Reorder struct layouts to minimize padding

[Algunenano]

The current LWGEOM structs have too much padding which makes them bigger than what they could be.

For example, currently they look like this:

typedef struct
{
    uint8_t type;
    uint8_t flags;
    GBOX *bbox;
    int32_t srid;
    void *data;
}
LWGEOM;

When you build them in a 64 bit machine which I'm considering the default, the struct looks like this:

{
    uint8_t type;     /* 1 byte */
    uint8_t flags;    /* 1 byte */
                      /* 6 bytes padding */
    GBOX *bbox;       /* 8 bytes */
    int32_t srid;     /* 4 bytes */
                      /* 4 bytes padding */
    void *data;       /* 8 bytes */
}

For a total of 32 bytes.

I'm proposing to use this instead:

{
	GBOX *bbox;    /* 8 bytes */
	void *data;    /* 8 bytes */
	int32_t srid;  /* 4 bytes */
	uint8_t type;  /* 1 byte */
	uint8_t flags; /* 1 byte */
	char pad[2];   /* 2 bytes of padding */
}

So the new way uses 24 bytes instead (25% less). I expect this to have a impact in big multigeometries as it reduces their memory footprint which should improve performance. OTOH, I want to test it first as changing the position of certain elements might be harmful in certain scenarios.

[Algunenano]

Initial WIP in ​https://github.com/postgis/postgis/pull/390

If I see a clear benefit I might investigate further and check other structs too.

[Algunenano]

With Komzpa's idea, I decided to hack both ST_Subdivide and ST_Collect to print memory stats and test what was the impact using the bigpolygon table:

diff --git a/postgis/lwgeom_accum.c b/postgis/lwgeom_accum.c
index db7ccd3fe..1d9c5844e 100644
--- a/postgis/lwgeom_accum.c
+++ b/postgis/lwgeom_accum.c
@@ -240,6 +240,7 @@ pgis_geometry_collect_finalfn(PG_FUNCTION_ARGS)
 
     geometry_array = pgis_accum_finalfn(p, CurrentMemoryContext, fcinfo);
     result = PGISDirectFunctionCall1( LWGEOM_collect_garray, geometry_array );
+    MemoryContextStats(CurrentMemoryContext);
     if (!result)
         PG_RETURN_NULL();
 
diff --git a/postgis/lwgeom_dump.c b/postgis/lwgeom_dump.c
index 133a2d7d0..92789c675 100644
--- a/postgis/lwgeom_dump.c
+++ b/postgis/lwgeom_dump.c
@@ -400,6 +400,7 @@ Datum ST_Subdivide(PG_FUNCTION_ARGS)
     else
     {
         /* do when there is no more left */
+        MemoryContextStats(funcctx->multi_call_memory_ctx);
         SRF_RETURN_DONE(funcctx);
     }
 }

SQL: Select ST_Collect(geom) from ( select ST_Subdivide(geom) As geom FROM big_polygon ) _a;

• Before changes:

ST_Subdivide:

 multi-call context: 11795280 total in 1722 blocks; 3172560 free (9560 chunks); 8622720 used
 total: 11795280 bytes in 1722 blocks; 3172560 free (9560 chunks); 8622720 used

ST_Collect:

 8069592 total in 11 blocks; 494824 free (2 chunks); 7574768 used
 total: 8069592 bytes in 11 blocks; 494824 free (2 chunks); 7574768 used

• After changes:

Subdivide uses 2 blocks less (1710/1712). ~1% less total bytes used:

 multi-call context: 11696976 total in 1710 blocks; 3134384 free (8883 chunks); 8562592 used
 total: 11696976 bytes in 1710 blocks; 3134384 free (8883 chunks); 8562592 used

ST_Collect uses 1 block less (10 vs 11). ~6.5% less total bytes used:

 7545304 total in 10 blocks; 30032 free (2 chunks); 7515272 used
 total: 7545304 bytes in 10 blocks; 30032 free (2 chunks); 7515272 used

I don't see any measurable impact in performance in my machine

[algunenano]

In 17407:

Reorder LWGEOM struct members to minimize extra padding

Closes ​https://github.com/postgis/postgis/pull/390
References #4368

[komzpa]

seems done and committed?