Re: [PoC] Improve dead tuple storage for lazy vacuum

On Mon, Apr 8, 2024 at 7:42 PM Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com> wrote:
>
>> I pushed both of these and see that mylodon complains that anonymous
>> unions are a C11 feature. I'm not actually sure that the union with
>> uintptr_t is actually needed, though, since that's not accessed as
>> such here. The simplest thing seems to get rid if the union and name
>> the inner struct "header", as in the attached.
>
>
> Provided uintptr_t is not accessed it might be good to get rid of it.
>
> Maybe this patch also need correction in this:
> +#define NUM_FULL_OFFSETS ((sizeof(uintptr_t) - sizeof(uint8) - sizeof(int8)) / sizeof(OffsetNumber))

For full context the diff was

-#define NUM_FULL_OFFSETS ((sizeof(bitmapword) - sizeof(uint16)) /
sizeof(OffsetNumber))
+#define NUM_FULL_OFFSETS ((sizeof(uintptr_t) - sizeof(uint8) -
sizeof(int8)) / sizeof(OffsetNumber))

I wanted the former, from f35bd9bf35 , to be independently useful (in
case the commit in question had some unresolvable issue), and its
intent is to fill struct padding when the array of bitmapword happens
to have length zero. Changing to uintptr_t for the size calculation
reflects the intent to fit in a (local) pointer, regardless of the
size of a bitmapword. (If a DSA pointer happens to be a different size
for some odd platform, it should still work, BTW.)

My thinking with the union was, for big-endian, to force the 'flags'
member to where it can be set, but thinking again, it should still
work if by happenstance the header was smaller than the child pointer:
A different bit would get tagged, but I believe that's irrelevant. The
'flags' member makes sure a byte is reserved for the tag, but it may
not be where the tag is actually located, if that makes sense.