Why is valarray so slow on Visual Studio 2015? An rvalue pointer to member function can be converted to pointer to a different member function of a different type. A value of any integral or enumeration type can be converted to a pointer type. Copyright 2022 www.appsloveworld.com. Visual Studio fails to display some watched expressions. Accepted answer. So e.g. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. 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. 3. What are the basic rules and idioms for operator overloading? 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. Thank for help. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Visual Studio fails to instantiate cast (conversion) operator template (T=bool) in the context of boolean operations. Why does this compile with Visual Studio 2013 but not g++-4.8.1? 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*. Using a bitwise AND on more than two bits. I've heard from gcc developers that they chose that implementation because it was required for compatibility with Intel. Not a dereferenced __m256i* ; that would be safe if the only __m256i accesses were via __m256i*. dont confuse C with C++, they are two different languages. Is 'Reinterpret_Cast'Ing Between Hardware Simd Vector Pointer and the Corresponding Type an Undefined Behavior. But especially for float/double, it's often easier to use the intrinsics because they take care of casting from float*, too. Just creating unaligned pointers, or pointers outside an object, is UB in ISO C++, even if you don't dereference them. std::cout << px; Would output the address. This is important because the capabilities of constexpr was vastly increased with the C++14 standard compared to C++11. (C++11 feature). No temporary is created, no copy is made, no constructors or conversion functions are called. How to allocate memory for std::vector and then call constructor for some elements later? Any pointer to function can be converted to a pointer to a different function type. So that code that does fail in Clang, try it with --std=c++14 or --std=c++17. PSE Advent Calendar 2022 (Day 11): The other side of Christmas. 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. An expression if integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. Does illicit payments qualify as transaction costs? building jsoncpp (Linux) - an instruction for us mere mortals? [C++14: 5.2.10/2]: The reinterpret_cast operator shall not cast away constness (5.2.11). 1) static_cast<Y*>(r.get()). (2.5) the result of a reinterpret_cast of a safely-derived pointer value; 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. a reinterpret_cast is a conversion operator. 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"? 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. (Unfortunately even current GCC still has that bug; _mm_loadu_si32 was fixed in GCC11.3 but not the older _ss and _sd loads.) Just like it's not safe to do char buf[1024]; int *p = (int*)buf;. 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. Why does this code emit buffer overrun warnings(C6385/C6386) in code analysis on Visual Studio 2012? Find centralized, trusted content and collaborate around the technologies you use most. Pick 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. 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: 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. How to get the address of the std::vector buffer start most elegantly? 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. Why does this C-style cast not consider static_cast followed by const_cast? But it seems that the error is "means that you tried to access memory that you do not have access to." Why does Visual Studio 2010 throw this error with Boost 1.42.0? Or store to an array and read the array. cpprefjpMarkdown. reinterpret_cast is a very special and dangerous type of casting operator. 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? Why does Visual Studio compile this function correctly without optimisation, but incorrectly with optimisation? Attempting to dereference such a pointer value results in undefined behavior. 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. This does actually optimize away to vector extract instructions. __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. In your case you set it to an arbitrary memory location, 20000. Intel intrinsics are defined that way (e.g. __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. Why does Visual Studio 2013 error on C4996? 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. 3) const_cast<Y*>(r.get()). (I have no idea if any specific hardware exists where more efficient code is possible because of this UB. (And with AVX-512, may be able to fold _mm512_set1_pd(x) broadcast-loads for instructions with a matching element width.). 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. 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. Connect and share knowledge within a single location that is structured and easy to search. Failing to parse different math operators, C++: Extracting symbols/variables of an analytical mathematical expression. be performed explicitly using, An expression of integral, enumeration, pointer, or pointer-to-member type But the drafts are close enough for practical usage. Not sure if it was just me or something she sent to the whole team. On some implementations, a function pointer can be converted to an object pointer or vice versa. 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. If not, it wouldn't be safe. 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 ). The resulting value is the same as the value of expression. 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 . Contribute to cpprefjp/site development by creating an account on GitHub. type (, An object pointer 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. _mm_load_ps is aliasing-safe, so it makes no sense that _mm_load_ss wouldn't be, when they both take float*. The compiler can fold a load into a later ALU instruction (or optimize it away) regardless of how you write it. It can typecast any pointer to any other data type. 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. Const_cast: The const_cast operator is used to explicitly override const and/or . Thanks for contributing an answer to Stack Overflow! 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. 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. It also allows. What are the differences between a pointer variable and a reference variable? So yes, it's safe to dereference a __m256* instead of using a _mm256_load_ps() aligned-load intrinsic. Visual Studio C++ compiler options: Why does /O2 define /Gs? The consent submitted will only be used for data processing originating from this website. How to share image data between applications? The Armadillo do not seems to talk about this point in the documentation so it is left unspecified. GNU C vector syntax provides the [] operator for vectors, like __m256 v = ; v[3] = 1.25;. 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. The exact aliasing semantics of Intel Intrinsics are not AFAIK documented anywhere. Including misaligned float* for _mm_loadu_ps, which supports any alignment, not just multiples of 4. And yes, deref of an __m256* is exactly equivalent to _mm256_load_ps, and is in fact how most compilers implement _mm256_load_ps. I think it's officially safe in MSVC, too, because I think the way they define __m128 as a normal union. What is a smart pointer and when should I use one? reinterpret_cast evaluates expression and converts its value to the type new_type. How to read an intermittent hard drive consistently? Otherwise, the result is a prvalue and lvalue-to-rvalue, array-to-pointer, or function-to-pointer implicit conversion is performed if necessary. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. 3-4) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed. 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. Reinterpreting memory is only legal . Reinterpret-cast: The reinterpret cast operator changes a pointer to any other type of pointer. ), But implementations which support Intel's intrinsics must define the behaviour, at least for the __m* types and float*/double*. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? it's like gcc -fno-strict-aliasing, defining the behaviour of stuff like *(int*)my_float and even encouraging it for type-punning. 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). 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. Segmentation fault (core dumped). 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. (2.3) the result of well-defined pointer arithmetic using a safely-derived pointer value; To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 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. 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. . 10. 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. 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. can be explicitly converted to its own type; such a cast yields the value of And nobody would want to use a compiler that broke it for a cases where memcpy with the same source pointer would be safe. Thus, vector data are likely not ensured to be 32-bytes aligned. 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. 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. 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. The code must be portable and strictly follow the C90 standard. Why is the Visual Studio Community 2017 C++ standard C++98? This is the sizeof of any C pointer in x64 build. Why does the Visual Studio conversion wizard 2010 create a massive SDF database file? In this program: ./converForTyEn 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 If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. And _mm_storeu_ps to a location that isn't 4-byte aligned requires creating an under-aligned float*. "There's no Qt version assigned to this project for platform Win32" - visual studio plugin for Qt. No. 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. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Seems to me that C++ member function pointer is just another C function pointer, so why the conversion is impossible? Interconvertibility between Wrapper* and float* isn't really relevant; you're not derefing a float*. 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. 9. CGAC2022 Day 10: Help Santa sort presents! That's strange. The actual standard is expensive to purchase. even without the may_alias, you could safely cast between float* and a hypothetical v8sf type. Why is visual studio code telling me that cout is not a member of std namespace? Acorn 23513 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. 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. [ Note: Asking for help, clarification, or responding to other answers. 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.). (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.). Or maybe pass it to a builtin function. Why this reinterpret_cast fails in Visual Studio? Why does this explicit conversion operator work with g++ but not Visual Studio 2013? 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. casting from pointer to an integer type and vice versa. Share Improve this answer Follow (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. The resulting reference can only be accessed safely if allowed by the type aliasing rules. 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. Plain dereference is still strict-aliasing safe because __mm* types are special. In Visual Studio 2010 why is the .NETFramework,Version=v4.0.AssemblyAttributes.cpp file created, and can I disable this? I'm talking about how GCC implements Intel's intrinsics only because that's what I'm familiar with. 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 however the compiler makes it happen, it's equivalent to a memcpy, including the lack of alignment requirement.). Except as described in 6.6.4.4.3, the result of such a conversion will not be a safely-derived pointer value. Why doesn't Stockfish announce when it solved a position as a book draw similar to how it announces a forced mate? 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?). Difference Between Char A[] = String; and Char *P = String; How to Pass a Member Function Where a Free Function Is Expected, Store Derived Class Objects in Base Class Variables, Adding External Library into Qt Creator Project, Is Returning by Rvalue Reference More Efficient, About Us | Contact Us | Privacy Policy | Free Tutorials. They're exactly identical, no diff in generated asm. 8 was printed on the cmd terminal window. 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. 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++. Is this an at-all realistic configuration for a DHC-2 Beaver? Visual Studio No Symbols have been loaded for this document. The Working Draft, Standard for Programming Language C ++ is 1448 pages. 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. All rights reserved. But may_alias makes it safe to load from an array of int[], char[], or whatever. 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). Designed by Colorlib. 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).. Manage SettingsContinue with Recommended Cookies. Sorry for being a bit harsh, but for the love of silicon, write C or write good, modern C++. An example of valid pointer usage would be int x = 42; int *px = &x; This puts the address of x into px. I dont know C that much but afaik with very few exceptions your code is not valid C (eg. 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 10. How is Jesus God when he sits at the right hand of the true God? The result is an lvalue or xvalue referring to the same object as the original lvalue, but with a different type. There's no guarantee you'll get efficient code from any of these element-access methods, though. GCC/clang use __attribute__((may_alias)).) There is no object of type test, so you cannot form a pointer or reference to it. Why is this code allowed to compile under Visual Studio 2013? 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. The result of this Does aliquot matter for final concentration? The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type. (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. It's strict-aliasing UB to dereference a uint16_t* that doesn't point to uint16_t objects. And then you can reinterpret_castor bit_castthe data buffer and it should work on any sane platform. I am new in computer science and new in C++. An example of valid pointer usage would be. In your case you set it to an arbitrary memory location, 20000. 7.6.1.10 Reinterpret cast [expr.reinterpret.cast] 1. Any pointer to object of type T1 can be converted to pointer to object of another type T2. @JMuzhen in the firtst reinterpret_cast std::byte is DynamicType and not AliasedType. An lvalue expression of type T1 can be converted to reference to another type T2. 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. That just makes your C look more like asm if you want it to. So this . 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. Why does Visual Studio not perform return value optimization (RVO) in this case. rev2022.12.11.43106. 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. like int to pointer and pointer to int etc. Not the answer you're looking for? 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. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . k : 0x4e20 different pointer-to-member-function type and back to its original type (C++11 feature). I'd guess that any reasonable implementation of the load/store intrinsics is going to have to provide the same sort of aliasing guarantees that memcpy does since it's more or less the kosher way to go from straight line to vectorized code in x86. when the result refers to the same object as the source glvalue. (Related: Is `reinterpret_cast`ing between hardware SIMD vector pointer and the corresponding type an undefined behavior? 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. What is the difference between const int*, const int * const, and int const *? How to make voltage plus/minus signs bolder? Do non-Segwit nodes reject Segwit transactions with invalid signature? A pointer must point to valid memory to be used. So you're 100% fine: your code never creates a misaligned uint16_t*, and doesn't directly dereference it. 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. An rvalue pointer to member function can be converted to pointer to a different member function of a different type. 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). To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. type or vice versa is [C++14: 5.2.10/2]: The reinterpret_cast operator shall not cast away constness (5.2.11). Dec 5 at 10:10. Ready to optimize your JavaScript with Rust? Why won't this code compile and run in Visual Studio 2010? I assume that ARM load/store intrinsics are defined similar to memcpy, being able to read/write the bytes of any object. A pointer converted to an integer of sufficient size (if Only the following conversions can be done with reinrepret_cast, except when such conversion would cast away constness or volatility. 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. any such exists on the implementation) and back to the same pointer type 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. the conversions that can be performed using a reinterpret_cast are limited; the permissible ones are specified by the standard. So a load intrinsic is literally already doing this. SeeIs `reinterpret_cast`ing between hardware SIMD vector pointer and the corresponding type an undefined behavior? 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. Instead, the functions std::static_pointer_cast, std::const_pointer_cast, std::dynamic_pointer_cast and std::reinterpret_pointer_cast should be used: 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. reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. 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. you could vld2q_u16 on an array of int, double, or char. Why is the Visual Studio C++ Compiler rejecting an enum as a template parameter? Yes, this behaviour is well-defined precisely because std::byte is an "AliasedType". How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? Since you are using reinterpret_cast on an integer literal (20000), the result of the conversion is not a safely-derived pointer value. its operand, It is intended to be unsurprising to those who know the addressing Conversions that can be performed explicitly Going the other way: element access of a vector. 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 compiler allow violation of private inheritance in this example? using, No other conversion can conditionally-supported. If _mm_load_ps() is supported, the implementation must also define the behaviour of the code in the question. structure of the underlying machine. Pointer to membercopy constructor memory leakcrtls valid heap pointerblock By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. AFAIK, the performance of _mm256_load_ps and _mm256_loadu_ps is about the same on relatively-new x86 processors. It is used when we want to work with bits. 1. C ++static_cast no constructors (, This is sometimes referred to as a type pun 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. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. 5. 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. Is it possible to get height of window's title bar? int g = 10; int * k = reinterpret_cast and #include "filename"? 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. An rvalue pointer to member object of some class T1 can be converted to a pointer to another member object of another class T2. Footnote 1: __attribute__((vector_size(16), may_alias)) in GNU C, and MSVC doesn't do type-based alias analysis. std::cout << *px; Would output the value of x. 8. How do I set, clear, and toggle a single bit? On the other hand, it is clear that, for embedded users and specialized compilers/flags/environments, it could be useful to some degree. Does integrating PDOS give total charge of a system? Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. MSVC defines vector types as a union with a .m128_f32[] member for per-element access. Here is one link: I think I have to use working draft and books to understand and write correct program in C++. Why does Visual C++ warn on implicit cast from const void ** to void * in C, but not in C++? conversion is unspecified, except in the following cases: Converting a prvalue of type pointer to member function to a Or at least that their intrinsics were supposed to work that way. 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 We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. 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. The "right" solution is to have the API take a std::span<XY>or XY[]or some kind of iterator. Making statements based on opinion; back them up with references or personal experience. Conversion to the original type yields the original value, otherwise the resulting pointer cannot be used safely. Any pointer can be converted to any integral type large enough to hold the value of the pointer. experimental::boyer_moore_horspool_searcher experimental::make_boyer_moore_horspool_searcher For C++11 it was defined as: . Do not use inside inner loops, and have a look at the resulting asm if performance matters. To learn more, see our tips on writing great answers. As a matter of fact, crafting an invalid pointer is UB by itself. 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. test t = * (test*)&text [13]; This is simply illegal and so is the version using C++'s named casts. Why does std::mutex create a C2248 when used in a struct with WIndows SOCKET? Why does this class declaration not work on Visual Studio. 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. What are the algorithms for real-time search engine? It's quite nice, especially for integer types where having different types for different element widths make v1 + v2 work with the right size. Was the ZX Spectrum used for number crunching? 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. 2022 ITCodar.com. (2.6) the result of a reinterpret_cast of an integer representation of a safely-derived pointer value; 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. (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; (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*. 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. Perhaps some char members of a struct would be a better example, then. And is suggested to use it using proper data type i.e., (pointer data type should be same as original data type). ISO C++ doesn't define __m256, so we need to look at what does define their behaviour on the implementations that support them. For float* and double*, the load/store intrinsics basically exist to wrap this reinterpret cast and communicate alignment info to the compiler. Is the code above considered dangerous in any way? 2. The approach I present at the end can be used, if you only have non-static member function pointers. Since std::size_t is an integral type, and Alias is std::size_t, I'd say . 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. 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. (For example, Intel intrinsics for unaligned loads depend on creating an unaligned __m128i*.) Dont do whatever this is. (Erci Finn). Demonstrates some uses of reinterpret_cast, // pointer to function to another and back, http://en.cppreference.com/w/cpp/language/reinterpret_cast. 4. I am learning C and C++. 6.6.4.4.3 Safely-derived pointers [basic.stc.dynamic.safety] A lot of x86-specific code has been developed with MSVC, which doesn't enforce strict aliasing at all, i.e. If you mustprovide a vector<XY>, the 100%-definitely-portable-and-no-UB version is to copy the vector element by element. A value of integral type or enumeration type can be explicitly converted to a pointer. How can I change 'long' to 'int' without changing 'long long' using perl/awk/etc. But any dereferencing only happens inside intrinsic "functions", so you don't have to worry about the strict-aliasing rule. discusses that for x86, but your case is a bit different; your pointers are aligned). 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. 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 . will have its original value; The effect of calling a function through a pointer to a function But that is a compiler bug, IMO. (Although that may not help a static analyzer). (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.). (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). Although when I printed the C++ member function pointer itself, 1 was printed on the cmd terminal window on x64 build. reinterpret_cast <new_type> (expression) Returns a value of type new_type. 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