![]() This example demonstrates the use of _declspec(align(#)): _declspec(align(32)) struct Str1 a Data in classes or structures is aligned in the class or structure at the minimum of its natural alignment and the current packing setting (from #pragma pack or the /Zp compiler option). Without _declspec(align(#)), the compiler generally aligns data on natural boundaries based on the target processor and the size of the data, up to 4-byte boundaries on 32-bit processors, and 8-byte boundaries on 64-bit processors. If data alignment is important in the called function, copy the parameter into correctly aligned memory before use. When you pass data that has an alignment attribute by value on the stack, its alignment is controlled by the calling convention. You can't specify alignment for function parameters. To guarantee that the destination of a copy or data transformation operation is correctly aligned, use _aligned_malloc. Ordinary allocators (for example, malloc, C++ operator new, and the Win32 allocators) typically return memory that isn't sufficiently aligned for _declspec(align(#)) structures or arrays of structures. For example, memcpy can copy a struct declared with _declspec(align(#)) to any location. The compiler doesn't guarantee or attempt to preserve the alignment attribute of data during a copy or data transform operation. You can use _declspec(align(#)) when you define a struct, union, or class, or when you declare a variable. #Microsoft word align middle how toFor information about how to declare unaligned pointers when targeting 64-bit processors, see _unaligned. declarator is the data that you're declaring as aligned.įor information about how to return a value of type size_t that is the alignment requirement of the type, see alignof. Valid entries are integer powers of two from 1 to 8192 (bytes), such as 2, 4, 8, 16, 32, or 64. For example, if you define a structure whose size is less than 32 bytes, you may want 32 byte alignment to make sure that objects of that structure type are efficiently cached. Additionally, by aligning frequently used data to the processor's cache line size, you improve cache performance. Many new instructions require data that's aligned to 16-byte boundaries. Writing applications that use the latest processor instructions introduces some new constraints and issues. _declspec( align( # ) ) declarator Remarks Use _declspec(align(#)) to precisely control the alignment of user-defined data (for example, static allocations or automatic data in a function). In Visual Studio 2015 and later, use the C++11 standard alignas specifier to control alignment.
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