Re: Ensuring hash tuples are properly maxaligned

Hi,

On 2018-01-02 19:08:49 -0500, Tom Lane wrote:
> Now, the existing definition of the struct seems safe on all
> architectures we support, but it would not take much to break it.
> I think we ought to do what we did recently in the memory-context
> code: insert an explicit padding calculation and a static assertion
> that it's right. Hence, the attached proposed patch (in which
> I also failed to resist the temptation to make the two code paths
> in dense_alloc() look more alike). Any objections?

Generally no objections.

> + /*
> + * It's required that data[] start at a maxaligned offset. This padding
> + * calculation is correct as long as int is not wider than size_t and
> + * dsa_pointer is not wider than a regular pointer, but there's a static
> + * assertion to check things in nodeHash.c.
> + */

But note that dsa_pointer can be wider than a regular pointer on
platforms without atomics support.

> +#define HASHMEMORYCHUNKDATA_RAWSIZE (SIZEOF_SIZE_T * 3 + SIZEOF_VOID_P)
> +
> + /* ensure proper alignment by adding padding if needed */
> +#if (HASHMEMORYCHUNKDATA_RAWSIZE % MAXIMUM_ALIGNOF) != 0
> + char padding[MAXIMUM_ALIGNOF - HASHMEMORYCHUNKDATA_RAWSIZE % MAXIMUM_ALIGNOF];
> +#endif
> +
> char data[FLEXIBLE_ARRAY_MEMBER]; /* buffer allocated at the end */
> } HashMemoryChunkData;

Wonder if this specific case wouldn't be easier handled with a union?

Greetings,

Andres Freund