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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c++
Subject: Re: Performance of unaligned memory-accesses
Date: Wed, 07 Aug 2019 17:52:04 -0700
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Xref: csiph.com comp.lang.c++:67415

Bart <bc@freeuk.com> writes:
[...]
> You have to keep an eye on what's going on, otherwise you may end up 
> with a struct that needs 65 bytes. Will the compiler say anything about 
> that, or just accept it, and try and create arrays with a stride of 65 
> bytes, or pad them to 128 bytes?
>
> I doubt it will re-design the struct to keep it to a power-of-two.
[...]

If the target imposes alignment constraints and you don't use any
compiler extensions to force packing, the compiler will do what it needs
to do to prevent unaligned access.  There's unlikely to be any need to
pad a 65-byte structure to 128 bits.  It would probably just be padded
to 68 or perhaps 72 bytes, depending on the alignment requirements of
its members.

For example:

#include <stdio.h>
#include <stdint.h>
int main(void) {
    struct foo {
        uint32_t arr[16];
        char c;
    };
    struct bar {
        uint64_t arr[8];
        char c;
    };
    printf("struct foo is %zu bytes\n", sizeof (struct foo));
    printf("struct bar is %zu bytes\n", sizeof (struct bar));
}

On my system:

struct foo is 68 bytes
struct bar is 72 bytes

The stride of an array of FOO is always equal to sizeof (FOO); there is
no padding between array elements.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */