memory-alignment

问题 I was wondering whether the compiler would use different padding on 32-bit and 64-bit systems, so I wrote the code below in a simple VS2019 C++ console project: struct Z { char s; __int64 i; }; int main() { std::cout << sizeof(Z) <<"\n"; } What I expected on each "Platform" setting: x86: 12 X64: 16 Actual result: x86: 16 X64: 16 Since the memory word size on x86 is 4 bytes, this means it has to store the bytes of i in two different words. So I thought the compiler would do padding this way:

问题 I'm writing a program (in C++) in which I need to allocate arrays whose starting addresses should be aligned with the cache line size. When I allocate these arrays I also want the memory initialized to zero. Right now I have it working using the posix_memalign function. This works well for getting memory aligned arrays but the arrays are uninitilized. Is there a better function I can use to zero out the arrays when I initialize them or do I just have to settle for writing a separate loop to

问题 Consider the following C code: #include <stdint.h> void func(void) { uint32_t var = 0; return; } The unoptimized (i.e.: -O0 option) assembly code generated by GCC 4.7.2 for the code above is: func: pushl %ebp movl %esp, %ebp subl $16, %esp movl $0, -4(%ebp) nop leave ret According to the stack alignment requirements of the System V ABI , the stack must be aligned by 16 bytes before every call instruction (the stack boundary is 16 bytes by default when not changed with the option -mpreferred

问题 I have the following data segment .data a: .byte 0x11 .align 1 b: .word 0x22334455 Assuming that address "a" is 0x10010000, then the expected address for the word at b is 0x10010002, but MARS stores the word at 0x10010004, ignoring the explicit ".align" directive. By the way, I used MARS MIPS simulator (Version 4.5 on a MacBook Pro) to assemble the above code. Therefore, my question is: Is this a bug, or is it expected that the behavior of MARS differs from SGI's 1992 documentation for MIPS

问题 I have the following data segment .data a: .byte 0x11 .align 1 b: .word 0x22334455 Assuming that address "a" is 0x10010000, then the expected address for the word at b is 0x10010002, but MARS stores the word at 0x10010004, ignoring the explicit ".align" directive. By the way, I used MARS MIPS simulator (Version 4.5 on a MacBook Pro) to assemble the above code. Therefore, my question is: Is this a bug, or is it expected that the behavior of MARS differs from SGI's 1992 documentation for MIPS

问题 The way I'm currently doing it (I'd prefer to get rid of the memcpy call): uint64_t integer; uint8_t string[8]; ... memcpy(&integer, &string, 8); //or swap the parameters Assuming integer array length to always be a multiple of 8 (64 bits total allocation) is a straight cast possible given compiler padding / alignment concerns? 回答1: There is absolutely no need to avoid or replace a memcpy() call if you're striving for optimization. Every modern optimizing compiler won't emit a call and

问题 I was creating this simple test program to demonstrate the way alignment works when allocating memory using standard new... #include <iostream> #include <iomanip> #include <cstdint> // // Print a reserved block: its asked size, its start address // and the size of the previous reserved block // void print(uint16_t num, uint16_t size_asked, uint8_t* p) { static uint8_t* last = nullptr; std::cout << "BLOCK " << num << ": "; std::cout << std::setfill('0') << std::setw(2) << size_asked << "b, ";

问题 Recently I observed that on Clang 9.0 alignof and __alignof are returning different values for unsigned long long and the same has been discussed at https://reviews.llvm.org/D54814: Starting in Clang 8.0 and GCC 8.0, alignof and __alignof return different values in same cases. Specifically alignof and _Alignof return the minimum alignment for a type, where as __alignof returns the preferred alignment. I know about type alignment but never came across "minimum alignment" and "preferred

问题 Is sizeof(Type) always divisible by alignof(Type) such that this statement will always be true? sizeof(Type) % alignof(Type) == 0 回答1: Yes, sizeof(Type) % alignof(Type) == 0 is true for all class types. The standard draft says: [dcl.array] ... An object of array type contains a contiguously allocated non-empty set of N subobjects of type T. [expr.sizeof] ... When applied to a class, the result is the number of bytes in an object of that class including any padding required for placing objects

问题 Is sizeof(Type) always divisible by alignof(Type) such that this statement will always be true? sizeof(Type) % alignof(Type) == 0 回答1: Yes, sizeof(Type) % alignof(Type) == 0 is true for all class types. The standard draft says: [dcl.array] ... An object of array type contains a contiguously allocated non-empty set of N subobjects of type T. [expr.sizeof] ... When applied to a class, the result is the number of bytes in an object of that class including any padding required for placing objects