Thread: alignment (classes, structs, members)

so i'm using sse2 and i'd like to use movaps (and similar instructions) which requires objects to be laid out at 16 byte boundaries (otherwise it segfaults).

Code:

so far i have:
struct Vector4 {
  float c[4];
...
};

how can i force the compiler to put all instances at 16 byte bounds?
i'm using g++. and simply putting a pragma pack around that struct didnt help (wrong usage i guess ^^).

so if anyone knows how to force alignments of all instances then please tell me.
(i couldn't find any satisfying anwer with google)

How did you attempt the pragma pack statement? IIRC it should have gone something like:

Code:

#pragma pack(16)

I could be wrong though... it's been awhile since I've needed to mess with struct packing/alingment issues.

You could try padding your structs with a dummy char array to the necessary size:

Code:

struct Vector4 {
    float c[4];
    ...
    char dummy[???];  // Pad as much as needed to get correct struct size
};

> float c[4]
I'm assuming this is your way of getting a 16 byte object?

If so, maybe something like

Code:

struct Vector4 __attribute__((aligned(16))) {
  unsigned char d[16];
};

ya, i figured that out with that "__attribute__ aligned" before i went to bed.
but i also figured out that "new" ignores that attribute and does not return aligned memory... sigh...

Create a static array of correctly aligned objects, and overload new to allocate from this pool of objects ?