fatal error C1010 - "stdafx.h" in Visual Studio how can this be corrected? As a matter of fact, crafting an invalid pointer is UB by itself. There is no object of type test, so you cannot form a pointer or reference to it. An rvalue pointer to member object of some class T1 can be converted to a pointer to another member object of another class T2. 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. (I'm not sure if the major implementations on x86 do define that behaviour, but you don't need to rely on it because they optimize away memcpy of 4 bytes into an int32_t. Not sure if it was just me or something she sent to the whole team. 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). 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. (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.). Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. 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. 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).. So e.g. How to use this drafts? Why this reinterpret_cast fails in Visual Studio? 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. 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"? (Related: Is `reinterpret_cast`ing between hardware SIMD vector pointer and the corresponding type an undefined behavior? Why does Visual Studio compiler allow violation of private inheritance in this example? The resulting value is the same as the value of expression. Why does this compile with Visual Studio 2013 but not g++-4.8.1? On the other hand, it is clear that, for embedded users and specialized compilers/flags/environments, it could be useful to some degree. 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. Intel intrinsics are defined that way (e.g. (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); Acorn 23513 Why is valarray so slow on Visual Studio 2015? 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. 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. A pointer must point to valid memory to be used. How to get the address of the std::vector buffer start most elegantly? In C, it's safe to cast between pointer types (without dereferencing) as long as you never create a pointer with insufficient alignment for its type. Designed by Colorlib. You've correctly shown that it's 100% safe to create that float* in ISO C, and pass it to an opaque function. You can tell from the void* arg that it's recent: Intel previously did insane stuff like _mm_loadl_epi64(__m128i*) for a movq load, taking a pointer to a 16-byte object but only loading the low 8 bytes (with no alignment requirement). It's quite nice, especially for integer types where having different types for different element widths make v1 + v2 work with the right size. 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. - J Muzhen. Why is this code allowed to compile under Visual Studio 2013? No temporary is created, no copy is made, no constructors or conversion functions are called. I think it's officially safe in MSVC, too, because I think the way they define __m128 as a normal union. 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. 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. A casting operator for abnormal casting cases. Do not use inside inner loops, and have a look at the resulting asm if performance matters. Manage SettingsContinue with Recommended Cookies. I assume that ARM load/store intrinsics are defined similar to memcpy, being able to read/write the bytes of any object. How to allocate memory for std::vector and then call constructor for some elements later? 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. Interconvertibility between Wrapper* and float* isn't really relevant; you're not derefing a float*. What are the differences between a pointer variable and a reference variable? It might not even be safe to use float* to read a __m256. Since std::size_t is an integral type, and Alias is std::size_t, I'd say . Pointer to membercopy constructor memory leakcrtls valid heap pointerblock Thus, vector data are likely not ensured to be 32-bytes aligned. So you're 100% fine: your code never creates a misaligned uint16_t*, and doesn't directly dereference it. 3) const_cast<Y*>(r.get()). (C++11 feature). That violates the strict-aliasing rule. As an analogy, a page number in a book's . (m68k has address vs. data registers, but it never faults from keeping bit-patterns that aren't valid addresses in A registers, as long as you don't deref them.). And yes, deref of an __m256* is exactly equivalent to _mm256_load_ps, and is in fact how most compilers implement _mm256_load_ps. Thank for help. 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. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . Yes, this behaviour is well-defined precisely because std::byte is an "AliasedType". k : 0x4e20 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. 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 GNU C vector syntax provides the [] operator for vectors, like __m256 v = ; v[3] = 1.25;. 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 . And BTW, the char*-can-alias-anything rule only works one way. When would I give a checkpoint to my D&D party that they can return to if they die? 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. dont confuse C with C++, they are two different languages. 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. test t = * (test*)&text [13]; This is simply illegal and so is the version using C++'s named casts. It's only safe to go the other way, using __m128i* dereference or more normally _mm_load_si128( (const __m128i*)ptr ). Pick one. Is the code above considered dangerous in any way? Const_cast: The const_cast operator is used to explicitly override const and/or . (Erci Finn). reinterpret_cast evaluates expression and converts its value to the type new_type. The exact aliasing semantics of Intel Intrinsics are not AFAIK documented anywhere. _mm_load_ps is aliasing-safe, so it makes no sense that _mm_load_ss wouldn't be, when they both take float*. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. 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. discusses that for x86, but your case is a bit different; your pointers are aligned). We and our partners use cookies to Store and/or access information on a device.We and our partners use data for Personalised ads and content, ad and content measurement, audience insights and product development.An example of data being processed may be a unique identifier stored in a cookie. you could vld2q_u16 on an array of int, double, or char. conditionally-supported. 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. 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. What are the algorithms for real-time search engine? What is the difference between const int*, const int * const, and int const *? Why does this C-style cast not consider static_cast followed by const_cast? 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. But any dereferencing only happens inside intrinsic "functions", so you don't have to worry about the strict-aliasing rule. 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. Received a 'behavior reminder' from manager. I'm talking about how GCC implements Intel's intrinsics only because that's what I'm familiar with. But especially for float/double, it's often easier to use the intrinsics because they take care of casting from float*, too. 4. Copyright 2022 www.appsloveworld.com. ), Intel's intrinsics API effectively requires compilers to support creating misaligned pointers as long as you don't deref them yourself. Why does Visual Studio compile this function correctly without optimisation, but incorrectly with optimisation? 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 (2.5) the result of a reinterpret_cast of a safely-derived pointer value; int g = 10; int * k = reinterpret_cast includes): (In case you were wondering, this is why dereferencing a __m256* is like _mm256_store_ps, not storeu.). Why is the Visual Studio Community 2017 C++ standard C++98? Making statements based on opinion; back them up with references or personal experience. Was the ZX Spectrum used for number crunching? 1) static_cast<Y*>(r.get()). like int to pointer and pointer to int etc. (By comparison, _mm256_loadu_ps would cast to a pointer to a less-aligned 32-byte vector type which isn't part of the documented API, like GCC's __m256_u*. 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. Instead, the functions std::static_pointer_cast, std::const_pointer_cast, std::dynamic_pointer_cast and std::reinterpret_pointer_cast should be used: Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. I expect 20000 and then 10. Ready to optimize your JavaScript with Rust? And then you can reinterpret_castor bit_castthe data buffer and it should work on any sane platform. 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 Except as described in 6.6.4.4.3, the result of such a conversion will not be a safely-derived pointer value. Perhaps some char members of a struct would be a better example, then. Conversions that can be performed explicitly (print a __m128i variable). Is 'Reinterpret_Cast'Ing Between Hardware Simd Vector Pointer and the Corresponding Type an Undefined Behavior. Visual Studio No Symbols have been loaded for this document. I am new in computer science and new in C++. using, No other conversion can Why is Visual Studio 2013 having trouble with this class member decltype? Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. GNU C native vectors without may_alias are allowed to alias their scalar type, e.g. The Working Draft, Standard for Programming Language C ++ is 1448 pages. (2.6) the result of a reinterpret_cast of an integer representation of a safely-derived pointer value; Dec 5 at 10:10. Find centralized, trusted content and collaborate around the technologies you use most. See GCC AVX _m256i cast to int array leads to wrong values for a real-world example of GCC breaking code that points an int* into a __m256i vec; object. cpprefjpMarkdown. If you wanted a load with normal C aliasing/alignment semantics to promise more to the optimizer, you'd just deref yourself, using _mm_set_ss( *foo ). There's no guarantee you'll get efficient code from any of these element-access methods, though. Demonstrates some uses of reinterpret_cast, // pointer to function to another and back, http://en.cppreference.com/w/cpp/language/reinterpret_cast. What are the basic rules and idioms for operator overloading? Nope, not at all. __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. 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. The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type. This is the sizeof of any C pointer in x64 build. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? 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? Otherwise, the result is a prvalue and lvalue-to-rvalue, array-to-pointer, or function-to-pointer implicit conversion is performed if necessary. But may_alias makes it safe to load from an array of int[], char[], or whatever. 10. (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; (C++11 feature). AFAIK, the performance of _mm256_load_ps and _mm256_loadu_ps is about the same on relatively-new x86 processors. C-style pointer casting detected. GCC/clang use __attribute__((may_alias)).) structure of the underlying machine. (I have no idea if any specific hardware exists where more efficient code is possible because of this UB. And nobody would want to use a compiler that broke it for a cases where memcpy with the same source pointer would be safe. 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. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. 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. __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. Just creating unaligned pointers, or pointers outside an object, is UB in ISO C++, even if you don't dereference them. What is the difference between #include and #include "filename"? Reinterpreting memory is only legal . SeeIs `reinterpret_cast`ing between hardware SIMD vector pointer and the corresponding type an undefined behavior? In this program: ./converForTyEn 5. 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. C++ latest working draft will be available online. 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. No. That's strange. std::cout << *px; Would output the value of x. Although when I printed the C++ member function pointer itself, 1 was printed on the cmd terminal window on x64 build. For C++11 it was defined as: . reinterpret_cast is a very special and dangerous type of casting operator. 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. Visual Studio C++ compiler options: Why does /O2 define /Gs? An rvalue pointer to member function can be converted to pointer to a different member function of a different type. Why does Visual Studio not perform return value optimization (RVO) in this case. 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 The only standard blessed way to do type punning is with memcpy: I prefer just using aligned memory of the correct type directly: I'm not positive that this setup is actually standard-blessed (it definitely is for _mm_load_ps since you can do it without type punning at all) but it does seem to also fix the issue. Why does Visual Studio 2013 error on C4996? Why does Visual Studio 2010 throw this error with Boost 1.42.0? 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. For float* and double*, the load/store intrinsics basically exist to wrap this reinterpret cast and communicate alignment info to the compiler. It's strict-aliasing UB to dereference a uint16_t* that doesn't point to uint16_t objects. The working draft of the standard (N4713) states regarding the usage of, 8.5.1.10 Reinterpret cast Dont do whatever this is. 7. Asking for help, clarification, or responding to other answers. 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. . Including misaligned float* for _mm_loadu_ps, which supports any alignment, not just multiples of 4. Also for AVX-512, to allow masked loads and masked stores. The Armadillo do not seems to talk about this point in the documentation so it is left unspecified. I guess this allows implementations on exotic hardware which do some kinds of checks on pointers when creating them (possibly instead of when dereferencing), or maybe which can't store the low bits of pointers. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Why does this code emit buffer overrun warnings(C6385/C6386) in code analysis on Visual Studio 2012? 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). On some implementations, a function pointer can be converted to an object pointer or vice versa. How do I set, clear, and toggle a single bit? [C++14: 5.2.10/2]: The reinterpret_cast operator shall not cast away constness (5.2.11). If not, it wouldn't be safe. 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. 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. That just makes your C look more like asm if you want it to. (Unfortunately even current GCC still has that bug; _mm_loadu_si32 was fixed in GCC11.3 but not the older _ss and _sd loads.) To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 3-4) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed. Segmentation fault (core dumped). experimental::boyer_moore_horspool_searcher experimental::make_boyer_moore_horspool_searcher 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. So a load intrinsic is literally already doing this. building jsoncpp (Linux) - an instruction for us mere mortals? Why does this class declaration not work on Visual Studio. 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. be performed explicitly using, An expression of integral, enumeration, pointer, or pointer-to-member type 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 . Any pointer to object of type T1 can be converted to pointer to object of another type T2. Or maybe pass it to a builtin function. If _mm_load_ps() is supported, the implementation must also define the behaviour of the code in the question. An example of valid pointer usage would be. If you mustprovide a vector<XY>, the 100%-definitely-portable-and-no-UB version is to copy the vector element by element. Why should I use a pointer rather than the object itself? conversion is unspecified, except in the following cases: Converting a prvalue of type pointer to member function to a 2. Visual Studio fails to display some watched expressions. type or vice versa is reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. it's like gcc -fno-strict-aliasing, defining the behaviour of stuff like *(int*)my_float and even encouraging it for type-punning. How can I change 'long' to 'int' without changing 'long long' using perl/awk/etc. [C++14: 5.2.10/2]: The reinterpret_cast operator shall not cast away constness (5.2.11). Some of our partners may process your data as a part of their legitimate business interest without asking for consent. How is Jesus God when he sits at the right hand of the true God? 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. So yes, it's safe to dereference a __m256* instead of using a _mm256_load_ps() aligned-load intrinsic. - n. m. 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. 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? 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. (For example, Intel intrinsics for unaligned loads depend on creating an unaligned __m128i*.) In Visual Studio 2010 why is the .NETFramework,Version=v4.0.AssemblyAttributes.cpp file created, and can I disable this? Why does the Visual Studio conversion wizard 2010 create a massive SDF database file? A tag already exists with the provided branch name. 8 was printed on the cmd terminal window. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. You're not directly dereffing the float*, only passing it to _mm256_load_ps which does an aliasing-safe load. Why does this explicit conversion operator work with g++ but not Visual Studio 2013? 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. Connect and share knowledge within a single location that is structured and easy to search. But that is a compiler bug, IMO. Why does Visual C++ warn on implicit cast from const void ** to void * in C, but not in C++? Does aliquot matter for final concentration? type (, An object pointer 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. 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. "There's no Qt version assigned to this project for platform Win32" - visual studio plugin for Qt. even without the may_alias, you could safely cast between float* and a hypothetical v8sf type. To insert/extract vector elements, use shuffle intrinsics, SSE2 _mm_insert_epi16 / _mm_extract_epi16 or SSE4.1 insert / _mm_extract_epi8/32/64. So that code that does fail in Clang, try it with --std=c++14 or --std=c++17. If T2's alignment is not stricter than T1's, conversion to the original type yields the original value, otherwise the resulting pointer cannot be used safely. plus everything implied by the presence of ARM NEON intrinsics! 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. 3-4) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed. How to make voltage plus/minus signs bolder? Or at least that their intrinsics were supposed to work that way. 8. Do non-Segwit nodes reject Segwit transactions with invalid signature? Is it possible to get height of window's title bar? 10. [ Note: For conversions between object . (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). a reinterpret_cast is a conversion operator. 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. 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. its operand, It is intended to be unsurprising to those who know the addressing 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. The consent submitted will only be used for data processing originating from this website. 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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). CGAC2022 Day 10: Help Santa sort presents! Going the other way: element access of a vector. 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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. 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. The code must be portable and strictly follow the C90 standard. 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. The resulting reference can only be accessed safely if allowed by the type aliasing rules. 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. Not a dereferenced __m256i* ; that would be safe if the only __m256i accesses were via __m256i*. when the result refers to the same object as the source glvalue. Thanks for contributing an answer to Stack Overflow! And in practice, compilers targeting byte-addressable machines do more or less define the behaviour even for creating misaligned pointers. Why doesn't Stockfish announce when it solved a position as a book draw similar to how it announces a forced mate? Sorry for being a bit harsh, but for the love of silicon, write C or write good, modern C++. Why won't this code compile and run in Visual Studio 2010? How to read an intermittent hard drive consistently? (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. 2022 ITCodar.com. So this . How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? PSE Advent Calendar 2022 (Day 11): The other side of Christmas. The result is an lvalue or xvalue referring to the same object as the original lvalue, but with a different type. Reinterpret-cast: The reinterpret cast operator changes a pointer to any other type of pointer. __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? any such exists on the implementation) and back to the same pointer type different pointer-to-member-function type and back to its original type All Rights Reserved. 1. To learn more, see our tips on writing great answers. Failing to parse different math operators, C++: Extracting symbols/variables of an analytical mathematical expression. It can typecast any pointer to any other data type. Since you are using reinterpret_cast on an integer literal (20000), the result of the conversion is not a safely-derived pointer value. no constructors (, This is sometimes referred to as a type pun Only the following conversions can be done with reinrepret_cast, except when such conversion would cast away constness or volatility. the conversions that can be performed using a reinterpret_cast are limited; the permissible ones are specified by the standard. The standard does state that reinterpret_cast is not what the standard calls a core constant expression. Using a bitwise AND on more than two bits. @JMuzhen in the firtst reinterpret_cast std::byte is DynamicType and not AliasedType. Visual Studio fails to instantiate cast (conversion) operator template (T=bool) in the context of boolean operations. 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++. Or store to an array and read the array. The actual standard is expensive to purchase. C++ offers four different kinds of casts as replacements: static_cast, const_cast, dynamic_cast and reinterpret_cast. Accepted answer. (And with AVX-512, may be able to fold _mm512_set1_pd(x) broadcast-loads for instructions with a matching element width.). Why is the Visual Studio C++ Compiler rejecting an enum as a template parameter? 11. ), But implementations which support Intel's intrinsics must define the behaviour, at least for the __m* types and float*/double*. 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. 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. 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. 9. A pointer must point to valid memory to be used. An lvalue expression of type T1 can be converted to reference to another type T2. But it seems that the error is "means that you tried to access memory that you do not have access to." rev2022.12.11.43106. 2) dynamic_cast<Y*>(r.get()) (If the result of the dynamic_cast is a null pointer value, the returned shared_ptr will be empty). (2.3) the result of well-defined pointer arithmetic using a safely-derived pointer value; A value of integral type or enumeration type can be explicitly converted to a pointer. The reinterpret_cast allows the pointer to be treated as an integral . Here is one link: I think I have to use working draft and books to understand and write correct program in C++. If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. But however the compiler makes it happen, it's equivalent to a memcpy, including the lack of alignment requirement.). Why is visual studio code telling me that cout is not a member of std namespace? It is used when we want to work with bits. This is important because the capabilities of constexpr was vastly increased with the C++14 standard compared to C++11. (2.4) the result of a well-defined pointer conversion of a safely-derived pointer value; Any pointer can be converted to any integral type large enough to hold the value of the pointer. Just like it's not safe to do char buf[1024]; int *p = (int*)buf;. What is a smart pointer and when should I use one? Conversion to the original type yields the original value, otherwise the resulting pointer cannot be used safely. They're exactly identical, no diff in generated asm. But the drafts are close enough for practical usage. Any pointer to function can be converted to a pointer to a different function type. Attempting to dereference such a pointer value results in undefined behavior. If new_type is an rvalue reference to object, the result is an xvalue. MSVC defines vector types as a union with a .m128_f32[] member for per-element access. 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