alignas is an attribute introduced in C++11, which informs the compiler that the attributed object shall have a certain memory alignment. It replaces non-portable, compiler-specific alignment attributing, such as __declspec(align(16)).
Questions tagged [alignas]
37 questions
36
votes
4 answers
Where can I use alignas() in C++11?
In an effort to standardize my code and make it more portable, I replaced
#ifdef __GNUC__
typedef __attribute__((aligned(16))) float aligned_block[4];
#else
typedef __declspec(align(16)) float aligned_block[4];
#endif
with
typedef float alignas(16)…
Walter
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18
votes
3 answers
How to use alignas to replace pragma pack?
I am trying to understand how alignas should be used, I wonder if it can be a replacement for pragma pack, I have tried hard to verify it but with no luck. Using gcc 4.8.1 (http://ideone.com/04mxpI) I always get 8 bytes for below STestAlignas, while…
marcinj
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13
votes
1 answer
gcc over-aligned new support (alignas )
I'm having some difficulty finding more information about GCC's aligned-new warning and the gcc -faligned-new option. Compiling on gcc 7.2.0 (without --std=c++17) and trying to define an aligned struct such as:
struct alignas(64) Foo { int x;…
ByteMe95
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11
votes
4 answers
treating memory returned by operator new(sizeof(T) * N) as an array
In C one can allocate dynamic arrays using malloc(sizeof(T) * N) and then use pointer arithmetic to get elements at i offset in this dynamic array.
In C++ one can do similar using operator new() in the same way as malloc() and then placement new…
xor256
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8
votes
1 answer
alignas in structs on 32-bit platforms
I am getting unexpected results when running the following code for 32-bit x86 linux (compiler flags: g++ -std=c++14 -m32). I tried gcc and clang.
#include
using namespace std;
struct S1
{
uint64_t a;
uint32_t b;
};
struct S2
{
…
user3809741
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- 2
8
votes
1 answer
Struggling with alignas syntax
I am trying to use alignas for pointers that are class members, and frankly I am not sure where I supposed to put it.
For instance:
class A
{
private:
int n;
alignas(64) double* ptr;
public:
A(const int num) : n(num), ptr(new…
user1683586
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7
votes
2 answers
alignas keyword not respected
I want to overalign my type on a cache boundary, so I used alignas:
struct alignas(64) W { };
This compiles fine. But then, to my surprise, when I try to allocate a bunch of Ws, they're not 64-byte aligned but actually 16-byte aligned:
#include…
Barry
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5
votes
2 answers
How to replace aligned_storage with std::byte[]?
The use of aligned_storage is deprecated in C++23 and suggested to be replaced with an aligned std::byte[] (see here). I have two questions about this:
1. How to align this?
The document suggests to replace
std::aligned_storage_t
TilmannZ
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4
votes
1 answer
alignas() effect on sizeof() - mandatory?
This program:
struct alignas(4) foo {};
int main() { return sizeof(foo); }
returns 4, with GCC 10.1 and clang 10.1, and icc 19.0.1 .
That makes me wonder - is it mandatory for alignas() to affect sizeof() this way? i.e. increase the size beyond…
einpoklum
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4
votes
1 answer
Is gcc missing an optimization opportunity on this comparison of overaligned struct of 2 shorts?
I was wondering if GCC will use the knowledge that the struct I created is overaligned and consider it simply as an integer when it comes to the generation of comparison asm.
It does not.
I wonder if this is due to ABI issues, some other factor I…
NoSenseEtAl
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4
votes
1 answer
How to create std::vector of char/std::byte where first byte is aligned to 16 byes, but there is no padding?
There is an existing question that requires C++03 and has no answer, so I will open a new one.
Problem I have is that I want to have std::vector of std::byte, but so that .data()(first element of data array) is 16 byte aligned.
alignas on wrapped…
NoSenseEtAl
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4
votes
1 answer
Do I need alignment in C++?
I am sure I do. The matrix library Eigen3 is likely much faster than any of my own matrix implementations thanks to alignment.
I recently started investigating on alignment using C++. I stumbled on the alignas and alignof functions, and decided to…
mfnx
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4
votes
0 answers
Why doesn't alignas compile when used in a static declaration with clang?
I have a compilation error on clang and a warning with gcc with this code:
static alignas(16) int one_s = 1; // clang: error: an attribute list cannot appear here; gcc: warning: attribute ignored;
static…
bluespeck
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3
votes
2 answers
Is it correct to use alignas(16) for an array[8] of m128?
This is the code I usually write:
alignas(16) __m128 myBuffer[8];
But maybe (since the object-array is 128*8 bit = 128 byte) I should write:
alignas(128) __m128 myBuffer[8];
Or, "since the first 16 byte are aligned" in the first example, the rest…
markzzz
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3
votes
2 answers
How does std::alignas optimize the performance of a program?
In 32-bit machine, One memory read cycle gets 4 bytes of data.
So for reading below buffer, It should take 32 read-cycle to read a buffer of 128 bytes mentioned below.
char buffer[128];
Now, Suppose if I have aligned this buffer as mentioned below…
Gaurav
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