[PATCH 0/5] MR201: ntdll: Implement RtlSetUserValueHeap.

[Rémi Bernon (@rbernon)]

On Tue Jun  7 19:33:23 2022 +0000, **** wrote:
> Zebediah Figura replied on the mailing list:
> ```
> On 6/7/22 13:33, Rémi Bernon wrote:
> >   typedef struct
> >   {
> > +    SIZE_T __pad[sizeof(SIZE_T) / sizeof(DWORD)];
> >       struct list           entry;      /* entry in heap large blocks
> list */
> >       SIZE_T                data_size;  /* size of user data */
> >       SIZE_T                block_size; /* total size of virtual
> memory block */
> > -    DWORD                 pad[2];     /* padding to ensure 16-byte
> alignment of data */
> > +    void                 *user_value;
> >       DWORD                 size;       /* fields for compatibility
> with normal arenas */
> >       DWORD                 magic;      /* these must remain at the
> end of the structure */
> >   } ARENA_LARGE;
> Doesn't this end up wasting space on 64 bits? I.e. the structure is 
> already aligned to a multiple of 16 bytes without any padding.
> (Also, somewhat out of curiosity, why move the member?)
> ```
It makes the large block header size match native size, of `8 * sizeof(void *)`.

Moving the member makes the struct layout closer to SUBHEAP, and I think the same struct could be use for both later. Native has a concept of heap region, which seems to correspond to both large blocks and SUBHEAP at the same time. For instance only growable heap let you allocate large blocks.

-- 
https://gitlab.winehq.org/wine/wine/-/merge_requests/201#note_1655