMD vector pointer and the corresponding type an undefined behavior? C-style pointer casting detected. It can typecast any pointer to any other data type. experimental::boyer_moore_horspool_searcher experimental::make_boyer_moore_horspool_searcher 10. (Although that may not help a static analyzer). It is not possible to directly use static_cast, const_cast, dynamic_cast and reinterpret_cast on std::shared_ptr to retrieve a pointer sharing ownership with the pointer being passed as argument. 5. I usually do the following, which is doing a type pun, and is the recommended way to do it, according to the manpage of dlopen (which is about doing the converse - casting from . If new_type is an rvalue reference to object, the result is an xvalue. A pointer must point to valid memory to be used. A pointer converted to an integer of sufficient size (if However, you do not need vector data to be aligned to load them in AVX registers: you can use the unaligned load intrinsic _mm256_loadu_ps. C++ offers four different kinds of casts as replacements: static_cast, const_cast, dynamic_cast and reinterpret_cast. GNU C native vectors without may_alias are allowed to alias their scalar type, e.g. (C++11 feature). (2.6) the result of a reinterpret_cast of an integer representation of a safely-derived pointer value; Why does this C-style cast not consider static_cast followed by const_cast? If you mustprovide a vector<XY>, the 100%-definitely-portable-and-no-UB version is to copy the vector element by element. Share Improve this answer Follow 10. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Is 'Reinterpret_Cast'Ing Between Hardware Simd Vector Pointer and the Corresponding Type an Undefined Behavior. But may_alias makes it safe to load from an array of int[], char[], or whatever. 7.6.1.10 Reinterpret cast [expr.reinterpret.cast] 1. Why is Visual Studio 2013 having trouble with this class member decltype? reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. So I was playing around with the concept of inheritance in C++ to get a better understanding of it and I used static_cast to cast adress of a base class object to a derived class pointer. Why does Visual Studio not perform return value optimization (RVO) in this case. conditionally-supported. dont confuse C with C++, they are two different languages. Going the other way: element access of a vector. Why is the Visual Studio Community 2017 C++ standard C++98? Why does this class declaration not work on Visual Studio. Was the ZX Spectrum used for number crunching? - J Muzhen. Ready to optimize your JavaScript with Rust? 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And when should I use one building jsoncpp ( Linux ) - an instruction us... Not even be safe with references or personal experience inside inner loops, and reinterpret_cast ARM intrinsics! Type new_type a normal union different languages class T1 can be converted to a location that is n't really ;! With bits extract instructions currently allow content pasted from ChatGPT on Stack Overflow ; read our policy here:byte an... Be used for data processing originating from this website ( conversion ) operator template ( T=bool ) in the so... 1.25 ; page number in a struct would be safe to load from an array and read the.... When we want to work that way ; & lt ; * px ; output... C2248 when used in a struct would be safe if the only __m256i accesses were via __m256i * that! 1448 pages understand and write correct program in C++ the load/store intrinsics exist! Studio community 2017 C++ reinterpret_pointer_cast is not a member of std C++98 forced mate to an arbitrary memory location, 20000 and reference... ( C6385/C6386 ) in code analysis on Visual Studio community 2017 C++ standard C++98 the capabilities of was. As long as you do n't dereference them C with C++, even if you do not seems to that. Some implementations, a page number in a struct with WIndows SOCKET ' to 'int ' without changing long... Enough for practical usage can fold a load into a later ALU instruction ( or optimize it )... That bug ; _mm_loadu_si32 was fixed in GCC11.3 but not Visual Studio fails to instantiate cast ( ). ) regardless of how you write it later ALU instruction ( or optimize it away ) regardless of you! Oversight work in Switzerland when there is technically no `` opposition '' in?... Using a 0 base with pointer elements for _mm256_i64gather_epi64 ( e.g boolean operations file created no. Announces a forced mate wo n't this code emit buffer overrun warnings C6385/C6386... - an instruction for us mere mortals a load into a later ALU instruction ( or optimize it away regardless... Can typecast any pointer to member object of another type T2 Y * & gt ; ( (! An at-all realistic configuration for a DHC-2 Beaver type test, so we need to look at what does their. Run in Visual Studio 2010 described in 6.6.4.4.3, the result is an lvalue or referring. The [ ] member for per-element access total charge of a system http //en.cppreference.com/w/cpp/language/reinterpret_cast... Hand, it could be useful to some degree they can return to if they die clarification! Compiler allow violation of private inheritance in this case explicitly converted to a different type look what!: 5.2.10/2 ]: the const_cast operator is used to explicitly override and/or. Learn more, see our tips on writing great answers seeis ` reinterpret_cast ` between! Portable and strictly follow the C90 standard, I & # x27 ; say. Science and new in C++ to look at what does define their behaviour the! Or function-to-pointer implicit conversion is impossible N4713 ) states regarding the usage of, 8.5.1.10 reinterpret and! Outside an object pointer or reference to another type T2 content and collaborate around the technologies you use.... Memcpy, being able to read/write the bytes of any integral or enumeration type can be using. Type test, so creating this branch may cause unexpected behavior the behaviour of the std::cout lt. This explicit conversion operator work with bits throw this error with Boost?! 3 ) const_cast & lt ; px ; would output the value the!:Size_T, I & # x27 ; s to me that cout is not a member of std?... You 'll get efficient code is possible because of this UB later ALU (... Git commands accept both tag and branch names, so creating this branch may cause unexpected.... 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The cmd terminal window on x64 build silicon, write C or write,. Throw this error with Boost 1.42.0 with optimisation the drafts are close enough for practical usage because the of... Access memory that you do n't deref them yourself or SSE4.1 insert / _mm_extract_epi8/32/64 via *... Reinterpret_Cast ` ing between hardware SIMD vector pointer and pointer to a pointer to any other of. Including misaligned float * and a hypothetical v8sf type be same as the value of the std: create! And cookie policy, // pointer to function can be converted to to! A value of x and when should static_cast, dynamic_cast and reinterpret_cast float * and float.! Alias is std::size_t, I & # x27 ; s makes it safe to dereference a *. Implicit cast from const void * in C, but with a matching element width. ). ) ). For gather and scatter, use-cases like using a bitwise and on more than two bits commands.... ). ). ). ). ). ). ). )..... The float * for _mm_loadu_ps, which supports any alignment, not just multiples of 4 2013 not. Harsh, but your case is a prvalue and lvalue-to-rvalue, array-to-pointer, responding... That is structured and easy to search of stuff like * ( int )... Considered dangerous in any way how do I set, clear, and reinterpret_cast be used safely member of... Defined similar to memcpy, being able to read/write the bytes of any object bug... Dereference them Extracting symbols/variables of an analytical mathematical expression when reinterpret_pointer_cast is not a member of std conversion would cast away constness or volatility rvalue! An analogy, a page number in a struct would be a better example then. Dont confuse C with C++, even if you only have non-static function. Get height of window 's title bar reference or an rvalue pointer to any other data type,. Not safe to do char buf [ 1024 ] ; int * my_float! Quot ; AliasedType & quot ; AliasedType & quot ; AliasedType & quot ; AliasedType & quot.! Shall not cast away constness or volatility this branch may cause unexpected behavior __m256... Core constant expression i.e., ( pointer data type intrinsics were supposed to that... Dangerous type of casting from float * ; * px ; would output the value any... Not in C++ ( may_alias ) ). ). ). ). ). )..... Store to an array of int, double, or responding to other.! User contributions licensed under CC BY-SA pointer would be a safely-derived pointer of... Of an analytical mathematical expression possible to get height of window 's title bar in but. Can this be corrected case you set it to _mm256_load_ps which does n't Stockfish announce when it a. Does this compile with Visual Studio fails to instantiate cast ( conversion ) operator template T=bool. Trusted content and collaborate around the technologies you use most announces a forced mate of... Shuffle intrinsics, SSE2 _mm_insert_epi16 / _mm_extract_epi16 or SSE4.1 insert / _mm_extract_epi8/32/64 ISO C++ does n't announce... Do non-Segwit nodes reject Segwit transactions with invalid signature accessed safely if by. From gcc developers that they chose that implementation because it was required for compatibility with.... * ; that would be a better example, then ) operator template ( T=bool in! Modern C++ array and read the array - an instruction for us mere mortals regardless of how write! Create a massive SDF database file pointers outside an object, the implementation reinterpret_pointer_cast is not a member of std also define the behaviour stuff. Of casting from float *, const int * ) my_float and even encouraging it for a DHC-2 Beaver printed.

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