C array padding

Question

I have many data array defined in my code with the length only specified in the array initializer.

int a[] = {1, 2, 3, 4};
int b[] = {1, 2, 3, 4, 5};

For cache line coherence consideration, I want to padding them to cache line size (32). The following code can be used without scarifying the exact size information

struct {
   int payload[4];
} __attribute__((aligned(32))) a = {
   .payload = {1, 2, 3, 4}
};

struct {
   int payload[5];
} __attribute__((aligned(32))) b = {
   .payload = {1, 2, 3, 4, 5}
};

// sizeof(a.payload) == 4 * sizeof(int), sizeof(b.payload) == 5 * sizeof(int)

However, there are quite a bundle of such data, it is hard to manually rewrite them one by one. Is there a way to define a macro to handle this, like

#define ARRAY_INITIALIZER (array, initializer) ...
#define ARRAY_PAYLOAD(array) array.payload
ARRAY_INITIALIZER(a, {1, 2, 3, 4});
ARRAY_INITIALIZER(b, {1, 2, 3, 4, 5});

The most difficulty thing here is how to calculate the length of initializer here, don't get any direction for it, can anyone help here?

This might be useful: stackoverflow.com/questions/2124339/… Yes, it is C++, but the macro definition might work for you too. — zerocukor287
– zerocukor287, Commented Aug 6, 2024 at 8:05
Could __attribute__((aligned(32))) be applied to the array as a single variable, but not a structure field? — Stas Simonov
– Stas Simonov, Commented Aug 6, 2024 at 8:15
Why place the arrays in a struct? Is your real question how to get the initial arrays 32-byte aligned? — nielsen
– nielsen, Commented Aug 6, 2024 at 10:56
@nielsen, putting it inside a struct has 2 effect 1) end data padding, not only align on start, but also align on end 2) sizeof can still get the actual size without the end padding — Eric Sun
– Eric Sun, Commented Aug 6, 2024 at 11:01
@StasSimonov, aligned to variable will only make it start aligned, say a array of 5 is start align on 32, the left 27 bytes are still available for other variables to allocate, we want it occupies the full 32 byte to avoid cache coherence issue — Eric Sun
– Eric Sun, Commented Aug 6, 2024 at 11:02

Stas Simonov · Accepted Answer · 2024-08-06 08:55:23Z

5

This initialization produces the aligned addresses either in C or C++:

#include <stdalign.h>

alignas(32) int a[] = {1, 2, 3, 4} ;
alignas(32) int b[] = {1, 2, 3, 4, 5};

edited Aug 6, 2024 at 8:55

answered Aug 6, 2024 at 8:21

Stas Simonov

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1 Comment

Eric Sun Over a year ago

sizeof(b) is 20 which is not 32 byte aligned, which leave the possibility that b and other variables are in the same cache line which may cause potential problems

Ted Lyngmo · Accepted Answer · 2024-08-06 09:05:03Z

3

Here's one idea using variadic macros:

#include <stdalign.h> // for "alignas" - not needed since C23

#define VA_COUNT(...) (sizeof((int[]){0, __VA_ARGS__}) / sizeof(int) - 1)

#define ARRAY_INITIALIZER(V, ...)                       \
    struct {                                            \
        alignas(32) int payload[VA_COUNT(__VA_ARGS__)]; \
    } V = {.payload = {__VA_ARGS__}}

Note that the standard _Alignas (alignas) is used to align the array. The payload size is calculated by using VA_COUNT.

Example use:

#include <stdio.h>

// the macro definitions from above goes here

#define SIZE(x) (sizeof(x) / sizeof *(x))

int main(void) {
    ARRAY_INITIALIZER(a, 1, 2, 3, 4);

    for (unsigned i = 0; i < SIZE(a.payload); ++i) {
        printf("%d\n", a.payload[i]);
    }
}

Output:

answered Aug 6, 2024 at 9:05

Ted Lyngmo

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3 Comments

Eric Sun Over a year ago

Hi Ted. In my environment I don't have alignas so I still use__attribute__((aligned(32))) after the struct. I am also changing the {0, __VA_ARGS__} to __VA_ARGS__ because I want the _VA_ARGS hold the full {...} initializer, and the result is OK. Any reason you add an extra 0 for it? I check sizeof((int[]){}) and it can compile OK

Ted Lyngmo Over a year ago

@EricSun Don't you have _Alignas either? It's in the language since C11, but perhaps you are using an older standard? I recommend that you do not remove the extra 0 and the -1 to compensate. Not that it matters much since you will not use it with empty parameter lists but there's no reason to make the macro fragile. Demo

Eric Sun Over a year ago

Understand Ted. BTW godbolt.org is new to me and seems very useful. Thank you for the help!

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