This does actually optimize away to vector extract instructions. Here is one link: I think I have to use working draft and books to understand and write correct program in C++. (2.3) the result of well-defined pointer arithmetic using a safely-derived pointer value; And in practice, compilers targeting byte-addressable machines do more or less define the behaviour even for creating misaligned pointers. A pointer must point to valid memory to be used. It can typecast any pointer to any other data type. Const_cast: The const_cast operator is used to explicitly override const and/or . 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 . So a load intrinsic is literally already doing this. Does illicit payments qualify as transaction costs? Does aliquot matter for final concentration? Making statements based on opinion; back them up with references or personal experience. Seems to me that C++ member function pointer is just another C function pointer, so why the conversion is impossible? 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. For float* and double*, the load/store intrinsics basically exist to wrap this reinterpret cast and communicate alignment info to the compiler. Perhaps some char members of a struct would be a better example, then. 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. Why should I use a pointer rather than the object itself? Failing to parse different math operators, C++: Extracting symbols/variables of an analytical mathematical expression. [C++14: 5.2.10/2]: The reinterpret_cast operator shall not cast away constness (5.2.11). cpprefjpMarkdown. (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.). no constructors (, This is sometimes referred to as a type pun 6.6.4.4.3 Safely-derived pointers [basic.stc.dynamic.safety] 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. (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.). (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); That just makes your C look more like asm if you want it to. The code must be portable and strictly follow the C90 standard. What are the basic rules and idioms for operator overloading? Find centralized, trusted content and collaborate around the technologies you use most. [ Note: The resulting reference can only be accessed safely if allowed by the type aliasing rules. be performed explicitly using, An expression of integral, enumeration, pointer, or pointer-to-member type For conversions between object . 4. its operand, It is intended to be unsurprising to those who know the addressing __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. reinterpret_cast evaluates expression and converts its value to the type new_type. Thanks for contributing an answer to Stack Overflow! How to use this drafts? An expression if integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. C++ offers four different kinds of casts as replacements: static_cast, const_cast, dynamic_cast and reinterpret_cast. Going the other way: element access of a vector. Why is this code allowed to compile under Visual Studio 2013? The exact aliasing semantics of Intel Intrinsics are not AFAIK documented anywhere. can be explicitly converted to its own type; such a cast yields the value of That's strange. 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. Why is visual studio code telling me that cout is not a member of std namespace? 4) reinterpret_cast<Y*>(r.get()) The behavior of these functions is undefined unless the corresponding cast from U* to T* is well formed: Why does this class declaration not work on Visual Studio. 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. 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. (2.4) the result of a well-defined pointer conversion of a safely-derived pointer value; But any dereferencing only happens inside intrinsic "functions", so you don't have to worry about the strict-aliasing rule. If _mm_load_ps() is supported, the implementation must also define the behaviour of the code in the question. If not, it wouldn't be safe. I am learning C and C++. plus everything implied by the presence of ARM NEON intrinsics! How is Jesus God when he sits at the right hand of the true God? Connect and share knowledge within a single location that is structured and easy to search. Although when I printed the C++ member function pointer itself, 1 was printed on the cmd terminal window on x64 build. But it seems that the error is "means that you tried to access memory that you do not have access to." What are the differences between a pointer variable and a reference variable? An rvalue pointer to member object of some class T1 can be converted to a pointer to another member object of another class T2. Yes, this behaviour is well-defined precisely because std::byte is an "AliasedType". A tag already exists with the provided branch name. Including misaligned float* for _mm_loadu_ps, which supports any alignment, not just multiples of 4. I assume that ARM load/store intrinsics are defined similar to memcpy, being able to read/write the bytes of any object. 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*. Why does Visual Studio 2013 error on C4996? 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 is the Visual Studio Community 2017 C++ standard C++98? Since you are using reinterpret_cast on an integer literal (20000), the result of the conversion is not a safely-derived pointer value. I am new in computer science and new in C++. 3-4) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed. 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. __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. __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? a reinterpret_cast is a conversion operator. conditionally-supported. The result is an lvalue or xvalue referring to the same object as the original lvalue, but with a different type. So that code that does fail in Clang, try it with --std=c++14 or --std=c++17. There's no guarantee you'll get efficient code from any of these element-access methods, though. Accepted answer. 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. k : 0x4e20 Do not use inside inner loops, and have a look at the resulting asm if performance matters. Designed by Colorlib. Attempting to dereference such a pointer value results in undefined behavior. Why does Visual Studio compile this function correctly without optimisation, but incorrectly with optimisation? Is 'Reinterpret_Cast'Ing Between Hardware Simd Vector Pointer and the Corresponding Type an Undefined Behavior. What is the difference between #include and #include "filename"? So yes, it's safe to dereference a __m256* instead of using a _mm256_load_ps() aligned-load intrinsic. Using a bitwise AND on more than two bits. All rights reserved. (print a __m128i variable). 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. type (, An object pointer Why does std::mutex create a C2248 when used in a struct with WIndows SOCKET? That violates the strict-aliasing rule. If new_type is an rvalue reference to object, the result is an xvalue. (Related: Is `reinterpret_cast`ing between hardware SIMD vector pointer and the corresponding type an undefined behavior? How to read an intermittent hard drive consistently? 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"? 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. Visual Studio fails to display some watched expressions. In your case you set it to an arbitrary memory location, 20000. Ready to optimize your JavaScript with Rust? No temporary is created, no copy is made, no constructors or conversion functions are called. A lot of x86-specific code has been developed with MSVC, which doesn't enforce strict aliasing at all, i.e. Why does Visual Studio 2010 throw this error with Boost 1.42.0? 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?). (Erci Finn). A value of any integral or enumeration type can be converted to a pointer type. When would I give a checkpoint to my D&D party that they can return to if they die? 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. Except as described in 6.6.4.4.3, the result of such a conversion will not be a safely-derived pointer value. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. 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 . 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. 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. 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. A pointer converted to an integer of sufficient size (if 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. An example of valid pointer usage would be. 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 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. Also for AVX-512, to allow masked loads and masked stores. Thus, vector data are likely not ensured to be 32-bytes aligned. 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. You've correctly shown that it's 100% safe to create that float* in ISO C, and pass it to an opaque function. (C++11 feature). 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.). AFAIK, the performance of _mm256_load_ps and _mm256_loadu_ps is about the same on relatively-new x86 processors. The working draft of the standard (N4713) states regarding the usage of, 8.5.1.10 Reinterpret cast 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. The Armadillo do not seems to talk about this point in the documentation so it is left unspecified. 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. rev2022.12.11.43106. The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type. In this program: ./converForTyEn 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. The standard does state that reinterpret_cast is not what the standard calls a core constant expression. Pick one. (2.6) the result of a reinterpret_cast of an integer representation of a safely-derived pointer value; How can I change 'long' to 'int' without changing 'long long' using perl/awk/etc. And is suggested to use it using proper data type i.e., (pointer data type should be same as original data type). But especially for float/double, it's often easier to use the intrinsics because they take care of casting from float*, too. It is used when we want to work with bits. An rvalue pointer to member function can be converted to pointer to a different member function of a different type. _mm_load_ps is aliasing-safe, so it makes no sense that _mm_load_ss wouldn't be, when they both take float*. 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). How to allocate memory for std::vector and then call constructor for some elements later? (2.5) the result of a reinterpret_cast of a safely-derived pointer value; 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). Why doesn't Stockfish announce when it solved a position as a book draw similar to how it announces a forced mate? Dont do whatever this is. Is this an at-all realistic configuration for a DHC-2 Beaver? 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).. (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.). The Working Draft, Standard for Programming Language C ++ is 1448 pages. 3. 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. It's only safe to go the other way, using __m128i* dereference or more normally _mm_load_si128( (const __m128i*)ptr ). 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. But the drafts are close enough for practical usage. 10. I think it's officially safe in MSVC, too, because I think the way they define __m128 as a normal union. 6. - n. m. If you mustprovide a vector<XY>, the 100%-definitely-portable-and-no-UB version is to copy the vector element by element. Why this reinterpret_cast fails in Visual Studio? Do non-Segwit nodes reject Segwit transactions with invalid signature? It might not even be safe to use float* to read a __m256. 9. 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. . Nope, not at all. How to share image data between applications? Visual Studio fails to instantiate cast (conversion) operator template (T=bool) in the context of boolean operations. The resulting value is the same as the value of expression. Not the answer you're looking for? Why does this explicit conversion operator work with g++ but not Visual Studio 2013? Interconvertibility between Wrapper* and float* isn't really relevant; you're not derefing a float*. conversion is unspecified, except in the following cases: Converting a prvalue of type pointer to member function to a reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . (Unfortunately even current GCC still has that bug; _mm_loadu_si32 was fixed in GCC11.3 but not the older _ss and _sd loads.) 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). any such exists on the implementation) and back to the same pointer type Any pointer can be converted to any integral type large enough to hold the value of the pointer. The approach I present at the end can be used, if you only have non-static member function pointers. (I have no idea if any specific hardware exists where more efficient code is possible because of this UB. MSVC defines vector types as a union with a .m128_f32[] member for per-element access. 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. Does integrating PDOS give total charge of a system? To learn more, see our tips on writing great answers. 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. It's strict-aliasing UB to dereference a uint16_t* that doesn't point to uint16_t objects. Any pointer to object of type T1 can be converted to pointer to object of another type T2. So this . A value of integral type or enumeration type can be explicitly converted to a pointer. 2022 ITCodar.com. Why is the Visual Studio C++ Compiler rejecting an enum as a template parameter? building jsoncpp (Linux) - an instruction for us mere mortals? 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. will have its original value; The effect of calling a function through a pointer to a function 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. 1. It also allows. All Rights Reserved. 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. 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? Received a 'behavior reminder' from manager. 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. As a matter of fact, crafting an invalid pointer is UB by itself. Footnote 1: __attribute__((vector_size(16), may_alias)) in GNU C, and MSVC doesn't do type-based alias analysis. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. 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 "right" solution is to have the API take a std::span<XY>or XY[]or some kind of iterator. discusses that for x86, but your case is a bit different; your pointers are aligned). casting from pointer to an integer type and vice versa. (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*. Thank for help. Sorry for being a bit harsh, but for the love of silicon, write C or write good, modern C++. 7.6.1.10 Reinterpret cast [expr.reinterpret.cast] 1. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. I expect 20000 and then 10. PSE Advent Calendar 2022 (Day 11): The other side of Christmas. 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. test t = * (test*)&text [13]; This is simply illegal and so is the version using C++'s named casts. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. 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. The actual standard is expensive to purchase. 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. Why does this C-style cast not consider static_cast followed by const_cast? 8 was printed on the cmd terminal window. Since std::size_t is an integral type, and Alias is std::size_t, I'd say . Why won't this code compile and run in Visual Studio 2010? Visual Studio C++ compiler options: Why does /O2 define /Gs? different pointer-to-member-function type and back to its original type Contribute to cpprefjp/site development by creating an account on GitHub. Is it possible to get height of window's title bar? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. 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. Why does the Visual Studio conversion wizard 2010 create a massive SDF database file? 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? The result of this GNU C native vectors without may_alias are allowed to alias their scalar type, e.g. 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. 8. 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. What is the difference between const int*, const int * const, and int const *? 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. Share Improve this answer Follow An rvalue pointer to member function can be converted to pointer to a different member function of a different type. GNU C vector syntax provides the [] operator for vectors, like __m256 v = ; v[3] = 1.25;. experimental::boyer_moore_horspool_searcher experimental::make_boyer_moore_horspool_searcher 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. you could vld2q_u16 on an array of int, double, or char. What is a smart pointer and when should I use one? Segmentation fault (core dumped). (And with AVX-512, may be able to fold _mm512_set1_pd(x) broadcast-loads for instructions with a matching element width.). 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. reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. 5. 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. Conversion to the original type yields the original value, otherwise the resulting pointer cannot be used safely. ), Intel's intrinsics API effectively requires compilers to support creating misaligned pointers as long as you don't deref them yourself. 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 (Although that may not help a static analyzer). 11. Any pointer to function can be converted to a pointer to a different function type. 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. No. Why does this compile with Visual Studio 2013 but not g++-4.8.1? (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. It's quite nice, especially for integer types where having different types for different element widths make v1 + v2 work with the right size. [C++14: 5.2.10/2]: The reinterpret_cast operator shall not cast away constness (5.2.11). Plain dereference is still strict-aliasing safe because __mm* types are special. (C++11 feature). 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. I'm talking about how GCC implements Intel's intrinsics only because that's what I'm familiar with. Manage SettingsContinue with Recommended Cookies. 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. But may_alias makes it safe to load from an array of int[], char[], or whatever. But however the compiler makes it happen, it's equivalent to a memcpy, including the lack of alignment requirement.). 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. Otherwise, the result is a prvalue and lvalue-to-rvalue, array-to-pointer, or function-to-pointer implicit conversion is performed if necessary. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? the conversions that can be performed using a reinterpret_cast are limited; the permissible ones are specified by the standard. 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 To insert/extract vector elements, use shuffle intrinsics, SSE2 _mm_insert_epi16 / _mm_extract_epi16 or SSE4.1 insert / _mm_extract_epi8/32/64. Why does this code emit buffer overrun warnings(C6385/C6386) in code analysis on Visual Studio 2012? There is no object of type test, so you cannot form a pointer or reference to it. Why is Visual Studio 2013 having trouble with this class member decltype? 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. If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. 2. 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. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. 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. And yes, deref of an __m256* is exactly equivalent to _mm256_load_ps, and is in fact how most compilers implement _mm256_load_ps. Reinterpreting memory is only legal . GCC/clang use __attribute__((may_alias)).) Or at least that their intrinsics were supposed to work that way. 5. 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. But that is a compiler bug, IMO. 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. Just like it's not safe to do char buf[1024]; int *p = (int*)buf;. 1) static_cast<Y*>(r.get()). 3) const_cast<Y*>(r.get()). Acorn 23513 How to get the address of the std::vector buffer start most elegantly? Was the ZX Spectrum used for number crunching? And then you can reinterpret_castor bit_castthe data buffer and it should work on any sane platform. In Visual Studio 2010 why is the .NETFramework,Version=v4.0.AssemblyAttributes.cpp file created, and can I disable this? To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. std::cout << px; Would output the address. std::cout << *px; Would output the value of x. Just creating unaligned pointers, or pointers outside an object, is UB in ISO C++, even if you don't dereference them. I dont know C that much but afaik with very few exceptions your code is not valid C (eg. 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. Why does Visual Studio not perform return value optimization (RVO) in this case. Why is valarray so slow on Visual Studio 2015? The reinterpret_cast allows the pointer to be treated as an integral . Dec 5 at 10:10. 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). For C++11 it was defined as: . 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 (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; An lvalue expression of type T1 can be converted to reference to another type T2. 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. Visual Studio No Symbols have been loaded for this document. 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. dont confuse C with C++, they are two different languages. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. 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. 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. An example of valid pointer usage would be int x = 42; int *px = &x; This puts the address of x into px. What are the algorithms for real-time search engine? So e.g. 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. This is the sizeof of any C pointer in x64 build. it's like gcc -fno-strict-aliasing, defining the behaviour of stuff like *(int*)my_float and even encouraging it for type-punning. CGAC2022 Day 10: Help Santa sort presents! By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Or store to an array and read the array. So you're 100% fine: your code never creates a misaligned uint16_t*, and doesn't directly dereference it. C++ latest working draft will be available online. reinterpret_cast is a very special and dangerous type of casting operator. 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 . Copyright 2022 www.appsloveworld.com. Not sure if it was just me or something she sent to the whole team. (Which may pointer-cast to some special type and deref, or may pass the pointer on to a __builtin_arm_whatever() compiler built-in.). Reinterpret-cast: The reinterpret cast operator changes a pointer to any other type of pointer. 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. I've heard from gcc developers that they chose that implementation because it was required for compatibility with Intel. A casting operator for abnormal casting cases. And nobody would want to use a compiler that broke it for a cases where memcpy with the same source pointer would be safe. using, No other conversion can 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. "There's no Qt version assigned to this project for platform Win32" - visual studio plugin for Qt. Only the following conversions can be done with reinrepret_cast, except when such conversion would cast away constness or volatility. like int to pointer and pointer to int etc. 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. 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. Why does Visual C++ warn on implicit cast from const void ** to void * in C, but not in C++? Not a dereferenced __m256i* ; that would be safe if the only __m256i accesses were via __m256i*. C-style pointer casting detected. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 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++. A pointer must point to valid memory to be used. 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. Is the code above considered dangerous in any way? C ++static_cast 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. 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. type or vice versa is Intel intrinsics are defined that way (e.g. How to make voltage plus/minus signs bolder? @JMuzhen in the firtst reinterpret_cast std::byte is DynamicType and not AliasedType. Pointer to membercopy constructor memory leakcrtls valid heap pointerblock (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. 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. Asking for help, clarification, or responding to other answers. And _mm_storeu_ps to a location that isn't 4-byte aligned requires creating an under-aligned float*. 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. fatal error C1010 - "stdafx.h" in Visual Studio how can this be corrected? 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. As an analogy, a page number in a book's . 7. 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. 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. 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. (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). ), But implementations which support Intel's intrinsics must define the behaviour, at least for the __m* types and float*/double*. The compiler can fold a load into a later ALU instruction (or optimize it away) regardless of how you write it. 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