Re: Making type Datum be 8 bytes everywhere

Tomas Vondra <tomas(at)vondra(dot)me> writes:
> While testing a different patch, I tried running with address sanitizer
> on rpi5, running the 32-bit OS (which AFAIK is 64-bit kernel and 32-bit
> user space). With that, stats_ext regression tests fail like this:

> extended_stats.c:1082:27: runtime error: store to misaligned address
> 0x036671dc for type 'Datum', which requires 8 byte alignment
> 0x036671dc: note: pointer points here
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 7e
> 7f 08 00 00 00 7f 7f 7f 7f
> ^

> This happens because build_sorted_items() does palloc(), and then
> accesses the pointer as array of structs, with a Datum field. And it
> apparently expects the pointer to be a multiple of 8 bytes. Isn't that a
> bit strange, with 32-bit user space? The pointer is indeed a multiple of
> 4B, so maybe the expected alignment is wrong?

I think build_sorted_items is plainly at fault here, where it does

/* Compute the total amount of memory we need (both items and values). */
len = data->numrows * sizeof(SortItem) + nvalues * (sizeof(Datum) + sizeof(bool));

/* Allocate the memory and split it into the pieces. */
ptr = palloc0(len);

/* items to sort */
items = (SortItem *) ptr;
ptr += data->numrows * sizeof(SortItem);

/* values and null flags */
values = (Datum *) ptr;
ptr += nvalues * sizeof(Datum);

This is silently assuming that sizeof(SortItem) is a multiple of
alignof(Datum), which on a 32-bit-pointer platform is not true
any longer. We ought to MAXALIGN the two occurrences of
data->numrows * sizeof(SortItem).

Do you want to fix it, or shall I?

regards, tom lane