any such exists on the implementation) and back to the same pointer type I'm talking about how GCC implements Intel's intrinsics only because that's what I'm familiar with. it's like gcc -fno-strict-aliasing, defining the behaviour of stuff like *(int*)my_float and even encouraging it for type-punning. 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. - J Muzhen. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? As a matter of fact, crafting an invalid pointer is UB by itself. (Erci Finn). 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. On some implementations, a function pointer can be converted to an object pointer or vice versa. 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. will have its original value; The effect of calling a function through a pointer to a function This is important because the capabilities of constexpr was vastly increased with the C++14 standard compared to C++11. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. [ Note: 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. 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. For C++11 it was defined as: . 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. Or at least that their intrinsics were supposed to work that way. 5. Just like it's not safe to do char buf[1024]; int *p = (int*)buf;. 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. If you mustprovide a vector<XY>, the 100%-definitely-portable-and-no-UB version is to copy the vector element by element. I am new in computer science and new in C++. 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 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. Why does Visual C++ warn on implicit cast from const void ** to void * in C, but not in C++? They're exactly identical, no diff in generated asm. Asking for help, clarification, or responding to other answers. end note ]. (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.). You've correctly shown that it's 100% safe to create that float* in ISO C, and pass it to an opaque function. Why does Visual Studio compiler allow violation of private inheritance in this example? 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++. Why does this compile with Visual Studio 2013 but not g++-4.8.1? An rvalue pointer to member function can be converted to pointer to a different member function of a different type. Yes, this behaviour is well-defined precisely because std::byte is an "AliasedType". So this . 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 working draft of the standard (N4713) states regarding the usage of, 8.5.1.10 Reinterpret cast How to share image data between applications? If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. __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? But however the compiler makes it happen, it's equivalent to a memcpy, including the lack of alignment requirement.). Why does the Visual Studio conversion wizard 2010 create a massive SDF database file? An lvalue expression of type T1 can be converted to reference to another type T2. An example of valid pointer usage would be int x = 42; int *px = &x; This puts the address of x into px. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. Why this reinterpret_cast fails in Visual Studio? (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. If new_type is an rvalue reference to object, the result is an xvalue. Connect and share knowledge within a single location that is structured and easy to search. 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. Just creating unaligned pointers, or pointers outside an object, is UB in ISO C++, even if you don't dereference them. Why does Visual Studio 2010 throw this error with Boost 1.42.0? 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. So yes, it's safe to dereference a __m256* instead of using a _mm256_load_ps() aligned-load intrinsic. "There's no Qt version assigned to this project for platform Win32" - visual studio plugin for Qt. Since you are using reinterpret_cast on an integer literal (20000), the result of the conversion is not a safely-derived pointer value. 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. What is the difference between #include and #include "filename"? The result of this Instead, the functions std::static_pointer_cast, std::const_pointer_cast, std::dynamic_pointer_cast and std::reinterpret_pointer_cast should be used: Any pointer to function can be converted to a pointer to a different function type. The compiler can fold a load into a later ALU instruction (or optimize it away) regardless of how you write it. MSVC defines vector types as a union with a .m128_f32[] member for per-element access. conditionally-supported. 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. 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?). The approach I present at the end can be used, if you only have non-static member function pointers. GNU C vector syntax provides the [] operator for vectors, like __m256 v = ; v[3] = 1.25;. What is the difference between const int*, const int * const, and int const *? Not sure if it was just me or something she sent to the whole team. But any dereferencing only happens inside intrinsic "functions", so you don't have to worry about the strict-aliasing rule. Copyright 2022 www.appsloveworld.com. But the drafts are close enough for practical usage. Dont do whatever this is. It's quite nice, especially for integer types where having different types for different element widths make v1 + v2 work with the right size. 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). 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. be performed explicitly using, An expression of integral, enumeration, pointer, or pointer-to-member type And BTW, the char*-can-alias-anything rule only works one way. (Although that may not help a static analyzer). It is used when we want to work with bits. (For example, Intel intrinsics for unaligned loads depend on creating an unaligned __m128i*.) For float, there are no insert/extract intrinsics that you should use with scalar float. Why should I use a pointer rather than the object itself? Visual Studio C++ compiler options: Why does /O2 define /Gs? How to make voltage plus/minus signs bolder? (And with AVX-512, may be able to fold _mm512_set1_pd(x) broadcast-loads for instructions with a matching element width.). Also for AVX-512, to allow masked loads and masked stores. So that code that does fail in Clang, try it with --std=c++14 or --std=c++17. 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? The standard does state that reinterpret_cast is not what the standard calls a core constant expression. 10. Since std::size_t is an integral type, and Alias is std::size_t, I'd say . Is this an at-all realistic configuration for a DHC-2 Beaver? 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. An rvalue pointer to member function can be converted to pointer to a different member function of a different type. How can I change 'long' to 'int' without changing 'long long' using perl/awk/etc. using, No other conversion can 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. An example of valid pointer usage would be. The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type. Do not use inside inner loops, and have a look at the resulting asm if performance matters. (C++11 feature). type or vice versa is structure of the underlying machine. 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. a reinterpret_cast is a conversion operator. For float* and double*, the load/store intrinsics basically exist to wrap this reinterpret cast and communicate alignment info to the compiler. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. reinterpret_cast is a very special and dangerous type of casting operator. 6.6.4.4.3 Safely-derived pointers [basic.stc.dynamic.safety] What are the basic rules and idioms for operator overloading? Why does this C-style cast not consider static_cast followed by const_cast? C++ latest working draft will be available online. (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); its operand, It is intended to be unsurprising to those who know the addressing GNU C native vectors without may_alias are allowed to alias their scalar type, e.g. Why does this explicit conversion operator work with g++ but not Visual Studio 2013? Perhaps some char members of a struct would be a better example, then. 11. An expression if integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. How do I set, clear, and toggle a single bit? 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. A pointer converted to an integer of sufficient size (if Visual Studio fails to instantiate cast (conversion) operator template (T=bool) in the context of boolean operations. can be explicitly converted to its own type; such a cast yields the value of 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. For conversions between object . 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. When would I give a checkpoint to my D&D party that they can return to if they die? How to read an intermittent hard drive consistently? So e.g. 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 reinterpret_cast allows the pointer to be treated as an integral . 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. 2. 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. when the result refers to the same object as the source glvalue. Why does this class declaration not work on Visual Studio. 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. Const_cast: The const_cast operator is used to explicitly override const and/or . conversion is unspecified, except in the following cases: Converting a prvalue of type pointer to member function to a 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). like int to pointer and pointer to int etc. AFAIK, the performance of _mm256_load_ps and _mm256_loadu_ps is about the same on relatively-new x86 processors. Pointer to membercopy constructor memory leakcrtls valid heap pointerblock (2.3) the result of well-defined pointer arithmetic using a safely-derived pointer value; 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. And nobody would want to use a compiler that broke it for a cases where memcpy with the same source pointer would be safe. 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: Designed by Colorlib. That just makes your C look more like asm if you want it to. 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. Does integrating PDOS give total charge of a system? 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. Is it possible to get height of window's title bar? Not the answer you're looking for? Received a 'behavior reminder' from manager. (I have no idea if any specific hardware exists where more efficient code is possible because of this UB. Except as described in 6.6.4.4.3, the result of such a conversion will not be a safely-derived pointer value. 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. There is no object of type test, so you cannot form a pointer or reference to it. even without the may_alias, you could safely cast between float* and a hypothetical v8sf type. 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. If not, it wouldn't be safe. dont confuse C with C++, they are two different languages. That violates the strict-aliasing rule. 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. Find centralized, trusted content and collaborate around the technologies you use most. All rights reserved. 3-4) The behavior is undefined unless dynamic_cast<T*>((U*)nullptr) is well formed. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. ), Intel's intrinsics API effectively requires compilers to support creating misaligned pointers as long as you don't deref them yourself. Plain dereference is still strict-aliasing safe because __mm* types are special. A tag already exists with the provided branch name. 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"? cpprefjpMarkdown. You're not directly dereffing the float*, only passing it to _mm256_load_ps which does an aliasing-safe load. An rvalue pointer to member object of some class T1 can be converted to a pointer to another member object of another class T2. As an analogy, a page number in a book's . Conversions that can be performed explicitly And then you can reinterpret_castor bit_castthe data buffer and it should work on any sane platform. The Armadillo do not seems to talk about this point in the documentation so it is left unspecified. 3. 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). This does actually optimize away to vector extract instructions. Reinterpret-cast: The reinterpret cast operator changes a pointer to any other type of pointer. Conversion to the original type yields the original value, otherwise the resulting pointer cannot be used safely. But especially for float/double, it's often easier to use the intrinsics because they take care of casting from float*, too. A pointer must point to valid memory to be used. 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. std::cout << *px; Would output the value of x. experimental::boyer_moore_horspool_searcher experimental::make_boyer_moore_horspool_searcher (Which may pointer-cast to some special type and deref, or may pass the pointer on to a __builtin_arm_whatever() compiler built-in.). 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. Is 'Reinterpret_Cast'Ing Between Hardware Simd Vector Pointer and the Corresponding Type an Undefined Behavior. A value of integral type or enumeration type can be explicitly converted to a pointer. GCC/clang use __attribute__((may_alias)).) 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. No temporary is created, no copy is made, no constructors or conversion functions are called. Attempting to dereference such a pointer value results in undefined behavior. So a load intrinsic is literally already doing this. It's only safe to go the other way, using __m128i* dereference or more normally _mm_load_si128( (const __m128i*)ptr ). k : 0x4e20 PSE Advent Calendar 2022 (Day 11): The other side of Christmas. 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.). 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